// -*- text -*-
/**
 * \file GeographicLib.dox
 * \brief Documentation for GeographicLib
 *
 * Written by Charles Karney <charles@karney.com> and licensed under the
 * MIT/X11 License.  For more information, see
 * https://geographiclib.sourceforge.io/
 **********************************************************************/
namespace GeographicLib {
/**
\mainpage GeographicLib library
\author Charles F. F. Karney (charles@karney.com)
\version @PROJECT_VERSION@
\date 2022-07-25

\section abstract Abstract

GeographicLib is a small C++ library for

- geodesic and rhumb line calculations;
- conversions between geographic, UTM, UPS, MGRS, geocentric, and local
  cartesian coordinates;
- gravity (e.g., EGM2008) and geomagnetic field (e.g., WMM2020)
  calculations.

The emphasis is on returning accurate results with errors close to
round-off (about 5--15 nanometers).  Accurate algorithms for \ref
geodesic and \ref transversemercator have been developed for this
library.  The functionality of the library can be accessed from user
code or from the \ref utilities provided.

It is licensed under the MIT License; see
<a href="../../LICENSE.txt">LICENSE.txt</a>.

This library is <i>not</i> a general purpose projection library nor does
it perform datum conversions; instead use
<a href="https://proj.org">PROJ</a>.

<b>NOTE:</b> Starting with version 2.0, this library *only* includes the
C++ implementation.  If you're interested in the
[C](../../C/doc/index.html),
[Fortran](../../Fortran/doc/index.html),
[Python](../../Python/doc/index.html),
[Octave/MATLAB](https://github.com/geographiclib/geographiclib-octave#readme),
[Java](../../Java/doc/index.html), or
[JavaScript](../../JavaScript/doc/index.html)
implementations, go
[here](../../doc/library.html#languages) on the main [GeographicLib web
page](../../index.html).

\section links Links

- Library documentation (all versions):
  <a href="../">https://geographiclib.sourceforge.io/C++</a>
- Git repository: https://github.com/geographiclib/geographiclib
  - Releases are on the `release` branch, and specific
    releases are tagged as, e.g., `r1.52`,
    `r2.0`, etc.  This is the appropriate branch
    for most *users* of GeographicLib.
  - The main branch is `main` and most development is done on the
    `devel` branch.  These branches are for the *developers* of
    GeographicLib.  Tags `v1.52`, `v2.0`, etc., are aligned with the
    corresponding release tags `r1.52`, `r2.0`, etc.
- Source distribution:
  https://sourceforge.net/projects/geographiclib/files/distrib-C++
- Main GeographicLib web page:
  <a href="../../index.html">https://geographiclib.sourceforge.io</a>
- SourceForge project page: https://sourceforge.net/projects/geographiclib
- The library has been implemented in a few other
  <a href="../../doc/library.html#languages">languages</a>.

\section citint Citing GeographicLib

When citing GeographicLib, use (adjust the version number and date as
appropriate)
- C. F. F. Karney, GeographicLib, Version @PROJECT_VERSION@ (2022-05-06),
  https://geographiclib.sourceforge.io/C++/@PROJECT_VERSION@

\section contents Contents
 - \ref intro
 - \ref install
 - \ref start
 - \ref utilities
 - \ref organization
 - \ref other
 - \ref geoid
 - \ref gravity
 - \ref normalgravity
 - \ref magnetic
 - \ref geodesic
 - \ref nearest
 - \ref triaxial
 - \ref jacobi
 - \ref rhumb
 - \ref greatellipse
 - \ref transversemercator
 - \ref geocentric
 - \ref auxlat
 - \ref highprec
 - \ref changes

<center>
Forward to \ref intro.
</center>

**********************************************************************/
/**
\page intro Introduction

<center>
Forward to \ref install.  Up to \ref contents.
</center>

GeographicLib offers a C++ interfaces to a small (but important!) set
of geographic transformations.  It grew out of a desire to improve on
the <a href="http://earth-info.nga.mil/GandG/geotrans/">GEOTRANS</a>
package for transforming between geographic and MGRS coordinates.  At
present, GeographicLib provides UTM, UPS, MGRS, geocentric, and local
cartesian projections, gravity and geomagnetic models, and classes for
geodesic calculations.

The goals of GeographicLib are:
 - Accuracy.  In most applications the accuracy is close to round-off,
   about 5&nbsp;nm (5 nanometers).  Even though in many geographic
   applications 1 cm is considered "accurate enough", there is little
   penalty in providing much better accuracy.  In situations where a
   faster approximate algorithm is necessary, GeographicLib offers an
   accurate benchmark to guide the development.
 - Completeness.  For each of the projections included, an attempt is
   made to provide a complete solution.  For example,
   Geodesic::Inverse works for anti-podal points.
   Similarly, Geocentric::Reverse will return accurate
   geodetic coordinates even for points close to the center of the
   earth.
 - C++ interface.  For the projection methods, this allows encapsulation
   of the ellipsoid parameters.
 - Emphasis on projections necessary for analyzing military data.
 - Uniform treatment of UTM/UPS.  The UTMUPS class treats
   UPS as zone 0.  This simplifies conversions between UTM and UPS
   coordinates, etc.
 - Well defined and stable conventions for the conversion between
   UTM/UPS to MGRS coordinates.
 - Detailed internal documentation on the algorithms.  For the most part
   GeographicLib uses published algorithms and references are given.  If
   changes have been made (usually to improve the numerical accuracy),
   these are described in the code.

Various \ref utilities are provided with the library.  These illustrate
the use of the library and are useful in their own right.  This library
and the utilities have been tested with C++11 compliant versions of g++
under Linux, with Apple LLVM 7.0.2 under Mac OS X, and with MS Visual
Studio 14 (2015), 15 (2017), and 16 (2019) compiled for 32 bit and 64
bit on Windows.

The section \ref geodesic documents the method of solving the geodesic
problem.

The section \ref transversemercator documents various properties of this
projection.

The bulk of the testing has used geographically relevant values of the
flattening.  Thus, you can expect close to full accuracy for &minus;0.01
&le; \e f &le; 0.01 (but note that TransverseMercatorExact is restricted
to \e f &gt; 0).  However, reasonably accurate results can be expected if
&minus;0.1 &le; \e f &le; 0.1.  Outside this range, you should attempt
to verify the accuracy of the routines independently.  Two types of
problems may occur with larger values of <i>f</i>:
 - Some classes, specifically Geodesic, GeodesicLine, and
   TransverseMercator, use series expansions using \e f as a small
   parameter.  The accuracy of these routines will degrade as \e f
   becomes large.
 - Even when exact formulas are used, many of the classes need to invert
   the exact formulas (e.g., to invert a projection), typically, using
   Newton's method.  This usually provides an essentially exact
   inversion.  However, the choice of starting guess and the exit
   conditions have been tuned to cover small values of \e f and the
   inversion may be incorrect if \e f is large.

Undoubtedly, bugs lurk in this code and in the documentation.  Please
report any you find to charles@karney.com.

<center>
Forward to \ref install.  Up to \ref contents.
</center>

**********************************************************************/
/**
\page install Installing GeographicLib

<center>
Back to \ref intro.  Forward to \ref start.  Up to \ref contents.
</center>

GeographicLib has been developed under Linux with the g++ compiler,
under Mac OS X with Xcode, and under Windows with Visual Studio 2015 and
later.  It should compile on any systems with a C++11 compliant
compiler.  First download one of
- <a href="https://sourceforge.net/projects/geographiclib/files/distrib-C++/GeographicLib-@PROJECT_FULLVERSION@.tar.gz">
GeographicLib-@PROJECT_VERSION@.tar.gz</a> tar'ed sources
- <a href="https://sourceforge.net/projects/geographiclib/files/distrib-C++/GeographicLib-@PROJECT_FULLVERSION@.zip">
GeographicLib-@PROJECT_VERSION@.zip</a> zipped sources
- <a href="https://sourceforge.net/projects/geographiclib/files/distrib-C++/GeographicLib-@PROJECT_FULLVERSION@-win32.exe">
GeographicLib-@PROJECT_VERSION@-win32.exe</a> 32-bit binary installer for Windows
- <a href="https://sourceforge.net/projects/geographiclib/files/distrib-C++/GeographicLib-@PROJECT_FULLVERSION@-win64.exe">
GeographicLib-@PROJECT_VERSION@-win64.exe</a> 64-bit binary installer for Windows
.
Then pick one of the first five options below:
- \ref cmake.  This is the preferred installation method as it will work
  on the widest range of platforms.  However it requires that you have
  <a href="https://www.cmake.org">cmake</a> installed.
- \ref autoconf.  This method works for most Unix-like systems,
  including Linux and Mac OS X.
- \ref binaryinst.  Use this installation method if you only need to use
  the \ref utilities supplied with GeographicLib.
- \ref maintainer.  This describes addition tasks of interest only to
  the maintainers of this code.
.
This section documents only how to install the <i>software</i>.  If you
wish to use GeographicLib to evaluate geoid heights or the earth's
gravitational or magnetic fields, then you must also install the
relevant data files.  See \ref geoidinst, \ref gravityinst, and \ref
magneticinst for instructions.

The first two installation methods use two important techniques which
make software maintenance simpler
- <b>Out-of-source builds:</b>  This means that you create a separate
  directory for compiling the code.  In the description here the
  directories are called BUILD and are located in the top-level of the
  source tree.  You might want to use a suffix to denote the type of
  build, e.g., BUILD-vc14 for Visual Studio 14 (2015), or BUILD-shared
  for a build which creates a shared library.  The advantages of
  out-of-source builds are:
  - You don't mess up the source tree, so it's easy to "clean up".
    Indeed the source tree might be on a read-only file system.
  - Builds for multiple platforms or compilers don't interfere with each
    other.
- <b>The library is installed:</b> After compilation, there is a
  separate <i>install</i> step which copies the headers, libraries,
  tools, and documentation to a "central" location.  You may at this
  point delete the source and build directories.  If you have
  administrative privileges, you can install GeographicLib for the use
  of all users (e.g., in /usr/local).  Otherwise, you can install it for
  your personal use (e.g., in $HOME/packages).

\section cmake Installation with cmake

This is the recommended method of installation; however it requires that
<a href="https://www.cmake.org">cmake</a>, version 3.7.0 or later, be
installed on your system.  This permits GeographicLib to be built either
as a shared or a static library on a wide variety of systems.  cmake can
also determine the capabilities of your system and adjust the
compilation of the libraries and examples appropriately.

cmake is available for most computer platforms.  On Linux systems cmake
will typically be one of the standard packages and can be installed by a
command like
  \verbatim
  yum install cmake \endverbatim
(executed as root).  The minimum version of cmake supported for building
GeographicLib is 3.7.0 (which was released on 2016-11-11).

On other systems, download a binary installer from https://www.cmake.org
click on download, and save and run the appropriate installer.  Run the
cmake command with no arguments to get help.  Other useful tools are
ccmake and cmake-gui which offer curses and graphical interfaces to
cmake.  Building under cmake depends on whether it is targeting an IDE
(interactive development environment) or generating Unix-style
makefiles.  The instructions below have been tested with makefiles and
g++ on Linux and with the Visual Studio IDE on Windows.  It is known to
work also for Visual Studio 2017 Build Tools.

Here are the steps to compile and install GeographicLib:
- Unpack the source, running one of \verbatim
  tar xfpz GeographicLib-@PROJECT_VERSION@.tar.gz
  unzip -q GeographicLib-@PROJECT_VERSION@.zip \endverbatim
  then enter the directory created with \verbatim
  cd GeographicLib-@PROJECT_VERSION@ \endverbatim
- Create a separate build directory and enter it, for example, \verbatim
  mkdir BUILD
  cd BUILD \endverbatim
- Run cmake, pointing it to the source directory (..).  On Linux, Unix,
  and MacOSX systems, the command is \verbatim
  cmake .. \endverbatim
  For Windows, the command is typically something like \verbatim
  cmake -G "Visual Studio 14" -A win32 \
    -D CMAKE_INSTALL_PREFIX="C:/Program Files (x86)/GeographicLib-@PROJECT_VERSION@" \
    ..
  cmake -G "Visual Studio 14" -A x64 \
    -D CMAKE_INSTALL_PREFIX="C:/Program Files/GeographicLib-@PROJECT_VERSION@" \
    ..\endverbatim
  The definitions of CMAKE_INSTALL_PREFIX point to system directories.
  You might instead prefer to install to a user directory such as
  <code>C:/Users/jsmith/projects/GeographicLib-@PROJECT_VERSION@</code>.
  The settings given above are recommended to ensure that packages that
  use GeographicLib use the version compiled with the right compiler.
  If you need to rerun cmake, use \verbatim
  cmake . \endverbatim
  possibly including some options via <code>-D</code> (see the next step).
- cmake allows you to configure how GeographicLib is built and installed by
  supplying options, for example `\verbatim
  cmake -D CMAKE_INSTALL_PREFIX=/tmp/geographic . \endverbatim
  The options you might need to change are
  - <code>CMAKE_INSTALL_PREFIX</code> (default: <code>/usr/local</code>
    on non-Windows systems, <code>C:/Program Files/GeographicLib</code>
    on Windows systems) specifies where the library will be installed.
    For Windows systems, you might want to use a prefix which includes
    the compiler version, in order to keep the libraries built with
    different versions of the compiler in distinct locations.  If you
    just want to try the library to see if it suits your needs, pick,
    for example, `CMAKE_INSTALL_PREFIX=/tmp/geographic`.  Where under
    `CMAKE_INSTALL_PREFIX` the various components of the library are
    installed is governed by the following variables (a value of an
    empty string or "OFF" disables installation):
    - `INCDIR` header files (the `GeographicLib` subdirectory is used)
    - `BINDIR` tools
    - `SBINDIR` admin tools
    - `LIBDIR` libraries
    - `DLLDIR` dlls
    - `MANDIR` the man pages (the `man1` and `man8` subdirectories are used)
    - `CMAKEDIR` cmake configs
    - `PKGDIR` package config
    - `DOCDIR` documentation
    - `EXAMPLEDIR` examples
  - <code>GEOGRAPHICLIB_DATA</code> (default:
    /usr/local/share/GeographicLib for non-Windows systems,
    C:/ProgramData/GeographicLib for Windows systems) specifies the default
    location for the various datasets for use by Geoid,
    GravityModel, and MagneticModel.
    See \ref geoidinst, \ref gravityinst, and \ref magneticinst for more
    information.
  - <code>BUILD_SHARED_LIBS</code> (default ON) and
    <code>BUILD_BOTH_LIBS</code> (default OFF) specify the types of
    libraries to build.  If `BUILD_BOTH_LIBS` is set, then both shared
    and static libraries are building.  Otherwise a single library is
    built with the type determined by `BUILD_SHARED_LIBS`
  - <code>CMAKE_BUILD_TYPE</code> (default: Release).  This
    flags only affects non-IDE compile environments (like make + g++).
    The default is actually blank, but this is treated as
    Release.  Choose one of
    \verbatim
  Debug
  Release
  RelWithDebInfo
  MinSizeRel
\endverbatim
    (With IDE compile environments, you get to select the build type in
    the IDE.)
  - <code>BUILD_DOCUMENTATION</code> (default: OFF).  If set to
    ON, then html documentation is created from the source files,
    provided a sufficiently recent version of doxygen can be found.
    Otherwise, the html documentation will redirect to the appropriate
    version of the online documentation.
  - <code>GEOGRAPHICLIB_PRECISION</code> specifies the precision to be
    used for "real" (i.e., floating point) numbers. See \ref highprec
    for additional information about using quad and arbitrary precision.
    Nearly all the testing has been carried out with doubles and that's
    the recommended configuration.  In particular you should avoid
    "installing" the library with a precision different from double.
    Here are the possible values
    -# float (24-bit precision); typically this is far to inaccurate
       for geodetic applications.
    -# double precision (53-bit precision, the default).
    -# long double (64-bit precision); this does not apply for Visual
       Studio (long double is the same as double with that compiler).
    -# quad precision (113-bit precision).
    -# arbitrary precision.
  - <code>USE_BOOST_FOR_EXAMPLES</code> (default: OFF).  If set to ON,
    then the Boost library is searched for in order to build the
    NearestNeighbor example.
  - <code>APPLE_MULTIPLE_ARCHITECTURES</code> (default: OFF).  If set to
    ON, build for i386 and x86_64 and Mac OS X systems.  Otherwise,
    build for the default architecture.
  - <code>CONVERT_WARNINGS_TO_ERRORS</code> (default: OFF).  If set to
    ON, then compiler warnings are treated as errors.
  .
- Build and install the software.  In non-IDE environments, run
  \verbatim
  make         # compile the library and utilities
  make test    # run some tests
  make install # as root, if CMAKE_INSTALL_PREFIX is a system directory
\endverbatim
  Possible additional targets are \verbatim
  make dist
  make exampleprograms \endverbatim
  On IDE environments, run your IDE (e.g., Visual Studio), load
  GeographicLib.sln, pick the build type (e.g., Release), and select
  "Build Solution".  If this succeeds, select "RUN_TESTS" to build;
  finally, select "INSTALL" to install (RUN_TESTS and INSTALL are in
  the CMakePredefinedTargets folder).  Alternatively (for example, if
  you are using the Visual Studio 2017 Build Tools), you run the Visual
  Studio compiler from the command line with \verbatim
  cmake --build . --config Release --target ALL_BUILD
  cmake --build . --config Release --target RUN_TESTS
  cmake --build . --config Release --target INSTALL \endverbatim
  For maximum flexibility, it's a good idea to build and install both
  the Debug and Release versions of the library (in that order).  The
  installation directories will then contain the release versions of the
  tools and <i>both</i> versions (debug and release) of the libraries.
  If you use cmake to configure and build your programs, then the right
  version of the library (debug vs. release) will automatically be used.
- The headers, library, and utilities are installed in the
  `include/GeographicLib`, `lib`, and `bin` directories under
  <code>CMAKE_INSTALL_PREFIX</code>.  (dll dynamic libraries are
  installed in `bin`.)  For documentation, open in a web browser
  <a href="index.html">`share/doc/GeographicLib/html/index.html`</a>.
- With cmake 3.13 and later, cmake can create the build directory for
  you.  This allows you to configure and run the build on Windows with
  \verbatim
  cmake -G "Visual Studio 14" -A x64 -S . -B BUILD
  cmake --build BUILD --config Release --target ALL_BUILD \endverbatim
  or on Linux with \verbatim
  cmake -S . -B BUILD
  make -C BUILD -j4 \endverbatim

\section autoconf Installation using the autoconfigure tools

The method works on most Unix-like systems including Linux and Mac OS X.
Here are the steps to compile and install GeographicLib:
- Unpack the source, running \verbatim
  tar xfpz GeographicLib-@PROJECT_VERSION@.tar.gz \endverbatim
  then enter the directory created \verbatim
  cd GeographicLib-@PROJECT_VERSION@ \endverbatim
- Create a separate build directory and enter it, for example, \verbatim
  mkdir BUILD
  cd BUILD \endverbatim
- Configure the software, specifying the path of the source directory,
  with \verbatim
  ../configure \endverbatim
- By default GeographicLib will be installed under /usr/local.
  You can change this with, for example \verbatim
  ../configure --prefix=/tmp/geographic \endverbatim
- Compile and install the software with \verbatim
  make
  make install \endverbatim
- The headers, library, and utilities are installed in the
  include/GeographicLib, lib, and bin directories under
  <code>prefix</code>.  For documentation, open <a href="index.html">
  `share/doc/GeographicLib/html/index.html`</a> in a web browser.

\section binaryinst Using a binary installer

Binary installers are available for some platforms

\subsection binaryinstwin Windows

Use this method if you only need to use the GeographicLib utilities.
The header files and static and shared versions of library are also
provided; these can only be used by Visual Studio 14 2015 or later
(2017, 2019, or 2022) (in either release or debug mode).  However, if
you plan to use the library, it may be advisable to build it with the
compiler you are using for your own code using \ref cmake.

Download and run
<a href="https://sourceforge.net/projects/geographiclib/files/distrib-C++/GeographicLib-@PROJECT_VERSION@-win32.exe">
GeographicLib-@PROJECT_VERSION@-win32.exe</a> or
<a href="https://sourceforge.net/projects/geographiclib/files/distrib-C++/GeographicLib-@PROJECT_VERSION@-win64.exe">
GeographicLib-@PROJECT_VERSION@-win64.exe</a>:
 - read the MIT/X11 License agreement,
 - select whether you want your PATH modified,
 - select the installation folder, by default
   `C:\\Program Files\\GeographicLib-@PROJECT_VERSION@` or
   `C:\\Program Files (x86)\\GeographicLib-@PROJECT_VERSION@`,
 - select the start menu folder,
 - and install.
 .
(Note that the default installation folder adheres the the convention
given in \ref cmake.)  The start menu will now include links to the
documentation for the library and for the utilities (and a link for
uninstalling the library).  If you ask for your PATH to be modified, it
will include
<code>C:/Program FilesGeographicLib-@PROJECT_VERSION@/bin</code>
where the utilities are installed.  The headers and library are
installed in the `include/GeographicLib` and `lib` folders.  The libraries
were built using using Visual Studio 14 2015 in release and debug modes.
The utilities were linked against the release-mode shared library.

\subsection binaryinstosx MacOSX

GeographicLib is available as Homebrew package
[geographiclib](https://formulae.brew.sh/formula/geographiclib).  Follow
the directions on https://brew.sh/ for installing this package manager.
Then type \verbatim
brew install geographiclib \endverbatim
Now links to GeographicLib are installed under /usr/local.

\subsection binaryinstlin Linux

Some Linux distributions, Fedora, Debian, and Ubuntu, offer
GeographicLib as a standard package.  Typically these will be one or two
versions behind the latest.  It's also available with the follow package
managers
- Homebrew package [geographiclib](
  https://formulae.brew.sh/formula/geographiclib) (also for MacOSX)
- conda-forge package [geographiclib-cpp](
  https://anaconda.org/conda-forge/geographiclib-cpp) (also for MacOSX
  and Windows)
- vcpkg package [geographiclib](https://vcpkg.info/port/geographiclib)
  (also for MacOSX and Windows)

\section maintainer Maintainer tasks

Some aspects of library maintenance are covered in [HOWTO-RELEASE.txt](
https://github.com/geographiclib/geographiclib/blob/main/HOWTO-RELEASE.txt).

Check the code out of git with \verbatim
  git clone git@github.com:geographiclib/geographiclib.git
\endverbatim
There are three branches in the git repository:
- <b>main</b>: the main branch for code maintenance.  Releases are
  tagged on this branch as, e.g., v@PROJECT_VERSION@.
- <b>devel</b>: the development branch; changes made here are merged
  into main.
- <b>release</b>: the release branch created by unpacking the source
  releases (git is \e not used to merge changes from the other
  branches into this branch).  This differs from the main branch in that
  some administrative files are excluded while some intermediate files
  are included (in order to aid building on as many platforms as
  possible).  Releases are tagged on this branch as, e.g.,
  r@PROJECT_VERSION@.
.
The autoconf configuration script and the formatted man pages are not
checked into main branch of the repository.  In order to create these,
configure with cmake and run \verbatim
  make dist \endverbatim
which will copy the man pages from the build directory back into the
source tree and package the resulting source tree for distribution as
\verbatim
  GeographicLib-@PROJECT_VERSION@.tar.gz
  GeographicLib-@PROJECT_VERSION@.zip \endverbatim
Finally, \verbatim
  make package \endverbatim
or building PACKAGE in Visual Studio will create a binary installer for
GeographicLib.  For Windows, this requires in the installation of
<a href="https://nsis.sourceforge.io">NSIS</a>.  On Windows, you should
configure GeographicLib with <code>PACKAGE_DEBUG_LIBS</code>=ON, build both
Release and Debug versions of the library and finally build PACKAGE in
Release mode.  This will get the release and debug versions of the
library included in the package.  For example, the 64-bit binary
installer is created with \verbatim
  cmake -G "Visual Studio 14" -A x64 \
    -D BUILD_BOTH_LIBS=ON \
    -D PACKAGE_DEBUG_LIBS=ON \
    ..
  cmake --build . --config Debug --target ALL_BUILD
  cmake --build . --config Release --target ALL_BUILD
  cmake --build . --config Release --target PACKAGE \endverbatim

Prior to making a release, the script
<code>develop/test-distribution.sh</code> is run on a Fedora system.  This
checked that the library compiles correctly is multiple configurations.
In addition it creates a directory and scripts for checking the
compilation on Windows.

The following Fedora packages are required by
<code>tests/test-distribution.sh</code>
- cmake
- automake
- autoconf-archive
- libtool
- gcc-c++
- ccache
- doxygen
- boost-devel
- mpfr-devel
- mpreal.h, version 3.6.9 with PR #15, from
  https://github.com/advanpix/mpreal and installed in the
  <code>include/</code> directory.

For all the tests to be run, the following datasets need to be installed
- geoid models: `egm96-5`
- magnetic models: `wmm2010` `emm2015`
- gravity models: `egm2008` `grs80`

<center>
Back to \ref intro.  Forward to \ref start.  Up to \ref contents.
</center>

**********************************************************************/
/**
\page start Getting started

<center>
Back to \ref install.  Forward to \ref utilities.  Up to \ref contents.
</center>

Much (but not all!) of the useful functionality of GeographicLib is
available via simple command line utilities.  Interfaces to some of them
are available via the web.  See \ref utilities for documentation on
these.

In order to use GeographicLib from C++ code, you will need to
- Include the header files for the GeographicLib classes in your code.
  E.g., \code
  #include <GeographicLib/LambertConformalConic.hpp> \endcode
- Include the GeographicLib:: namespace prefix to the GeographicLib classes,
  or include \code
  using namespace GeographicLib; \endcode
  in your code.
- Build your code with cmake.  In brief, the necessary steps are:
  - include in your CMakeLists.txt files \verbatim
    find_package (GeographicLib REQUIRED)
    include_directories (${GeographicLib_INCLUDE_DIRS})
    add_executable (program source1.cpp source2.cpp)
    target_link_libraries (program ${GeographicLib_LIBRARIES}) \endverbatim
  - configure your package, e.g., with \verbatim
    mkdir BUILD
    cd BUILD
    cmake -G "Visual Studio 14" -A x64 \
      -D CMAKE_PREFIX_PATH="C:/Program Files" \
      -D CMAKE_INSTALL_PREFIX="C:/Program Files/testgeog" \
      .. \endverbatim
    Note that you almost always want to configure and build your code
    somewhere other than the source directory (in this case, we use the
    BUILD subdirectory).  Also, on Windows, make sure that the version
    of Visual Studio (14 in the example above) architecture (x64 in the
    example above) is compatible with that used to build GeographicLib.
    In this example, it's not necessary to specify
    <code>CMAKE_PREFIX_PATH</code>, because <code>C:/Program
    Files</code> is one of the system paths which is searched
    automatically.
  - build your package.  On Linux and MacOS this usually involves just
    running make.  On Windows, you can load the solution file created by
    cmake into Visual Studio; alternatively, you can get cmake to run
    build your code with \verbatim
    cmake --build . --config Release --target ALL_BUILD \endverbatim
    You might also want to install your package (using "make install" or
    build the "INSTALL" target with the command above).
    With cmake 3.13 and later, cmake can create the build directory for
    you.  This allows you to configure and run the build on Windows with
    \verbatim
    cmake -G "Visual Studio 14" -A x64 \
      -D CMAKE_PREFIX_PATH="C:/Program Files" \
      -D CMAKE_INSTALL_PREFIX="C:/Program Files/testgeog" \
      -S . -B BUILD
    cmake --build BUILD --config Release --target ALL_BUILD \endverbatim
    or on Linux with \verbatim
    cmake -D CMAKE_INSTALL_PREFIX="/tmp/testgeog" \
      -S . -B BUILD
    make -C BUILD -j4 \endverbatim

NOTE: The inclusion of \verbatim
  include_directories (${GeographicLib_INCLUDE_DIRS}) \endverbatim
is only necessary if `find_package` found GeographicLib via the
`FindGeographicLib.cmake` "module".  However, it's OK to include this
line even for "config" lookups.

The most import step in using cmake is the `find_package` command.  The cmake
documentation describes the locations searched by find_package (the
appropriate rule for GeographicLib are those for "Config" mode lookups).
In brief, the locations that are searched are (from least specific to
most specific, i.e., in <i>reverse</i> order) are
- under the system paths, i.e., locations such as <code>C:/Program
  Files</code> and <code>/usr/local</code>);
- frequently, it's necessary to search within a "package directory"
  (or set of directories) for external dependencies; this is given by
  a (semicolon separated) list of directories specified by the cmake
  variable <code>CMAKE_PREFIX_PATH</code> (illustrated above);
- the package directory for GeographicLib can be overridden with the
  <i>environment variable</i> <code>GeographicLib_DIR</code> (which is the
  directory under which GeographicLib is installed);
- finally, if you need to point to a particular build of GeographicLib,
  define the <i>cmake variable</i> <code>GeographicLib_DIR</code>, which
  specifies the absolute path of the directory containing the
  configuration file <code>geographiclib-config.cmake</code> (for
  debugging this may be the top-level <i>build</i> directory, as
  opposed to <i>installation</i> directory, for GeographicLib).
.
Typically, specifying nothing or <code>CMAKE_PREFIX_PATH</code>
suffices.  However the two <code>GeographicLib_DIR</code> variables allow
for a specific version to be chosen.  On Windows systems (with Visual
Studio), find_package will only find versions of GeographicLib built with
the right version of the compiler.  (If you used a non-cmake method of
installing GeographicLib, you can try copying cmake/FindGeographicLib.cmake
to somewhere in your <code>CMAKE_MODULE_PATH</code> in order for
find_package to work.  However, this method has not been thoroughly
tested.)

If GeographicLib is not found, check the values of
<code>GeographicLib_CONSIDERED_CONFIGS</code> and
<code>GeographicLib_CONSIDERED_VERSIONS</code>; these list the
configuration files and corresponding versions which were considered
by find_package.

If GeographicLib is found, then the following cmake variables are set:
- <code>GeographicLib_FOUND</code> = 1
- <code>GeographicLib_VERSION</code> = @PROJECT_VERSION@
- <code>GeographicLib_INCLUDE_DIRS</code>
- <code>GeographicLib_LIBRARIES</code> = one of the following two:
- <code>GeographicLib_SHARED_LIBRARIES</code> = GeographicLib::GeographicLib_SHARED
- <code>GeographicLib_STATIC_LIBRARIES</code> = GeographicLib::GeographicLib_STATIC
- <code>GeographicLib_LIBRARY_DIRS</code>
- <code>GeographicLib_BINARY_DIRS</code>
- <code>GEOGRAPHICLIB_DATA</code> = value of this compile-time parameter
- <code>GEOGRAPHICLIB_PRECISION</code> = value of this compile-time
  parameter (usually 2).  You can set this parameter before calling
  `find_package` and only versions with a matching value of
  `GEOGRAPHICLIB_PRECISION` will be found.
.
Either of <code>GeographicLib_SHARED_LIBRARIES</code> or
<code>GeographicLib_STATIC_LIBRARIES</code> may be empty, if that version
of the library is unavailable.  If you require a specific version,
SHARED or STATIC, of the library, add a <code>COMPONENTS</code> clause
to find_package, e.g.,
\verbatim
  find_package (GeographicLib 2.0 REQUIRED COMPONENTS SHARED) \endverbatim
causes only packages which include the shared library to be found.  If
the package includes both versions of the library, then
<code>GeographicLib_LIBRARIES</code> is set to the shared version,
unless you include \verbatim
  set (GeographicLib_USE_STATIC_LIBS ON) \endverbatim
<i>before</i> the find_package command.  You can check whether
<code>GeographicLib_LIBRARIES</code> refers to the shared or static
library with \verbatim
  get_target_property (_LIBTYPE ${GeographicLib_LIBRARIES} TYPE) \endverbatim
which results in <code>_LIBTYPE</code> being set to
<code>SHARED_LIBRARY</code> or <code>STATIC_LIBRARY</code>.
On Windows, cmake takes care of linking to the release or debug
version of the library as appropriate.  (This assumes that the Release
and Debug versions of the libraries were built and installed.  This is
true for the Windows binary installer for GeographicLib version 1.34 and
later.)

You can also use the pkg-config utility to specify compile and link
flags.  This requires that pkg-config be installed and that it's
configured to search, e.g., /usr/local/lib/pgkconfig; if not, define
the environment variable <code>PKG_CONFIG_PATH</code> to include
this directory.  The compile and link steps under Linux would
typically be
\verbatim
g++ -c -g -O3 `pkg-config --cflags geographiclib` testprogram.cpp
g++ -g -o testprogram testprogram.o `pkg-config --libs geographiclib`
\endverbatim

Here is a very simple test code, which uses the Geodesic
class:
\include example-Geodesic-small.cpp
This example is <code>examples/example-Geodesic-small.cpp</code>.  If you
compile, link, and run it according to the instructions above, it should
print out \verbatim
  5551.76 km \endverbatim
Here is a complete CMakeList.txt files you can use to build this test
code using the installed library: \verbatim
project (geodesictest)
cmake_minimum_required (VERSION 3.1.0)

find_package (GeographicLib REQUIRED)
include_directories (${GeographicLib_INCLUDE_DIRS})

if (NOT MSVC)
  set (CMAKE_INSTALL_RPATH_USE_LINK_PATH TRUE)
endif ()

add_executable (${PROJECT_NAME} example-Geodesic-small.cpp)
target_link_libraries (${PROJECT_NAME} ${GeographicLib_LIBRARIES})

if (MSVC)
  get_target_property (_LIBTYPE ${GeographicLib_LIBRARIES} TYPE)
  if (_LIBTYPE STREQUAL "SHARED_LIBRARY")
    # On Windows systems, copy the shared library to build directory
    add_custom_command (TARGET ${PROJECT_NAME} POST_BUILD
      COMMAND ${CMAKE_COMMAND} -E
      copy $<TARGET_FILE:${GeographicLib_LIBRARIES}> ${CMAKE_CFG_INTDIR}
      COMMENT "Copying shared library for GeographicLib")
  endif ()
endif () \endverbatim

The next steps are:
 - Learn about and run the \ref utilities.
 - Read the section, \ref organization, for an overview of the library.
 - Browse the <a href="annotated.html">Class List</a> for full documentation
   on the classes in the library.
 - Look at the example code in the examples directory.  Each file
   provides a very simple standalone example of using one GeographicLib
   class.  These are included in the descriptions of the classes.
 - Look at the source code for the utilities in the tools directory for
   more examples of using GeographicLib from C++ code, e.g.,
   GeodesicProj.cpp is a program to performing various geodesic
   projections.

Here's a list of some of the abbreviations used here with links to the
corresponding Wikipedia articles:
 - <a href="https://en.wikipedia.org/wiki/WGS84">
   WGS84</a>, World Geodetic System 1984.
 - <a href="https://en.wikipedia.org/wiki/Universal_Transverse_Mercator_coordinate_system">
   UTM</a>, Universal Transverse Mercator coordinate system.
 - <a href="https://en.wikipedia.org/wiki/Universal_Polar_Stereographic">
   UPS</a>, Universal Polar Stereographic coordinate system.
 - <a href="https://en.wikipedia.org/wiki/Military_grid_reference_system">
   MGRS</a>, Military Grid Reference System.
 - <a href="https://en.wikipedia.org/wiki/Geoid">
   EGM</a>, Earth Gravity Model.
 - <a href="https://en.wikipedia.org/wiki/World_Magnetic_Model">
   WMM</a>, World Magnetic Model.
 - <a href="https://en.wikipedia.org/wiki/IGRF">
   IGRF</a>, International Geomagnetic Reference Field.

<center>
Back to \ref install.  Forward to \ref utilities.  Up to \ref contents.
</center>

**********************************************************************/
/**
\page utilities Utility programs

<center>
Back to \ref start.  Forward to \ref organization.  Up to \ref contents.
</center>

Various utility programs are provided with GeographicLib.  These should
be installed in a directory included in your PATH (e.g.,
/usr/local/bin).  These programs are wrapper programs that invoke
the underlying functionality provided by the library.

The utilities are
 - <a href="GeoConvert.1.html">
   <b>GeoConvert</b></a>: convert geographic coordinates using
   GeoCoords.  See \ref GeoConvert.cpp.
 - <a href="GeodSolve.1.html">
   <b>GeodSolve</b></a>: perform geodesic calculations using
   Geodesic and GeodesicLine.  See \ref GeodSolve.cpp.
 - <a href="Planimeter.1.html">
   <b>Planimeter</b></a>: compute the area of geodesic polygons using
   PolygonAreaT.  See \ref Planimeter.cpp.
 - <a href="TransverseMercatorProj.1.html">
   <b>TransverseMercatorProj</b></a>: convert between geographic
   and transverse Mercator.  This is for testing
   TransverseMercatorExact and
   TransverseMercator.  See \ref TransverseMercatorProj.cpp.
 - <a href="CartConvert.1.html">
   <b>CartConvert</b></a>: convert geodetic coordinates to geocentric or
   local cartesian using Geocentric and
   LocalCartesian.  See \ref CartConvert.cpp.
 - <a href="GeodesicProj.1.html">
   <b>GeodesicProj</b></a>: perform projections based on geodesics
   using AzimuthalEquidistant, Gnomonic,
   and CassiniSoldner.  See \ref GeodesicProj.cpp.
 - <a href="ConicProj.1.html">
   <b>ConicProj</b></a>: perform conic projections using
   LambertConformalConic and
   AlbersEqualArea.  See \ref ConicProj.cpp.
 - <a href="GeoidEval.1.html">
   <b>GeoidEval</b></a>: look up geoid heights using
   Geoid.  See \ref GeoidEval.cpp.
 - <a href="Gravity.1.html">
   <b>Gravity</b></a>: compute the earth's gravitational field using
   GravityModel and GravityCircle.  See \ref Gravity.cpp.
 - <a href="MagneticField.1.html">
   <b>MagneticField</b></a>: compute the earth's magnetic field using
   MagneticModel and MagneticCircle.  See \ref MagneticField.cpp.
 - <a href="RhumbSolve.1.html">
   <b>RhumbSolve</b></a>: perform rhumb line calculations using Rhumb
   and RhumbLine.  See \ref RhumbSolve.cpp.
 .
The documentation for these utilities is in the form of man pages.  This
documentation can be accessed by clicking on the utility name in the
list above, running the man command on Unix-like systems, or by invoking
the utility with the <code>\--help</code> option.  A brief summary of
usage is given by invoking the utility with the <code>-h</code> option.
The version of the utility is given by the <code>\--version</code>
option.

The utilities all accept data on standard input, transform it in some
way, and print the results on standard output.  This makes the utilities
easy to use within scripts to transform tabular data; however they can
also be used interactively, often with the input supplied via a pipe,
e.g.,
 - echo 38SMB4488 | GeoConvert -d

Online versions of four of these utilities are provided:
 - <a href="https://geographiclib.sourceforge.io/cgi-bin/GeoConvert">GeoConvert</a>
 - <a href="https://geographiclib.sourceforge.io/cgi-bin/GeodSolve">GeodSolve</a>
 - <a href="https://geographiclib.sourceforge.io/cgi-bin/Planimeter">Planimeter</a>
 - <a href="https://geographiclib.sourceforge.io/cgi-bin/GeoidEval">GeoidEval</a>
 - <a href="https://geographiclib.sourceforge.io/cgi-bin/RhumbSolve">RhumbSolve</a>

<center>
Back to \ref start.  Forward to \ref organization.  Up to \ref contents.
</center>

**********************************************************************/
/**
\page organization Code organization

<center>
Back to \ref utilities.  Forward to \ref other.  Up to \ref contents.
</center>

Here is a brief description of the relationship between the various
components of GeographicLib.  All of these are defined in the
GeographicLib namespace.

TransverseMercator, PolarStereographic, LambertConformalConic, and
AlbersEqualArea provide the basic projections.  The constructors for
these classes specify the ellipsoid and the forward and reverse
projections are implemented as const member functions.
TransverseMercator uses Kr&uuml;ger's series which have been extended to
sixth order in the square of the eccentricity.  PolarStereographic,
LambertConformalConic, and AlbersEqualArea use the exact formulas for
the projections (e.g., from Snyder).

TransverseMercator::UTM and PolarStereographic::UPS are const static
instantiations specific for the WGS84 ellipsoid with the UTM and UPS
scale factors.  (These do \e not add the standard false eastings or
false northings for UTM and UPS.)  Similarly
LambertConformalConic::Mercator is a const static instantiation of this
projection for a WGS84 ellipsoid and a standard parallel of 0 (which
gives the Mercator projection).  AlbersEqualArea::CylindricalEqualArea,
AzimuthalEqualAreaNorth, and AzimuthalEqualAreaSouth, likewise provide
special cases of the equal area projection.

UTMUPS uses TransverseMercator::UTM and PolarStereographic::UPS to
perform the UTM and UPS projections.  The class offers a uniform
interface to UTM and UPS by treating UPS as UTM zone 0.  This class
stores no internal state and the forward and reverse projections are
provided via static member functions.  The forward projection offers the
ability to override the standard UTM/UPS choice and the UTM zone.

MGRS transforms between UTM/UPS coordinates and MGRS.
UPS coordinates are handled as UTM zone 0.  This class stores no
internal state and the forward (UTM/UPS to MGRS) and reverse (MGRS to
UTM/UPS) conversions are provided via static member functions.

GeoCoords holds a single geographic location which may be
specified as latitude and longitude, UTM or UPS, or MGRS.  Member
functions are provided to convert between coordinate systems and to
provide formatted representations of them.
<a href="GeoConvert.1.html">GeoConvert</a> is a simple command line
utility to provide access to the GeoCoords class.

TransverseMercatorExact is a drop in replacement for TransverseMercator
which uses the exact formulas, based on elliptic functions, for the
projection as given by Lee.
<a href="TransverseMercatorProj.1.html">TransverseMercatorProj</a> is a
simple command line utility to test to the TransverseMercator and
TransverseMercatorExact.

Geodesic and GeodesicLine perform geodesic calculations.  The
constructor for Geodesic specifies the ellipsoid and the direct and
inverse calculations are implemented as const member functions.
Geocentric::WGS84 is a const static instantiation of Geodesic specific
for the WGS84 ellipsoid.  In order to perform a series of direct
geodesic calculations on a single line, the GeodesicLine class can be
used.  This packages all the information needed to specify a geodesic.
A const member function returns the coordinates a specified distance
from the starting point.  <a href="GeodSolve.1.html">GeodSolve</a> is a
simple command line utility to perform geodesic calculations.
PolygonAreaT is a class which compute the area of geodesic polygons
using the Geodesic class and <a href="Planimeter.1.html">Planimeter</a>
is a command line utility for the same purpose.  AzimuthalEquidistant,
CassiniSoldner, and Gnomonic are projections based on the Geodesic
class.  <a href="GeodesicProj.1.html">GeodesicProj</a> is a command line
utility to exercise these projections.

GeodesicExact and GeodesicLineExact are drop in replacements for
Geodesic and GeodesicLine in which the solution is given in terms of
elliptic integrals (computed by EllipticFunction).  These classes should
be used if the absolute value of the flattening exceeds 0.02.  The -E
option to <a href="GeodSolve.1.html">GeodSolve</a> uses these classes.

NearestNeighbor is a header-only class for efficiently \ref nearest of a
collection of points where the distance function obeys the triangle
inequality.  The geodesic distance obeys this condition.

Geocentric and LocalCartesian convert between
geodetic and geocentric or a local cartesian system.  The constructor for
specifies the ellipsoid and the forward and reverse projections are
implemented as const member functions.  Geocentric::WGS84 is a
const static instantiation of Geocentric specific for the WGS84 ellipsoid.
<a href="CartConvert.1.html">CartConvert</a> is a simple command line
utility to provide access to these classes.

Geoid evaluates geoid heights by interpolation.  This is
provided by the operator() member function.
<a href="GeoidEval.1.html">GeoidEval</a> is a simple command line
utility to provide access to this class.  This class requires
installation of data files for the various geoid models; see \ref
geoidinst for details.

Ellipsoid is a class which performs latitude
conversions and returns various properties of the ellipsoid.

GravityModel evaluates the earth's gravitational field using a
particular gravity model.  Various member functions return the
gravitational field, the gravity disturbance, the gravity anomaly, and
the geoid height <a href="Gravity.1.html">Gravity</a> is a simple
command line utility to provide access to this class.  If the field
several points on a circle of latitude are sought then use
GravityModel::Circle to return a GravityCircle object whose member
functions performs the calculations efficiently.  (This is particularly
important for high degree models such as EGM2008.)  These classes
requires installation of data files for the various gravity models; see
\ref gravityinst for details.  NormalGravity computes the gravity of the
so-called level ellipsoid.

MagneticModel evaluates the earth's magnetic field using a particular
magnetic model.  The field is provided by the operator() member
function.  <a href="MagneticField.1.html">MagneticField</a> is a simple
command line utility to provide access to this class.  If the field
several points on a circle of latitude are sought then use
MagneticModel::Circle to return a MagneticCircle object whose operator()
member function performs the calculation efficiently.  (This is
particularly important for high degree models such as emm2010.)  These
classes requires installation of data files for the various magnetic
models; see \ref magneticinst for details.

Constants, Math, Utility, DMS, are general utility class which are used
internally by the library; in addition EllipticFunction is used by
TransverseMercatorExact, Ellipsoid, and GeodesicExact, and
GeodesicLineExact; Accumulator is used by PolygonAreaT, and
SphericalEngine, CircularEngine, SphericalHarmonic, SphericalHarmonic1,
and SphericalHarmonic2 facilitate the summation of spherical harmonic
series which is needed by and MagneticModel and MagneticCircle.  One
important definition is Math::real which is the type used for real
numbers.  This allows the library to be easily switched to using floats,
doubles, or long doubles.  However all the testing has been with real
set to double and the library should be installed in this way.

GeographicLib uniformly represents all angle-like variables (latitude,
longitude, azimuth) in terms of degrees.  To convert from degrees to
radians, multiple the quantity by Math::degree().  To convert from
radians to degrees , divide the quantity by Math::degree().

In general, the constructors for the classes in GeographicLib check
their arguments and throw GeographicErr exceptions with a
explanatory message if these are illegal.  The member functions, e.g.,
the projections implemented by TransverseMercator and
PolarStereographic, the solutions to the geodesic problem, etc.,
typically do <i>not</i> check their arguments; the calling program
should ensure that the arguments are legitimate.  However, the functions
implemented by UTMUPS, MGRS, and GeoCoords do check their arguments and
may throw GeographicErr exceptions.  Similarly Geoid may
throw exceptions on file errors.  If a function does throw an exception,
then the function arguments used for return values will not have been
altered.

GeographicLib attempts to act sensibly with NaNs.  NaNs in constructors
typically throw errors (an exception is GeodesicLine).  However, calling
the class functions with NaNs as arguments is not an error; NaNs are
returned as appropriate.  "INV" is treated as an invalid zone
designation by UTMUPS.  "INVALID" is the corresponding invalid MGRS
string (and similarly for GARS, Geohash, and Georef strings).  NaNs
allow the projection of polylines which are separated by NaNs; in this
format they can be easily plotted in MATLAB.

A note about portability.  For the most part, the code uses standard C++
and should be able to be deployed on any system with a modern C++
compiler.  System dependencies come into
 - Math -- GeographicLib needs to define functions such
   as atanh for systems that lack them.  The system dependence will
   disappear with the adoption of C++11 because the needed functions are
   part of that standard.
 - use of long double -- the type is used only for testing.  On systems
   which lack this data type the cmake and autoconf configuration
   methods should detect its absence and omit the code that depends on
   it.
 - Geoid, GravityModel, and
   MagneticModel -- these class uses system-dependent
   default paths for looking up the respective datasets.  It also relies
   on getenv to find the value of the environment variables.
 - Utility::readarray reads numerical data from binary
   files.  This assumes that floating point numbers are in IEEE format.
   It attempts to handled the "endianness" of the target platform using
   the GEOGRAPHICLIB_WORDS_BIGENDIAN macro (which sets the compile-time
   constant Math::bigendian).

An attempt is made to maintain backward compatibility with GeographicLib
(especially at the level of your source code).  Sometimes it's necessary
to take actions that depend on what version of GeographicLib is being
used; for example, you may want to use a new feature of GeographicLib,
but want your code still to work with older versions.  In that case, you
can test the values of the macros `GEOGRAPHICLIB_VERSION_MAJOR`,
`GEOGRAPHICLIB_VERSION_MINOR`, and `GEOGRAPHICLIB_VERSION_PATCH`; these
expand to numbers (and the last one is usually 0); these macros appeared
starting in version 1.31.  There's also a macro
`GEOGRAPHICLIB_VERSION_STRING` which expands to, e.g.,
"@PROJECT_VERSION@"; this macro has been defined since version 1.9.
Since version 1.37, there's also been a macro `GEOGRAPHICLIB_VERSION`
which encodes the version as a single number, e.g, 1003900.  Do not rely
on this particular packing; instead use the macro
GEOGRAPHICLIB_VERSION_NUM(a,b,c) which allows you to compare versions
with, e.g.,
\code
#if GEOGRAPHICLIB_VERSION >= GEOGRAPHICLIB_VERSION_NUM(1,37,0)
...
#endif \endcode

<center>
Back to \ref utilities.  Forward to \ref other.  Up to \ref contents.
</center>

**********************************************************************/
/**
\page other GeographicLib in other languages

<center>
Back to \ref organization.  Forward to \ref geoid.  Up to \ref contents.
</center>

Jump to
- \ref implement.
- \ref wrappers.
- \ref maxima.

\section implement Implementations in other languages

Various subsets of GeographicLib have been implemented in other
languages:
[C](../../C/doc/index.html),
[Fortran](../../Fortran/doc/index.html),
[Python](../../Python/doc/index.html),
[Octave/MATLAB](https://github.com/geographiclib/geographiclib-octave#readme),
[Java](../../Java/doc/index.html), and
[JavaScript](../../JavaScript/doc/index.html).
These are described [here](../../doc/library.html#languages) on the main
[GeographicLib web page](../../index.html).

For the most part, these implementations focus on the geodesic
capabilities of GeographicLib, i.e., the classes Geodesic, GeodesicLine,
and PolygonAreaT.  The exceptions are
- The [Java package GeographicLib-Java](
  https://search.maven.org/artifact/net.sf.geographiclib/GeographicLib-Java)
  includes an implementation of the Gnomonic projection
- The [Octave/MATLAB package
  geographiclib](https://github.com/geographiclib/geographiclib-octave#readme)
  also includes implementations of TransverseMercator,
  PolarStereographic, AzimuthalEquidistant, CassiniSoldner, Gnomonic,
  UTMUPS, MGRS, Geoid, Geocentric, LocalCartesian.  In addition this
  package includes calculations for \ref greatellipse.

\section wrappers Using wrappers to invoke GeographicLib

Alternatively, it is possible to call the C++ library directly from
certain other languages.  Examples of this idiom are given in the
[`wrapper`](https://github.com/geographiclib/geographiclib/blob/main/wrapper)
directory in the source distribution.

- The C example in [`wrapper/c`](
  https://github.com/geographiclib/geographiclib/blob/main/wrapper/c)
  gives a small example, which converts heights above the geoid to
  heights above the ellipsoid.

- For JavaScript, [`wrapper/javascript`](
  https://github.com/geographiclib/geographiclib/blob/main/wrapper/javascript),
  there is a minimal description of translating C++ source code to
  JavaScript.

- The Python example in [`wrapper/python`](
  https://github.com/geographiclib/geographiclib/blob/main/wrapper/python)
  showns how to convert heights above the geoid to heights above the
  ellipsoid using `boost-python`.  An alternation is to use
  [Cython](https::/cython.org).

- It is also possible to call the C++ version of GeographicLib directly
  from Octave or MATLAB.  The example in [`wrapper/octave`](
  https://github.com/geographiclib/geographiclib/blob/main/wrapper/octave)
  solves the inverse geodesic problem for ellipsoids with arbitrary
  flattening.

- The Excel example in [`wrapper/excel`](
  https://github.com/geographiclib/geographiclib/blob/main/wrapper/excel)
  shows how to call the C++ library from Visual Basic which, in turn,
  allows the functionality to be exposed in Excel.  Here computations
  involving geodesics and rhumb are illustrated.  At present this works
  only on Windows systems.

\section maxima Maxima routines

Maxima is a free computer algebra system which can be downloaded from
https://maxima.sourceforge.io.  Maxima was used to generate the series used by
TransverseMercator (<a href="tmseries.mac"> tmseries.mac</a>), Geodesic
(<a href="geod.mac"> geod.mac</a>), Rhumb (<a href="rhumbarea.mac">
rhumbarea.mac</a>), \ref gearea (<a href="gearea.mac"> gearea.mac</a>),
the relation between \ref auxlat (<a href="auxlat.mac"> auxlat.mac</a>),
and to generate accurate data for testing (<a href="tm.mac"> tm.mac</a>
and <a href="geodesic.mac"> geodesic.mac</a>).  The latter uses Maxima's
bigfloat arithmetic together with series extended to high order or
solutions in terms of elliptic integrals (<a href="ellint.mac">
ellint.mac</a>).  These files contain brief instructions on how to use
them.

<center>
Back to \ref organization.  Forward to \ref geoid.  Up to \ref contents.
</center>

**********************************************************************/
/**
\page geoid Geoid height

<center>
Back to \ref other.  Forward to \ref gravity.  Up to \ref contents.
</center>

The gravitational equipotential surface approximately coinciding with
mean sea level is called the geoid.  The Geoid class and
the <a href="GeoidEval.1.html">GeoidEval</a> utility evaluate the height
of the geoid above the WGS84 ellipsoid.  This can be used to convert
heights above mean sea level to heights above the WGS84 ellipsoid.
Because the normal to the ellipsoid differs from the normal to the geoid
(the direction of a plumb line) there is a slight ambiguity in the
measurement of heights; however for heights up to 10&nbsp;km this ambiguity
is only 1&nbsp;mm.

The geoid is usually approximated by an "earth gravity model" (EGM).
The models published by the NGA are:
- <b>EGM84</b>:
  https://earth-info.nga.mil/index.php?dir=wgs84&action=wgs84#tab_egm84
- <b>EGM96</b>:
  https://earth-info.nga.mil/index.php?dir=wgs84&action=wgs84#tab_egm96
- <b>EGM2008</b>:
  https://earth-info.nga.mil/index.php?dir=wgs84&action=wgs84#tab_egm2008
.
Geoid offers a uniform way to handle all 3 geoids models at a variety of
grid resolutions.  (In contrast, the software tools that NGA offers are
different for each geoid model, and the interpolation programs are
different for each grid resolution.  In addition these tools are written
in Fortran with is nowadays difficult to integrate with other software.)

The height of the geoid above the ellipsoid, \e N, is sometimes called
the geoid undulation.  It can be used to convert a height above the
ellipsoid, \e h, to the corresponding height above the geoid (the
orthometric height, roughly the height above mean sea level), \e H,
using the relations

&nbsp;&nbsp;&nbsp;\e h = \e N + \e H; &nbsp;&nbsp;\e H = &minus;\e N + \e h.

Unlike other components of GeographicLib, there is a appreciable error
in the results obtained (at best, the RMS error is 1&nbsp;mm).  However the
class provides methods to report the maximum and RMS errors in the
results.  The class also returns the gradient of the geoid.  This can be
used to estimate the direction of a plumb line relative to the WGS84
ellipsoid.

The GravityModel class calculates geoid heights using the
underlying gravity model.  This is slower then Geoid but
considerably more accurate.  This class also can accurately compute all
the components of the acceleration due to gravity (and hence the
direction of plumb line).

Go to
 - \ref geoidinst
 - \ref geoidformat
 - \ref geoidinterp
 - \ref geoidcache
 - \ref testgeoid

\section geoidinst Installing the geoid datasets

The geoid heights are computed using interpolation into a rectangular
grid.  The grids are read from data files which have been are computed
using the NGA synthesis programs in the case of the EGM84 and EGM96
models and using the NGA binary gridded data files in the case of
EGM2008.  These data files are available for download:
<center>
<table>
<caption>Available geoid data files</caption>
<tr>
 <th rowspan="2">name         <th rowspan="2">geoid    <th rowspan="2">grid
 <th rowspan="2">size\n(MB)
 <th colspan="3"><center>Download Links (size, MB)</center></th>
<tr>
 <th>tar file</th>
 <th>Windows\n installer</th>
 <th>zip file</th>
<tr>
 <td>egm84-30
 <td>EGM84
 <td><center>30'</center>
 <td><center>0.6</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/geoids-distrib/egm84-30.tar.bz2">
 link</a> (0.5)</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/geoids-distrib/egm84-30.exe">
 link</a> (0.8)</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/geoids-distrib/egm84-30.zip">
 link</a> (0.5)</center>
<tr>
 <td>egm84-15
 <td>EGM84
 <td><center>15'</center>
 <td><center>2.1</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/geoids-distrib/egm84-15.tar.bz2">
 link</a> (1.5)</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/geoids-distrib/egm84-15.exe">
 link</a> (1.9)</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/geoids-distrib/egm84-15.zip">
 link</a> (2.0)</center>
<tr>
 <td>egm96-15
 <td>EGM96
 <td><center>15'</center>
 <td><center>2.1</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/geoids-distrib/egm96-15.tar.bz2">
 link</a> (1.5)</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/geoids-distrib/egm96-15.exe">
 link</a> (1.9)</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/geoids-distrib/egm96-15.zip">
 link</a> (2.0)</center>
<tr>
 <td>egm96-5
 <td>EGM96
 <td><center>5'</center>
 <td><center> 19</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/geoids-distrib/egm96-5.tar.bz2">
 link</a> (11)</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/geoids-distrib/egm96-5.exe">
 link</a> (11)</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/geoids-distrib/egm96-5.zip">
 link</a> (17)</center>
<tr>
 <td>egm2008-5
 <td>EGM2008
 <td><center>5'</center>
 <td><center> 19</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/geoids-distrib/egm2008-5.tar.bz2">
 link</a> (11)</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/geoids-distrib/egm2008-5.exe">
 link</a> (11)</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/geoids-distrib/egm2008-5.zip">
 link</a> (17)</center>
<tr>
 <td>egm2008-2_5
 <td>EGM2008
 <td><center>2.5'</center>
 <td><center> 75</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/geoids-distrib/egm2008-2_5.tar.bz2">
 link</a> (35)</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/geoids-distrib/egm2008-2_5.exe">
 link</a> (33)</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/geoids-distrib/egm2008-2_5.zip">
 link</a> (65)</center>
<tr>
 <td>egm2008-1
 <td>EGM2008
 <td><center>1'</center>
 <td><center>470</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/geoids-distrib/egm2008-1.tar.bz2">
 link</a> (170)</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/geoids-distrib/egm2008-1.exe">
 link</a> (130)</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/geoids-distrib/egm2008-1.zip">
 link</a> (390)</center>
</table>
</center>
The "size" column is the size of the uncompressed data and it also
gives the memory requirements for caching the entire dataset using the
Geoid::CacheAll method.
At a minimum you should install egm96-5 and either egm2008-1 or
egm2008-2_5.  Many applications use the EGM96 geoid, however the use of
EGM2008 is growing. (EGM84 is rarely used now.)

For Linux and Unix systems, GeographicLib provides a shell script
geographiclib-get-geoids (typically installed in /usr/local/sbin) which
automates the process of downloading and installing the geoid data.  For
example
\verbatim
   geographiclib-get-geoids best # to install egm84-15, egm96-5, egm2008-1
   geographiclib-get-geoids -h   # for help
\endverbatim
This script should be run as a user with write access to the
installation directory, which is typically
/usr/local/share/GeographicLib (this can be overridden with the -p
flag), and the data will then be placed in the "geoids" subdirectory.

Windows users should download and run the Windows installers.  These
will prompt for an installation directory with the default being
\verbatim
   C:/ProgramData/GeographicLib
\endverbatim
(which you probably should not change) and the data is installed in the
"geoids" sub-directory.  (The second directory name is an alternate name
that Windows 7 uses for the "Application Data" directory.)

Otherwise download \e either the tar.bz2 file \e or the zip file (they
have the same contents).  If possible use the tar.bz2 files, since these
are compressed about 2 times better than the zip file.  To unpack these,
run, for example
\verbatim
   mkdir -p /usr/local/share/GeographicLib
   tar xofjC egm96-5.tar.bz2 /usr/local/share/GeographicLib
   tar xofjC egm2008-2_5.tar.bz2 /usr/local/share/GeographicLib
   etc.
\endverbatim
and, again, the data will be placed in the "geoids" subdirectory.

However you install the geoid data, all the datasets should
be installed in the same directory.  Geoid and
<a href="GeoidEval.1.html">GeoidEval</a> uses a compile time default to
locate the datasets.  This is
- /usr/local/share/GeographicLib/geoids, for non-Windows systems
- C:/ProgramData/GeographicLib/geoids, for Windows systems
.
consistent with the examples above.  This may be overridden at run-time
by defining the GEOGRAPHICLIB_GEOID_PATH or the GEOGRAPHICLIB_DATA
environment variables; see Geoid::DefaultGeoidPath() for
details.  Finally, the path may be set using the optional second
argument to the Geoid constructor or with the "-d" flag
to <a href="GeoidEval.1.html">GeoidEval</a>.  Supplying the "-h" flag to
<a href="GeoidEval.1.html">GeoidEval</a> reports the default geoid path
for that utility.  The "-v" flag causes GeoidEval to report the full
path name of the data file it uses.

\section geoidformat The format of the geoid data files

The gridded data used by the Geoid class is stored in
16-bit PGM files.  Thus the data for egm96-5 might be stored in the file
- /usr/local/share/GeographicLib/geoids/egm96-5.pgm
.
PGM simple graphic format with the following properties
- it is well documented
  <a href="http://netpbm.sourceforge.net/doc/pgm.html">here</a>;
- there are no separate "little-endian" and "big-endian" versions;
- it uses 1 or 2 bytes per pixel;
- pixels can be randomly accessed;
- comments can be added to the file header;
- it is sufficiently simple that it can be easily read without using the
  <a href="http://netpbm.sourceforge.net/doc/libnetpbm.html">libnetpbm</a>
  library (and thus we avoid adding a software dependency to
  GeographicLib).
.
The major drawback of this format is that since there are only 65535
possible pixel values, the height must be quantized to 3&nbsp;mm.  However,
the resulting quantization error (up to 1.5&nbsp;mm) is typically smaller
than the linear interpolation errors.  The comments in the header for
egm96-5 are
\verbatim
   # Geoid file in PGM format for the GeographicLib::Geoid class
   # Description WGS84 EGM96, 5-minute grid
   # URL https://earth-info.nga.mil/index.php?dir=wgs84&action=wgs84#tab_egm96
   # DateTime 2009-08-29 18:45:03
   # MaxBilinearError 0.140
   # RMSBilinearError 0.005
   # MaxCubicError 0.003
   # RMSCubicError 0.001
   # Offset -108
   # Scale 0.003
   # Origin 90N 0E
   # AREA_OR_POINT Point
   # Vertical_Datum WGS84
\endverbatim
Of these lines, the Scale and Offset lines are required and define the
conversion from pixel value to height (in meters) using \e height =
\e offset + \e scale \e pixel.  The Geoid constructor also reads the
Description, DateTime, and error lines (if present) and stores the
resulting data so that it can be returned by
Geoid::Description, Geoid::DateTime,
Geoid::MaxError, and Geoid::RMSError
methods.  The other lines serve as additional documentation but are not
used by this class.  Accompanying egm96-5.pgm (and similarly with the
other geoid data files) are two files egm96-5.wld and
egm96-5.pgm.aux.xml.  The first is an ESRI "world" file and the second
supplies complete projection metadata for use by
<a href="http://www.gdal.org">GDAL</a>.  Neither of these files is read
by Geoid.

You can use gdal_translate to convert the data files to a standard
GeoTiff, e.g., with
\verbatim
   gdal_translate -ot Float32 -scale 0 65000 -108 87 egm96-5.pgm egm96-5.tif
\endverbatim
The arguments to -scale here are specific to the Offset and Scale
parameters used in the pgm file (note 65000 * 0.003 &minus; 108 = 87).
You can check these by running <a href="GeoidEval.1.html">GeoidEval</a>
with the "-v" option.

Here is a sample script which uses GDAL to create a 1-degree
squared grid of geoid heights at 3&quot; resolution (matching DTED1) by
bilinear interpolation.
\verbatim
   #! /bin/sh
   lat=37
   lon=067
   res=3                           # resolution in seconds
   TEMP=`mktemp junkXXXXXXXXXX`    # temporary file for GDAL
   gdalwarp -q -te `echo $lon $lat $res | awk '{
       lon = $1; lat = $2; res = $3;
       printf "%.14f %.14f %.14f %.14f",
           lon  -0.5*res/3600, lat  -0.5*res/3600,
           lon+1+0.5*res/3600, lat+1+0.5*res/3600;
   }'` -ts $((3600/res+1)) $((3600/res+1)) -r bilinear egm96-5.tif $TEMP
   gdal_translate -quiet \
       -mo AREA_OR_POINT=Point \
       -mo Description="WGS84 EGM96, $res-second grid" \
       -mo Vertical_Datum=WGS84 \
       -mo Tie_Point_Location=pixel_corner \
       $TEMP e$lon-n$lat.tif
   rm -f $TEMP
\endverbatim

Because the pgm files are uncompressed, they can take up a lot of disk
space.  Some compressed formats compress in tiles and so might be
compatible with the requirement that the data can be randomly accessed.
In particular gdal_translate can be used to convert the pgm files to
compressed tiff files with
\verbatim
gdal_translate -co COMPRESS=LZW -co PREDICTOR=2 -co TILED=YES \
   –a_offset -108 -a_scale 0.003 \
   egmyy-g.pgm egmyy-g.tif
\endverbatim
(The <code>-a_offset</code> and <code>-a_scale</code> options were
introduced in gdal_translate version 2.3.)  The resulting files sizes
are
\verbatim
                  pgm      tif
    egm84-30      0.6 MB   0.5 MB
    egm84-15      2.1 MB   1.4 MB
    egm96-15      2.1 MB   1.5 MB
    egm96-5        19 MB   8.5 MB
    egm2008-5      19 MB   9.8 MB
    egm2008-2_5    75 MB    28 MB
    egm2008-1     470 MB    97 MB
\endverbatim
Currently, there are no plans for GeographicLib to support this
compressed format.

The Geoid class only handles world-wide geoid models.  The pgm provides
geoid height postings on grid of points with uniform spacing in latitude
(row) and longitude (column).  If the dimensions of the pgm file are
\e h &times; \e w, then latitude (resp. longitude) spacing is
&minus;180&deg;/(\e h &minus; 1) (resp. 360&deg;/\e w), with the (0,0)
pixel given the value at &phi; = 90&deg;, &lambda; = 0&deg;.  Models
covering a limited area will need to be "inserted" into a world-wide
grid before being accessible to the Geoid class.

\section geoidinterp Interpolating the geoid data

Geoid evaluates the geoid height using bilinear or cubic
interpolation.

The bilinear interpolation is based on the values at the 4 corners of
the enclosing cell.  The interpolated height is a continuous function of
position; however the gradient has discontinuities are cell boundaries.
The quantization of the data files exacerbates the errors in the
gradients.

The cubic interpolation is a least-squares fit to the values on a
12-point stencil with weights as follows:
\verbatim
   . 1 1 .
   1 2 2 1
   1 2 2 1
   . 1 1 .
\endverbatim
The cubic is constrained to be independent of longitude when evaluating
the height at one of the poles.  Cubic interpolation is considerably
more accurate than bilinear interpolation; however, in this
implementation there are small discontinuities in the heights are cell
boundaries.  The over-constrained cubic fit slightly reduces the
quantization errors on average.

The algorithm for the least squares fit is taken from, F. H. Lesh,
<a href="https://doi.org/10.1145/368424.368443">Multi-dimensional
least-squares polynomial curve fitting</a>, CACM 2, 29--30 (1959).
This algorithm is not part of Geoid; instead it is implemented as
<a href="https://en.wikipedia.org/wiki/Maxima_(software)">Maxima</a>
code which is used to precompute the matrices to convert the function
values on the stencil into the coefficients from the cubic polynomial.
This code is included as a comment in Geoid.cpp.

The interpolation methods are quick and give good accuracy.  Here is a
summary of the combined quantization and interpolation errors for the
heights.
<center><table>
<caption>Interpolation and quantization errors for geoid heights</caption>
<tr>
 <th rowspan="2">name
 <th rowspan="2">geoid
 <th rowspan="2">grid
 <th colspan="2"><center>bilinear error</center></th>
 <th colspan="2"><center>cubic error</center></th>
<tr>
 <th><center>max</center><th><center>rms</center>
 <th><center>max</center><th><center>rms</center>
<tr>
 <td>egm84-30   <td>EGM84  <td><center>30'  </center>
 <td><center>1.546 m</center><td><center>70 mm</center>
 <td><center>0.274 m</center><td><center>14 mm</center>
<tr>
 <td>egm84-15   <td>EGM84  <td><center>15'  </center>
 <td><center>0.413 m</center><td><center>18 mm</center>
 <td><center>0.021 m</center><td><center>1.2 mm</center>
<tr>
 <td>egm96-15   <td>EGM96  <td><center>15'  </center>
 <td><center>1.152 m</center><td><center>40 mm</center>
 <td><center>0.169 m</center><td><center>7.0 mm</center>
<tr>
 <td>egm96-5    <td>EGM96  <td><center> 5'  </center>
 <td><center>0.140 m</center><td><center>4.6 mm</center>
 <td><center>0.0032 m</center><td><center>0.7 mm</center>
<tr>
 <td>egm2008-5  <td>EGM2008<td><center> 5'  </center>
 <td><center>0.478 m</center><td><center>12 mm</center>
 <td><center>0.294 m</center><td><center>4.5 mm</center>
<tr>
 <td>egm2008-2_5<td>EGM2008<td><center> 2.5'</center>
 <td><center>0.135 m</center><td><center>3.2 mm</center>
 <td><center>0.031 m</center><td><center>0.8 mm</center>
<tr>
 <td>egm2008-1  <td>EGM2008<td><center> 1'  </center>
 <td><center>0.025 m</center><td><center>0.8 mm</center>
 <td><center>0.0022 m</center><td><center>0.7 mm</center>
</table></center>
The errors are with respect to the specific NGA earth gravity model
(not to any "real" geoid).  The RMS values are obtained using 5 million
uniformly distributed random points.  The maximum values are obtained by
evaluating the errors using a different grid with points at the
centers of the original grid.  (The RMS difference between EGM96 and
EGM2008 is about 0.5&nbsp;m.  The RMS difference between EGM84 and EGM96 is
about 1.5&nbsp;m.)

The errors in the table above include the quantization errors that arise
because the height data that Geoid uses are quantized to
3&nbsp;mm.  If, instead, Geoid were to use data files without
such quantization artifacts, the overall error would be reduced <i>but
only modestly</i> as shown in the following table, where only the
changed rows are included and where the changed entries are given in
bold:
<center><table>
<caption>Interpolation (only!) errors for geoid heights</caption>
<tr>
 <th rowspan="2">name
 <th rowspan="2">geoid
 <th rowspan="2">grid
 <th colspan="2"><center>bilinear error</center></th>
 <th colspan="2"><center>cubic error</center></th>
<tr>
 <th><center>max</center><th><center>rms</center>
 <th><center>max</center><th><center>rms</center>
<tr>
 <td>egm96-5    <td>EGM96  <td><center> 5'  </center>
 <td><center>0.140 m</center><td><center>4.6 mm</center>
 <td><center><b>0.0026 m</b></center><td><center><b>0.1 mm</b></center>
<tr>
 <td>egm2008-2_5<td>EGM2008<td><center> 2.5'</center>
 <td><center>0.135 m</center><td><center>3.2 mm</center>
 <td><center>0.031 m</center><td><center><b>0.4 mm</b></center>
<tr>
 <td>egm2008-1  <td>EGM2008<td><center> 1'  </center>
 <td><center>0.025 m</center><td><center><b>0.6 mm</b></center>
 <td><center><b>0.0010 m</b></center><td><center><b>0.011 mm</b></center>
</table></center>

\section geoidcache Caching the geoid data

A simple way of calling Geoid is as follows
\code
   #include <GeographicLib/Geoid.hpp>
   #include <iostream>
   ...
   GeographicLib::Geoid g("egm96-5");
   double lat, lon;
   while (std::cin >> lat >> lon)
      std::cout << g(lat, lon) << "\n";
   ...
\endcode

The first call to g(lat, lon) causes the data for the stencil points (4
points for bilinear interpolation and 12 for cubic interpolation) to be
read and the interpolated value returned.  A simple 0th-order caching
scheme is provided by default, so that, if the second and subsequent
points falls within the same grid cell, the data values are not reread
from the file.  This is adequate for most interactive use and also
minimizes disk accesses for the case when a continuous track is being
followed.

If a large quantity of geoid calculations are needed, the calculation
can be sped up by preloading the data for a rectangular block with
Geoid::CacheArea or the entire dataset with
Geoid::CacheAll.  If the requested points lie within the
cached area, the cached data values are used; otherwise the data is read
from disk as before.  Caching all the data is a reasonable choice for
the 5' grids and coarser.  Caching all the data for the 1' grid will
require 0.5 GB of RAM and should only be used on systems with sufficient
memory.

The use of caching does not affect the values returned.  Because of the
caching and the random file access, this class is \e not normally thread
safe; i.e., a single instantiation cannot be safely used by multiple
threads.  If multiple threads need to calculate geoid heights, there are
two alternatives:
 - they should all construct thread-local instantiations.
 - Geoid should be constructed with \e threadsafe = true.
   This causes all the data to be read at the time of construction (and
   if this fails, an exception is thrown), the data file to be closed
   and the single-cell caching to be turned off.  The resulting object
   may then be shared safely between threads.

\section testgeoid Test data for geoids

A test set for the geoid models is available at
 - <a href="https://sourceforge.net/projects/geographiclib/files/testdata/GeoidHeights.dat.gz">
   GeoidHeights.dat.gz</a>
 .
This is about 11 MB (compressed).  This test set consists of a set of
500000 geographic coordinates together with the corresponding geoid
heights according to various earth gravity models.

Each line of the test set gives 5 space delimited numbers
 - latitude (degrees, exact)
 - longitude (degrees, exact)
 - EGM84 height (meters, accurate to 1 &mu;m)
 - EGM96 height (meters, accurate to 1 &mu;m)
 - EGM2008 height (meters, accurate to 1 &mu;m)
 .
The latitude and longitude are all multiples of 10<sup>&minus;6</sup>
deg and should be regarded as exact.  The geoid heights are computed
using the harmonic NGA synthesis programs, where the programs were
compiled (with gfortran) and run under Linux.  In the case of the
EGM2008 program, a <code>SAVE</code> statement needed to be added to
subroutine <code>latf</code>, in order for the program to compile
correctly with a stack-based compiler.  Similarly the EGM96 code
requires a <code>SAVE</code> statement in subroutine
<code>legfdn</code>.

<center>
Back to \ref other.  Forward to \ref gravity.  Up to \ref contents.
</center>

**********************************************************************/
/**
\page gravity Gravity models

<center>
Back to \ref geoid.  Forward to \ref normalgravity.  Up to \ref contents.
</center>

GeographicLib can compute the earth's gravitational field with an earth
gravity model using the GravityModel and GravityCircle classes and with
the <a href="Gravity.1.html">Gravity</a> utility.  These models expand
the gravitational potential of the earth as sum of spherical harmonics.
The models also specify a reference ellipsoid, relative to which geoid
heights and gravity disturbances are measured.  Underlying all these
models is <i>normal gravity</i> which is the exact solution for an
idealized rotating ellipsoidal body.  This is implemented with the
NormalGravity class and some notes on are provided in section \ref
normalgravity

Go to
 - \ref gravityinst
 - \ref gravityformat
 - \ref gravitynga
 - \ref gravitygeoid
 - \ref gravityatmos
 - \ref gravityparallel
 - \ref normalgravity (now on its own page)

The supported models are
 - <b>egm84</b>, the
   <a href="https://earth-info.nga.mil/index.php?dir=wgs84&action=wgs84#tab_egm84">
   Earth Gravity Model 1984</a>, which includes terms up to degree 180.
 - <b>egm96</b>, the
   <a href="https://earth-info.nga.mil/index.php?dir=wgs84&action=wgs84#tab_egm96">
   Earth Gravity Model 1996</a>, which includes terms up to degree 360.
 - <b>egm2008</b>, the
   <a href="https://earth-info.nga.mil/index.php?dir=wgs84&action=wgs84#tab_egm2008">
   Earth Gravity Model 2008</a>, which includes terms up to degree 2190.
   See also <a href="https://doi.org/10.1029/2011JB008916"> Pavlis et
   al. (2012)</a>
 - <b>wgs84</b>, the
   <a href="https://web.archive.org/web/20161214054445/http://earth-info.nga.mil/GandG/publications/tr8350.2/tr8350_2.html">
   WGS84 Reference Ellipsoid</a>.  This is just reproduces the normal
   gravitational field for the reference ellipsoid.  This includes the
   zonal coefficients up to order 20.  Usually NormalGravity::WGS84()
   should be used instead.
 - <b>grs80</b>, the
   GRS80 Reference Ellipsoid.  This is just reproduces the normal
   gravitational field for the GRS80 ellipsoid.  This includes the
   zonal coefficients up to order 20.  Usually NormalGravity::GRS80()
   should be used instead.

See
 - W. A. Heiskanen and H. Moritz, Physical Geodesy (Freeman, San
   Francisco, 1967).
 .
for more information.

<b>Acknowledgment:</b> I would like to thank Mathieu Peyr&eacute;ga for
sharing EGM_Geoid_CalculatorClass from his Geo library with me.  His
implementation was the first I could easily understand and he and I
together worked through some of the issues with overflow and underflow
the occur while performing high-degree spherical harmonic sums.

\section gravityinst Installing the gravity models

These gravity models are available for download:
<center>
<table>
<caption>Available gravity models</caption>
<tr>
 <th rowspan="2">name         <th rowspan="2">max\n degree
 <th rowspan="2">size\n(kB)
 <th colspan="3"><center>Download Links (size, kB)</center></th>
<tr>
 <th>tar file</th>
 <th>Windows\n installer</th>
 <th>zip file</th>
<tr>
 <td>egm84
 <td><center>180</center>
 <td><center>27</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/gravity-distrib/egm84.tar.bz2">
 link</a> (26)</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/gravity-distrib/egm84.exe">
 link</a> (55)</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/gravity-distrib/egm84.zip">
 link</a> (26)</center>
<tr>
 <td>egm96
 <td><center>360</center>
 <td><center>2100</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/gravity-distrib/egm96.tar.bz2">
 link</a> (2100)</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/gravity-distrib/egm96.exe">
 link</a> (2300)</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/gravity-distrib/egm96.zip">
 link</a> (2100)</center>
<tr>
 <td>egm2008
 <td><center>2190</center>
 <td><center>76000</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/gravity-distrib/egm2008.tar.bz2">
 link</a> (75000)</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/gravity-distrib/egm2008.exe">
 link</a> (72000)</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/gravity-distrib/egm2008.zip">
 link</a> (73000)</center>
<tr>
 <td>wgs84
 <td><center>20</center>
 <td><center>1</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/gravity-distrib/wgs84.tar.bz2">
 link</a> (1)</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/gravity-distrib/wgs84.exe">
 link</a> (30)</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/gravity-distrib/wgs84.zip">
 link</a> (1)</center>
</table>
</center>
The "size" column is the size of the uncompressed data.

For Linux and Unix systems, GeographicLib provides a shell script
geographiclib-get-gravity (typically installed in /usr/local/sbin)
which automates the process of downloading and installing the gravity
models.  For example
\verbatim
   geographiclib-get-gravity all  # to install egm84, egm96, egm2008, wgs84
   geographiclib-get-gravity -h   # for help
\endverbatim
This script should be run as a user with write access to the
installation directory, which is typically
/usr/local/share/GeographicLib (this can be overridden with the -p
flag), and the data will then be placed in the "gravity" subdirectory.

Windows users should download and run the Windows installers.  These
will prompt for an installation directory with the default being
\verbatim
   C:/ProgramData/GeographicLib
\endverbatim
(which you probably should not change) and the data is installed in the
"gravity" sub-directory.  (The second directory name is an alternate name
that Windows 7 uses for the "Application Data" directory.)

Otherwise download \e either the tar.bz2 file \e or the zip file (they
have the same contents).  To unpack these, run, for example
\verbatim
   mkdir -p /usr/local/share/GeographicLib
   tar xofjC egm96.tar.bz2 /usr/local/share/GeographicLib
   tar xofjC egm2008.tar.bz2 /usr/local/share/GeographicLib
   etc.
\endverbatim
and, again, the data will be placed in the "gravity" subdirectory.

However you install the gravity models, all the datasets should be
installed in the same directory.  GravityModel and
<a href="Gravity.1.html">Gravity</a> uses a compile time default to
locate the datasets.  This is
- /usr/local/share/GeographicLib/gravity, for non-Windows systems
- C:/ProgramData/GeographicLib/gravity, for Windows systems
.
consistent with the examples above.  This may be overridden at run-time
by defining the GEOGRAPHICLIB_GRAVITY_PATH or the GEOGRAPHICLIB_DATA
environment variables; see
GravityModel::DefaultGravityPath() for details.  Finally,
the path may be set using the optional second argument to the
GravityModel constructor or with the "-d" flag to
<a href="Gravity.1.html">Gravity</a>.  Supplying the "-h" flag to
<a href="Gravity.1.html">Gravity</a> reports the default path for
gravity models for that utility.  The "-v" flag causes Gravity to report
the full path name of the data file it uses.

\section gravityformat The format of the gravity model files

The constructor for GravityModel reads a file called
NAME.egm which specifies various properties for the gravity model.  It
then opens a binary file NAME.egm.cof to obtain the coefficients of the
spherical harmonic sum.

The first line of the .egm file must consist of "EGMF-v" where EGMF
stands for "Earth Gravity Model Format" and v is the version number of
the format (currently "1").

The rest of the File is read a line at a time.  A # character and
everything after it are discarded.  If the result is just white space it
is discarded.  The remaining lines are of the form "KEY WHITESPACE
VALUE".  In general, the KEY and the VALUE are case-sensitive.

GravityModel only pays attention to the following
keywords
 - keywords that affect the field calculation, namely:
   - <b>ModelRadius</b> (required), the normalizing radius for the model
     in meters.
   - <b>ReferenceRadius</b> (required), the equatorial radius \e a for
     the reference ellipsoid meters.
   - <b>ModelMass</b> (required), the mass constant \e GM for the model
     in meters<sup>3</sup>/seconds<sup>2</sup>.
   - <b>ReferenceMass</b> (required), the mass constant \e GM for the
     reference ellipsoid in meters<sup>3</sup>/seconds<sup>2</sup>.
   - <b>AngularVelocity</b> (required), the angular velocity &omega; for
     the model \e and the reference ellipsoid in rad
     seconds<sup>&minus;1</sup>.
   - <b>Flattening</b>, the flattening of the reference ellipsoid; this
     can be given a fraction, e.g., 1/298.257223563.  One of
     <b>Flattening</b> and <b>DynamicalFormFactor</b> is required.
   - <b>DynamicalFormFactor</b>, the dynamical form factor
     <i>J</i><sub>2</sub> for the reference ellipsoid.  One of
     <b>Flattening</b> and <b>DynamicalFormFactor</b> is required.
   - <b>HeightOffset</b> (default 0), the constant height offset
     (meters) added to obtain the geoid height.
   - <b>CorrectionMultiplier</b> (default 1), the multiplier for the
     "zeta-to-N" correction terms for the geoid height to convert them
     to meters.
   - <b>Normalization</b> (default full), the normalization used for the
     associated Legendre functions (full or schmidt).
   - <b>ID</b> (required), 8 printable characters which serve as a
     signature for the .egm.cof file (they must appear as the first 8
     bytes of this file).
   .
   The parameters <b>ModelRadius</b>, <b>ModelMass</b>, and
   <b>AngularVelocity</b> apply to the gravity model, while
   <b>ReferenceRadius</b>, <b>ReferenceMass</b>, <b>AngularVelocity</b>,
   and either <b>Flattening</b> or <b>DynamicalFormFactor</b>
   characterize the reference ellipsoid.  <b>AngularVelocity</b>
   (because it can be measured precisely) is the same for the gravity
   model and the reference ellipsoid.  <b>ModelRadius</b> is merely a
   scaling parameter for the gravity model and there's no requirement
   that it be close to the equatorial radius of the earth (although
   that's typically how it is chosen).  <b>ModelMass</b> and
   <b>ReferenceMass</b> need not be the same and, indeed, they are
   slightly difference for egm2008.  As a result the disturbing
   potential \e T has a 1/\e r dependence at large distances.
 - keywords that store data that the user can query:
   - <b>Name</b>, the name of the model.
   - <b>Description</b>, a more descriptive name of the model.
   - <b>ReleaseDate</b>, when the model was created.
 - keywords that are examined to verify that their values are valid:
   - <b>ByteOrder</b> (default little), the order of bytes in the
     .egm.cof file.  Only little endian is supported at present.
 .
Other keywords are ignored.

The coefficient file NAME.egm.cof is a binary file in little endian
order.  The first 8 bytes of this file must match the ID given in
NAME.egm.  This is followed by 2 sets of spherical harmonic
coefficients.  The first of these gives the gravity potential and the
second gives the zeta-to-N corrections to the geoid height.  The format
for each set of coefficients is:
 - \e N, the maximum degree of the sum stored as a 4-byte signed integer.
   This must satisfy \e N &ge; &minus;1.
 - \e M, the maximum order of the sum stored as a 4-byte signed integer.
   This must satisfy \e N &ge; \e M &ge; &minus;1.
 - <i>C</i><sub><i>nm</i></sub>, the coefficients of the cosine
   coefficients of the sum in column (i.e., \e m) major order.  There
   are (\e M + 1) (2\e N - \e M + 2) / 2 elements which are stored as
   IEEE doubles (8 bytes).  For example for \e N = \e M = 3, there are
   10 coefficients arranged as
   <i>C</i><sub>00</sub>,
   <i>C</i><sub>10</sub>,
   <i>C</i><sub>20</sub>,
   <i>C</i><sub>30</sub>,
   <i>C</i><sub>11</sub>,
   <i>C</i><sub>21</sub>,
   <i>C</i><sub>31</sub>,
   <i>C</i><sub>22</sub>,
   <i>C</i><sub>32</sub>,
   <i>C</i><sub>33</sub>.
 - <i>S</i><sub><i>nm</i></sub>, the coefficients of the sine
   coefficients of the sum in column (i.e., \e m) major order starting
   at \e m = 1.  There are \e M (2\e N - \e M + 1) / 2 elements which
   are stored as IEEE doubles (8 bytes).  For example for \e N = \e M =
   3, there are 6 coefficients arranged as
   <i>S</i><sub>11</sub>,
   <i>S</i><sub>21</sub>,
   <i>S</i><sub>31</sub>,
   <i>S</i><sub>22</sub>,
   <i>S</i><sub>32</sub>,
   <i>S</i><sub>33</sub>.
 .
Although the coefficient file is in little endian order, GeographicLib
can read it on big endian machines.  It can only be read on machines
which store doubles in IEEE format.

As an illustration, here is egm2008.egm:
\verbatim
EGMF-1
# An Earth Gravity Model (Format 1) file.  For documentation on the
# format of this file see
# https://geographiclib.sourceforge.io/C++/doc/gravity.html#gravityformat
Name            egm2008
Publisher       National Geospatial Intelligence Agency
Description     Earth Gravity Model 2008
URL             https://earth-info.nga.mil/index.php?dir=wgs84&action=wgs84#tab_egm2008
ReleaseDate     2008-06-01
ConversionDate  2011-11-19
DataVersion     1
ModelRadius     6378136.3
ModelMass       3986004.415e8
AngularVelocity 7292115e-11
ReferenceRadius 6378137
ReferenceMass   3986004.418e8
Flattening      1/298.257223563
HeightOffset    -0.41

# Gravitational and correction coefficients taken from
# EGM2008_to2190_TideFree and Zeta-to-N_to2160_egm2008 from
# the egm2008 distribution.
ID              EGM2008A
\endverbatim

The code to produce the coefficient files for the wgs84 and grs80 models
is
\include make-egmcof.cpp

\section gravitynga Comments on the NGA harmonic synthesis code

GravityModel attempts to reproduce the results of NGA's
harmonic synthesis code for EGM2008, hsynth_WGS84.f.  Listed here are
issues that I encountered using the NGA code:
 -# A compiler which allocates local variables on the stack produces an
    executable with just returns NaNs.  The problem here is a missing
    <code>SAVE</code> statement in subroutine <code>latf</code>.
 -# Because the model and references masses for egm2008 differ (by about
    1 part in 10<sup>9</sup>), there should be a 1/\e r contribution to
    the disturbing potential \e T.  However, this term is set to zero in
    hsynth_WGS84 (effectively altering the normal potential).  This
    shifts the surface \e W = <i>U</i><sub>0</sub> outward by about
    5&nbsp;mm.  Note too that the reference ellipsoid is no longer a
    level surface of the effective normal potential.
 -# Subroutine <code>radgrav</code> computes the ellipsoidal coordinate
    &beta; incorrectly.  This leads to small errors in the deflection
    of the vertical, &xi; and &eta;, when the height above the
    ellipsoid, \e h, is non-zero (about 10<sup>&minus;7</sup> arcsec at
    \e h = 400&nbsp;km).
 -# There are several expressions which will return inaccurate results
    due to cancellation.  For example, subroutine <code>grs</code>
    computes the flattening using \e f = 1 &minus; sqrt(1 &minus;
    <i>e</i><sup>2</sup>).  Much better is to use \e f =
    <i>e</i><sup>2</sup>/(1 + sqrt(1 &minus; <i>e</i><sup>2</sup>)).  The
    expressions for \e q and \e q' in <code>grs</code> and
    <code>radgrav</code> suffer from similar problems.  The resulting
    errors are tiny (about 50&nbsp;pm in the geoid height); however, given
    that's there's essentially no cost to using more accurate
    expressions, it's preferable to do so.
 -# hsynth_WGS84 returns an "undefined" value for \e xi and \e eta at the
    poles.  Better would be to return the value obtained by taking the
    limit \e lat &rarr; &plusmn; 90&deg;.
 .
Issues 1--4 have been reported to the authors of hsynth_WGS84.
Issue 1 is peculiar to Fortran and is not encountered in C++ code and
GravityModel corrects issues 3--5.  On issue 2,
GravityModel neglects the 1/\e r term in \e T in
GravityModel::GeoidHeight and
GravityModel::SphericalAnomaly in order to produce
results which match NGA's for these quantities.  On the other hand,
GravityModel::Disturbance and
GravityModel::T <i>do</i> include this term.

\section gravitygeoid Details of the geoid height and anomaly calculations

Ideally, the geoid represents a surface of constant gravitational
potential which approximates mean sea level.  In reality some
approximations are taken in determining this surface.  The steps taking
by GravityModel in computing the geoid height are
described here (in the context of EGM2008).  This mimics NGA's code
hsynth_WGS84 closely because most users of EGM2008 use the gridded data
generated by this code (e.g., Geoid) and it is desirable
to use a consistent definition of the geoid height.
 - The model potential is band limited; the minimum wavelength that is
   represented is 360&deg;/2160 = 10' (i.e., about 10NM or
   18.5km).  The maximum degree for the spherical harmonic sum is 2190;
   however the model was derived using ellipsoidal harmonics of degree
   and order 2160 and the degree was increased to 2190 in order to
   capture the ellipsoidal harmonics faithfully with spherical
   harmonics.
 - The 1/\e r term is omitted from the \e T (this is issue 2 in \ref
   gravitynga).  This moves the equipotential surface by about 5mm.
 - The surface \e W = <i>U</i><sub>0</sub> is determined by Bruns'
   formula, which is roughly equivalent to a single iteration of
   Newton's method.  The RMS error in this approximation is about 1.5mm
   with a maximum error of about 10&nbsp;mm.
 - The model potential is only valid above the earth's surface.  A
   correction therefore needs to be included where the geoid lies
   beneath the terrain.  This is NGA's "zeta-to-N" correction, which is
   represented by a spherical harmonic sum of degree and order 2160 (and
   so it misses short wavelength terrain variations).  In addition, it
   entails estimating the isostatic equilibrium of the earth's crust.
   The correction lies in the range [&minus;5.05&nbsp;m, 0.05&nbsp;m],
   however for 99.9% of the earth's surface the correction is less than
   10&nbsp;mm in magnitude.
 - The resulting surface lies above the observed mean sea level,
   so &minus;0.41m is added to the geoid height.  (Better would be to change
   the potential used to define the geoid; but this would only change
   the result by about 2mm.)
 .
A useful discussion of the problems with defining a geoid is given by
Dru A. Smith in
<a href="https://www.ngs.noaa.gov/PUBS_LIB/EGM96_GEOID_PAPER/egm96_geoid_paper.html">
There is no such thing as "The" EGM96 geoid: Subtle points on the use of
a global geopotential model</a>, IGeS Bulletin No. 8, International
Geoid Service, Milan, Italy, pp. 17--28 (1998).

GravityModel::GeoidHeight reproduces the results of the
several NGA codes for harmonic synthesis with the following maximum
discrepancies:
 - egm84 = 1.1mm.  This is probably due to inconsistent parameters for the
   reference ellipsoid in the NGA code.  (In particular, the value of
   mass constant excludes the atmosphere; however, it's not clear
   whether the other parameters have been correspondingly adjusted.)
   Note that geoid heights predicted by egm84 differ from those of more
   recent gravity models by about 1 meter.
 - egm96 = 23nm.
 - egm2008 = 78pm.  After addressing some of the issues alluded to in
   issue 4 in \ref gravitynga, the maximum discrepancy becomes 23pm.

The formula for the gravity anomaly vector involves computing gravity
and normal gravity at two different points (with the displacement
between the points unknown <i>ab initio</i>).  Since the gravity anomaly
is already a small quantity it is sometimes acceptable to employ
approximations that change the quantities by \e O(\e f).  The NGA code
uses the spherical approximation described by Heiskanen and Moritz,
Sec. 2-14 and GravityModel::SphericalAnomaly uses the
same approximation for compatibility.  In this approximation, the
gravity disturbance <b>delta</b> = <b>grad</b> \e T is calculated.
Here, \e T once again excludes the 1/\e r term (this is issue 2 in \ref
gravitynga and is consistent with the computation of the geoid height).
Note that <b>delta</b> compares the gravity and the normal gravity at
the \e same point; the gravity anomaly vector is then found by
estimating the gradient of the normal gravity in the limit that the
earth is spherically symmetric.  <b>delta</b> is expressed in \e
spherical coordinates as \e deltax, \e deltay, \e deltaz where, for
example, \e deltaz is the \e radial component of <b>delta</b> (not the
component perpendicular to the ellipsoid) and \e deltay is similarly
slightly different from the usual northerly component.  The components
of the anomaly are then given by
 - gravity anomaly, \e Dg01 = \e deltaz &minus; 2<i>T</i>/\e R, where \e R
   distance to the center of the earth;
 - northerly component of the deflection of the vertical, \e xi = &minus;
   <i>deltay</i>/\e gamma, where \e gamma is the magnitude of the normal
   gravity;
 - easterly component of the deflection of the vertical, \e eta = &minus;
   <i>deltax</i>/\e gamma.
 .
NormalGravity computes the normal gravity accurately and
avoids issue 3 of \ref gravitynga.  Thus while
GravityModel::SphericalAnomaly reproduces the results for
\e xi and \e eta at \e h = 0, there is a slight discrepancy if \e h is
non-zero.

\section gravityatmos The effect of the mass of the atmosphere

All of the supported models use WGS84 for the reference ellipsoid.  This
has (see
<a href="https://web.archive.org/web/20161214054445/https://earth-info.nga.mil/GandG/publications/tr8350.2/wgs84fin.pdf">
TR8350.2</a>, table 3.1)
 - \e a = 6378137 m
 - \e f = 1/298.257223563
 - &omega; = 7292115 &times; 10<sup>&minus;11</sup> rad
   s<sup>&minus;1</sup>
 - \e GM = 3986004.418 &times; 10<sup>8</sup> m<sup>3</sup>
   s<sup>&minus;2</sup>.
 .
The value of \e GM includes the mass of the atmosphere and so strictly
only applies above the earth's atmosphere.  Near the surface of the
earth, the value of \e g will be less (in absolute value) than the value
predicted by these models by about &delta;\e g = (4&pi;<i>G</i>/\e
g) \e A = 8.552&nbsp;&times;&nbsp;10<sup>&minus;11</sup> \e
A&nbsp;m<sup>2</sup>/kg, where \e G is the gravitational constant, \e g
is the earth's gravity, and \e A is the pressure of the atmosphere.  At
sea level we have \e A = 101.3&nbsp;kPa, and &delta;\e g =
8.7&nbsp;&times;&nbsp;10<sup>&minus;6</sup>&nbsp;m&nbsp;s<sup>&minus;2</sup>,
approximately.  (In other words the effect is about 1&nbsp;part in a
million; by way of comparison, buoyancy effects are about 100 times
larger.)

\section gravityparallel Geoid heights on a multi-processor system

The egm2008 model includes many terms (over 2 million spherical
harmonics).  For that reason computations using this model may be slow;
for example it takes about 78 ms to compute the geoid height at a single
point.  There are two ways to speed up this computation:
 - Use a GravityCircle to compute the geoid height at
   several points on a circle of latitude.  This reduces the cost per
   point to about 92&nbsp;&mu;s (a reduction by a factor of over 800).
 - Compute the values on several circles of latitude in parallel.  One
   of the simplest ways of doing this is with
   <a href="https://www.openmp.org"> OpenMP</a>; on an 8-processor system,
   this can speed up the computation by another factor of 8.
 .
Both of these techniques are illustrated by the following code,
which computes a table of geoid heights on
a regular grid and writes on the result in a
<a href="https://vdatum.noaa.gov/docs/gtx_info.html#dev_gtx_binary">.gtx</a>
file.  On an 8-processor Intel 3.0 GHz machine using OpenMP
(-DHAVE_OPENMP=1), it takes about 10 minutes of elapsed time to compute
the geoid height for EGM2008 on a 1' grid.  (Without these
optimizations, the computation would have taken about 50 days!)
\include GeoidToGTX.cpp

cmake will add in support for OpenMP for
<code>examples/GeoidToGTX.cpp</code>, if it is available.

<center>
Back to \ref geoid.  Forward to \ref normalgravity.  Up to \ref contents.
</center>

**********************************************************************/
/**
\page normalgravity Normal gravity

<center>
Back to \ref gravity.  Forward to \ref magnetic.  Up to \ref contents.
</center>

The NormalGravity class computes "normal gravity" which refers to the
exact (classical) solution of an idealised system consisting of an
ellipsoid of revolution with the following properties:
 - \e M the mass interior to the ellipsoid,
 - \e a the equatorial radius,
 - \e b the polar semi-axis,
 - &omega; the rotation rate about the polar axis.
 .
(N.B. The mass always appears in the combination \e GM, with units
m<sup>3</sup>/s<sup>2</sup>, where \e G is the gravtitational constant.)
The distribution of the mass \e M within the ellipsoid is such that the
surface of the ellipsoid is at a constant normal potential where the
normal potential is the sum of the gravitational potential (due to the
gravitational attraction) and the centrifugal potential (due to the
rotation).  The resulting field exterior to the ellipsoid is called
<i>normal gravity</i> and was found by Somigliana (1929).  Because the
potential is constant on the ellipsoid it is sometimes referred to as
the <i>level ellipsoid</i>.

Go to
 - \ref normalgravcoords
 - \ref normalgravpot
 - \ref normalgravmass
 - \ref normalgravsurf
 - \ref normalgravmean
 - \ref normalgravj2

References:
 - C. Somigliana, Teoria generale del campo gravitazionale dell'ellissoide
   di rotazione, Mem. Soc. Astron. Ital, <b>4</b>, 541--599 (1929).
 - W. A. Heiskanen and H. Moritz, Physical Geodesy (Freeman, San
   Francisco, 1967), Secs. 1-19, 2-7, 2-8 (2-9, 2-10), 6-2 (6-3).
 - B. Hofmann-Wellenhof, H. Moritz, Physical Geodesy (Second edition,
   Springer, 2006). https://doi.org/10.1007/978-3-211-33545-1
 - H. Moritz, Geodetic Reference System 1980, J. Geodesy 54(3), 395--405
   (1980). https://doi.org/10.1007/BF02521480
 - H. Moritz, The Figure of the Earth (Wichmann, 1990).
 - M. Chasles, Solution nouvelle du probl&egrave;me de l’attraction d’un
   ellipso&iuml;de h&eacute;t&eacute;rog&egrave;ne sur un point
   exterieur, Jour. Liouville 5, 465--488 (1840), Note 2.
   http://sites.mathdoc.fr/JMPA/PDF/JMPA_1840_1_5_A41_0.pdf

\section normalgravcoords Ellipsoidal coordinates

Two set of formulas are presented: those of Heiskanen and Moritz (1967)
which are applicable to an oblate ellipsoid and a second set where the
variables are distinguished with primes which apply to a prolate
ellipsoid.  The primes are omitted from those variables which are the
same in the two cases.  In the text, the parenthetical "alt." clauses
apply to prolate ellipsoids.

Cylindrical coordinates \f$ R,Z \f$ are expressed in terms of
ellipsoidal coordinates
\f[
\begin{align}
  R &= \sqrt{u^2 + E^2} \cos\beta = u' \cos\beta,\\
  Z &= u \sin\beta = \sqrt{u'^2 + E'^2} \sin\beta,
\end{align}
\f]
where
\f[
\begin{align}
  E^2 = a^2 - b^2,&{} \qquad u^2 = u'^2 + E'^2,\\
  E'^2 = b^2 - a^2,&{} \qquad u'^2 = u^2 + E^2.
\end{align}
\f]
Surfaces of constant \f$ u \f$ (or \f$ u' \f$) are confocal ellipsoids.
The surface \f$ u = 0 \f$ (alt. \f$ u' = 0 \f$) corresponds to the
focal disc of diameter \f$ 2E \f$ (alt. focal rod of length
\f$ 2E' \f$).  The level ellipsoid is given by \f$ u = b \f$
(alt. \f$ u' = a \f$).  On the level ellipsoid, \f$ \beta \f$ is the
familiar parametric latitude.

In writing the potential and the gravity, it is useful to introduce the
functions
\f[
\begin{align}
  Q(z) &= \frac1{2z^3}
    \biggl[\biggl(1 + \frac3{z^2}\biggr)\tan^{-1}z - \frac3z\biggr],\\
  Q'(z') &= \frac{(1+z'^2)^{3/2}}{2z'^3}
    \biggl[\biggl(2 + \frac3{z'^2}\biggr)\sinh^{-1}z' -
      \frac{3\sqrt{1+z'^2}}{z'}\biggr],\\
  H(z) &= \biggl(3Q(z)+z\frac{dQ(z)}{dz}\biggr)(1+z^2)\\
       &= \frac1{z^4}\biggl[3(1+z^2)
         \biggl(1-\frac{\tan^{-1}z}z\biggr)-z^2\biggr],\\
  H'(z') &= \frac{3Q'(z')}{1+z'^2}+z'\frac{dQ'(z')}{dz'}\\
         &= \frac{1+z'^2}{z'^4}
           \biggl[3\biggl(1-\frac{\sqrt{1+z'^2}\sinh^{-1}z'}{z'}\biggr)
           +z'^2\biggr].
\end{align}
\f]
The function arguments are \f$ z = E/u \f$
(alt. \f$ z' = E'/u' \f$) and the unprimed and primed quantities are
related by
\f[
\begin{align}
Q'(z') = Q(z),&{} \qquad H'(z') = H(z),\\
z'^2 = -\frac{z^2}{1 + z^2},&{} \qquad z^2 = -\frac{z'^2}{1 + z'^2}.
\end{align}
\f]
The functions \f$ q(u) \f$ and \f$ q'(u) \f$ used by Heiskanen and
Moritz are given by
\f[
  q(u) = \frac{E^3}{u^3}Q\biggl(\frac Eu\biggr),\qquad
  q'(u) = \frac{E^2}{u^2}H\biggl(\frac Eu\biggr).
\f]
The functions \f$ Q(z) \f$, \f$ Q'(z') \f$, \f$ H(z) \f$, and
\f$ H'(z') \f$ are more convenient for use in numerical codes because
they are finite in the spherical limit \f$ E \rightarrow 0 \f$, i.e.,
\f$ Q(0) = Q'(0) = \frac2{15} \f$, and \f$ H(0) = H'(0) = \frac25 \f$.

\section normalgravpot The normal potential

The normal potential is the sum of three components, a mass term, a
quadrupole term and a centrifugal term,
\f[ U = U_m + U_q + U_r. \f]
The mass term is
\f[ U_m = \frac {GM}E \tan^{-1}\frac Eu
        = \frac {GM}{E'} \sinh^{-1}\frac{E'}{u'}; \f]
the quadrupole term is
\f[
\begin{align}
 U_q &= \frac{\omega^2}2 \frac{a^2 b^3}{u^3} \frac{Q(E/u)}{Q(E/b)}
        \biggl(\sin^2\beta-\frac13\biggr)\\
     &= \frac{\omega^2}2 \frac{a^2 b^3}{(u'^2+E'^2)^{3/2}}
        \frac{Q'(E'/u')}{Q'(E'/a)}
        \biggl(\sin^2\beta-\frac13\biggr);
\end{align}
\f]
finally, the rotational term is
\f[
U_r = \frac{\omega^2}2 R^2
    = \frac{\omega^2}2 (u^2 + E^2) \cos^2\beta
    = \frac{\omega^2}2 u'^2 \cos^2\beta.
\f]

\f$ U_m \f$ and \f$ U_q \f$ are both gravitational potentials (due to
mass within the ellipsoid).  The total mass contributing to \f$ U_m \f$
is \f$ M \f$; the total mass contributing to \f$ U_q \f$ vanishes (far
from the ellipsoid, the \f$ U_q \f$ decays inversely as the cube of the
distance).

\f$ U_m \f$ and \f$ U_q + U_r \f$ are separately both constant on the
level ellipsoid.  \f$ U_m \f$ is the normal potential for a massive
non-rotating ellipsoid.  \f$ U_q + U_r \f$ is the potential for a
massless rotating ellipsoid.  Combining all the terms, \f$ U \f$ is the
normal potential for a massive rotating ellipsoid.

The figure shows normal gravity for the case \f$ GM = 1 \f$,
\f$ a = 1 \f$, \f$ b = 0.8 \f$, \f$ \omega = 0.3 \f$.  The level
ellipsoid is shown in red.  Contours of constant gravity potential are
shown in blue; the contour spacing is constant outside the ellipsoid and
equal to 1/20 of the difference between the potentials on the ellipsoid
and at the geostationary point (\f$ R = 2.2536 \f$, \f$ Z = 0 \f$);
inside the ellipsoid the contour spacing is 5 times greater.  The green
lines are stream lines for the gravity; these are spaced at intervals of
10&deg; in parametric latitude on the surface of the ellipsoid.  The
normal gravity is continued into the level ellipsoid under the
assumption that the mass is concentrated on the focal disc, shown in
black.

\image html normal-gravity-potential-1.svg "Normal gravity"

\section normalgravmass The mass distribution

Typically, the normal potential, \f$ U \f$, is only of interest for
outside the ellipsoid \f$ u \ge b \f$ (alt. \f$ u' \ge a \f$).  In
planetary applications, an open problem is finding a mass distribution
which is in hydrostatic equilibrium (the mass density is non-negative
and a non-decreasing function of the potential interior to the
ellipsoid).

However it is possible to give singular mass distributions consistent
with the normal potential.

For a non-rotating body, the potential \f$ U = U_m \f$ is generated by a
sandwiching the mass \f$ M \f$ uniformly between the level ellipsoid
with semi-axes \f$ a \f$ and \f$ b \f$ and a close <i>similar</i>
ellipsoid with semi-axes \f$ (1-\epsilon)a \f$ and
\f$ (1-\epsilon)b \f$.  Chasles (1840) extends a theorem of Newton to
show that the field interior to such an ellipsoidal shell vanishes.
Thus the potential on the ellipsoid is constant, i.e., it is indeed a
level ellipsoid.  This result also holds for a non-rotating triaxial
ellipsoid.

Observing that \f$ U_m \f$ and \f$ U_q \f$ as defined above obey
\f$ \nabla^2 U_m = \nabla^2 U_q = 0 \f$ everywhere for \f$ u > 0 \f$
(alt. \f$ u' > 0 \f$), we see that these potentials correspond to
masses concentrated at \f$ u = 0 \f$ (alt. \f$ u' = 0 \f$).

In the oblate case, \f$ U_m \f$ is generated by a massive disc at
\f$ Z = 0 \f$, \f$ R < E \f$, with mass density (mass per unit area) \f$
\rho_m \f$ and moments of inertia about the equatorial (resp. polar)
axis of \f$ A_m \f$ (resp. \f$ C_m \f$) given by
\f[
\begin{align}
\rho_m &= \frac M{2\pi E\sqrt{E^2 - R^2}},\\
A_m &= \frac {ME^2}3, \\
C_m &= \frac {2ME^2}3, \\
C_m-A_m &= \frac {ME^2}3.
\end{align}
\f]
This mass distribution is the same as that produced by projecting a
uniform spherical shell of mass \f$ M \f$ and radius \f$ E \f$ onto the
equatorial plane.

In the prolate case, \f$ U_m \f$ is generated by a massive rod at \f$ R
= 0 \f$, \f$ Z < E' \f$ and now the mass density \f$ \rho'_m \f$ has
units mass per unit length,
\f[
\begin{align}
\rho'_m &= \frac M{2E'},\\
A_m &= \frac {ME'^2}3, \\
C_m &= 0, \\
C_m-A_m &= -\frac {ME'^2}3.
\end{align}
\f]
This mass distribution is the same as that produced by projecting a
uniform spherical shell of mass \f$ M \f$ and radius \f$ E' \f$ onto the
polar axis.

Similarly, \f$ U_q \f$ is generated in the oblate case by
\f[
\begin{align}
\rho_q &= \frac{a^2 b^3 \omega^2}G
   \frac{2E^2 - 3R^2}{6\pi E^5 \sqrt{E^2 - R^2} Q(E/b)}, \\
A_q &= -\frac{a^2 b^3 \omega^2}G \frac2{45\,Q(E/b)}, \\
C_q &= -\frac{a^2 b^3 \omega^2}G \frac4{45\,Q(E/b)}, \\
C_q-A_q &= -\frac{a^2 b^3 \omega^2}G \frac2{45\,Q(E/b)}.
\end{align}
\f]
The corresponding results for a prolate ellipsoid are
\f[
\begin{align}
\rho_q' &= \frac{a^2 b^3 \omega^2}G
   \frac{3Z^2 - E'^2}{12\,E'^5 Q'(E'/a)}, \\
A_q &= \frac{a^2 b^3 \omega^2}G \frac2{45\,Q'(E'/a)}, \\
C_q &= 0, \\
C_q-A_q &= -\frac{a^2 b^3 \omega^2}G \frac2{45\,Q'(E'/a)}.
\end{align}
\f]

Summing up the mass and quadrupole terms, we have
\f[
\begin{align}
A &= A_m + A_q, \\
C &= C_m + C_q, \\
J_2 & = \frac{C - A}{Ma^2},
\end{align}
\f]
where \f$ J_2 \f$ is the <i>dynamical form factor</i>.

\section normalgravsurf The surface gravity

Each term in the potential contributes to the gravity on the surface of
the ellipsoid
\f[
\gamma = \gamma_m + \gamma_q + \gamma_r;
\f]
These are the components of gravity normal to the ellipsoid and, by
convention, \f$ \gamma \f$ is positive downwards.  The tangential
components of the total gravity and that due to \f$ U_m \f$ vanish.
Those tangential components of the gravity due to \f$ U_q \f$ and \f$
U_r \f$ cancel one another.

The gravity \f$ \gamma \f$ has the following dependence on latitude
\f[
\begin{align}
\gamma &= \frac{b\gamma_a\cos^2\beta + a\gamma_b\sin^2\beta}
               {\sqrt{b^2\cos^2\beta + a^2\sin^2\beta}}\\
       &= \frac{a\gamma_a\cos^2\phi + b\gamma_b\sin^2\phi}
               {\sqrt{a^2\cos^2\phi + b^2\sin^2\phi}},
\end{align}
\f]
and the individual components, \f$ \gamma_m \f$, \f$ \gamma_q \f$, and
\f$ \gamma_r \f$, have the same dependence on latitude.  The equatorial
and polar gravities are
\f[
\begin{align}
\gamma_a &= \gamma_{ma} + \gamma_{qa} + \gamma_{ra},\\
\gamma_b &= \gamma_{mb} + \gamma_{qb} + \gamma_{rb},
\end{align}
\f]
where
\f[
\begin{align}
\gamma_{ma} &= \frac{GM}{ab},\qquad \gamma_{mb} = \frac{GM}{a^2},\\
\gamma_{qa} &= -\frac{\omega^2 a}6 \frac{H(E/b)}{Q(E/b)}
             = -\frac{\omega^2 a}6 \frac{H'(E'/a)}{Q'(E'/a)},\\
\gamma_{qb} &= \frac{\omega^2 b}3 \frac{H(E/b)}{Q(E/b)}
             = \frac{\omega^2 b}3 \frac{H'(E'/a)}{Q'(E'/a)},\\
\gamma_{ra} &= -\omega^2 a,\qquad \gamma_{rb} = 0.
\end{align}
\f]

\section normalgravmean The mean gravity

Performing an average of the surface gravity over the area of the
ellipsoid gives
\f[
\langle \gamma \rangle = \frac {4\pi a^2 b}A
  \biggl(\frac{2\gamma_a}{3a} + \frac{\gamma_b}{3b}\biggr),
\f]
where \f$ A \f$ is the area of the ellipsoid
\f[
\begin{align}
 A &= 2\pi\biggl( a^2 + ab\frac{\sinh^{-1}(E/b)}{E/b} \biggr)\\
   &= 2\pi\biggl( a^2 + b^2\frac{\tan^{-1}(E'/a)}{E'/a} \biggr).
\end{align}
\f]

The contributions to the mean gravity are
\f[
\begin{align}
\langle \gamma_m \rangle &= \frac{4\pi}A GM, \\
\langle \gamma_q \rangle &= 0 \quad \text{(as expected)}, \\
\langle \gamma_r \rangle &= -\frac{4\pi}A \frac{2\omega^2 a^2b}3,\\
\end{align}
\f]
resulting in
\f[
\langle \gamma \rangle = \frac{4\pi}A
    \biggl(GM - \frac{2\omega^2 a^2b}3\biggr).
\f]

\section normalgravj2 Possible values of the dynamical form factor

The solution for the normal gravity is well defined for arbitrary
\f$ M \f$, \f$ \omega \f$, \f$ a > 0\f$, and \f$ f < 1 \f$.  (Note that
arbitrary oblate and prolate ellipsoids are possible, although
hydrostatic equilibrium would not result in a prolate ellipsoid.)
However, it is much easier to measure the dynamical form factor
\f$ J_2 \f$ (from the motion of artificial satellites) than the
flattening \f$ f \f$.  (Note too that \f$ GM \f$ is typically
measured from satellite or astronomical observations and so it
includes the mass of the atmosphere.)

So a question for the software developer is: given values of \f$ M > 0\f$,
\f$ \omega \f$, and \f$ a > 0 \f$, what are the allowed values of
\f$ J_2 \f$?  We restrict the question to \f$ M > 0 \f$.  The
(unphysical) case \f$ M = 0 \f$ is problematic because \f$ M \f$ appears
in the denominator in the definition of \f$ J_2 \f$.  In the (also
unphysical) case \f$ M < 0 \f$, a given \f$ J_2 \f$ can result from two
distinct values of \f$ f \f$.

Holding \f$ M > 0\f$, \f$ \omega \f$, and \f$ a > 0 \f$ fixed and
varying \f$ f \f$ from \f$ -\infty \f$ to \f$ 1 \f$, we find that
\f$ J_2 \f$ monotonically increases from \f$ -\infty \f$ to
\f[
\frac13 - \frac8{45\pi} \frac{\omega^2 a^3}{GM}.
\f]
Thus any value of \f$ J_2 \f$ less that this value is permissible (but
some of these values may be unphysical).  In obtaining this limiting
value, we used the result
\f$ Q(z \rightarrow \infty) \rightarrow \pi/(4 z^3) \f$.  The value
\f[
J_2 = -\frac13 \frac{\omega^2 a^3}{GM}
\f]
results in a sphere (\f$ f = 0 \f$).

<center>
Back to \ref gravity.  Forward to \ref magnetic.  Up to \ref contents.
</center>

**********************************************************************/
/**
\page magnetic Magnetic models

<center>
Back to \ref normalgravity.  Forward to \ref geodesic.  Up to \ref contents.
</center>

GeographicLib can compute the earth's magnetic field by a magnetic
model using the MagneticModel and
MagneticCircle classes and with the
<a href="MagneticField.1.html">MagneticField</a> utility.  These models
expand the internal magnetic potential of the earth as sum of spherical
harmonics.  They neglect magnetic fields due to the ionosphere, the
magnetosphere, nearby magnetized materials, electric machinery, etc.
Users of MagneticModel are advised to read the
<a href="https://www.ngdc.noaa.gov/IAGA/vmod/igrfhw.html">"Health
Warning"</a> this is provided with igrf11.  Although the advice is
specific to igrf11, many of the comments apply to all magnetic field
models.

The supported models are
 - <b>wmm2010</b>, the
   <a href="https://ngdc.noaa.gov/geomag/WMM/DoDWMM.shtml"> World
   Magnetic Model 2010</a>, which approximates the main magnetic field
   for the period 2010--2015.
 - <b>wmm2015v2</b>, the
   <a href="https://ngdc.noaa.gov/geomag/WMM/DoDWMM.shtml"> World
   Magnetic Model 2015</a>, which approximates the main magnetic field
   for the period 2015--2020.
 - <b>wmm2015</b>, a deprecated version of wmm2015v2.
 - <b>wmm2020</b>, the
   <a href="https://ngdc.noaa.gov/geomag/WMM/DoDWMM.shtml"> World
   Magnetic Model 2020</a>, which approximates the main magnetic field
   for the period 2020--2025.
 - <b>igrf11</b>, the
   <a href="https://ngdc.noaa.gov/IAGA/vmod/igrf.html">International
   Geomagnetic Reference Field (11th generation)</a>, which approximates
   the main magnetic field for the period 1900--2015.
 - <b>igrf12</b>, the
   <a href="https://ngdc.noaa.gov/IAGA/vmod/igrf.html">International
   Geomagnetic Reference Field (12th generation)</a>, which approximates
   the main magnetic field for the period 1900--2020.
 - <b>igrf13</b>, the
   <a href="https://ngdc.noaa.gov/IAGA/vmod/igrf.html">International
   Geomagnetic Reference Field (13th generation)</a>, which approximates
   the main magnetic field for the period 1900--2025.
 - <b>emm2010</b>, the
   <a href="https://ngdc.noaa.gov/geomag/EMM/index.html"> Enhanced
   Magnetic Model 2010</a>, which approximates the main and crustal
   magnetic fields for the period 2010--2015.
 - <b>emm2015</b>, the
   <a href="https://ngdc.noaa.gov/geomag/EMM/index.html"> Enhanced
   Magnetic Model 2015</a>, which approximates the main and crustal
   magnetic fields for the period 2000--2020.
 - <b>emm2017</b>, the
   <a href="https://ngdc.noaa.gov/geomag/EMM/index.html"> Enhanced
   Magnetic Model 2017</a>, which approximates the main and crustal
   magnetic fields for the period 2000--2022.

Go to
 - \ref magneticinst
 - \ref magneticformat

\section magneticinst Installing the magnetic field models

These magnetic models are available for download:
<center>
<table>
<caption>Available magnetic models</caption>
<tr>
 <th rowspan="2">name         <th rowspan="2">max\n degree
 <th rowspan="2">time\n interval
 <th rowspan="2">size\n(kB)
 <th colspan="3"><center>Download Links (size, kB)</center></th>
<tr>
 <th>tar file</th>
 <th>Windows\n installer</th>
 <th>zip file</th>
<tr>
 <td>wmm2010
 <td><center>12</center>
 <td><center>2010--2015</center>
 <td><center>3</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/magnetic-distrib/wmm2010.tar.bz2">
 link</a> (2)</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/magnetic-distrib/wmm2010.exe">
 link</a> (300)</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/magnetic-distrib/wmm2010.zip">
 link</a> (2)</center>
<tr>
 <td>wmm2015
 <td><center>12</center>
 <td><center>2015--2020</center>
 <td><center>3</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/magnetic-distrib/wmm2015.tar.bz2">
 link</a> (2)</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/magnetic-distrib/wmm2015.exe">
 link</a> (300)</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/magnetic-distrib/wmm2015.zip">
 link</a> (2)</center>
<tr>
 <td>wmm2015v2
 <td><center>12</center>
 <td><center>2015--2020</center>
 <td><center>3</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/magnetic-distrib/wmm2015v2.tar.bz2">
 link</a> (2)</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/magnetic-distrib/wmm2015v2.exe">
 link</a> (300)</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/magnetic-distrib/wmm2015v2.zip">
 link</a> (2)</center>
<tr>
 <td>wmm2020
 <td><center>12</center>
 <td><center>2020--2025</center>
 <td><center>3</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/magnetic-distrib/wmm2020.tar.bz2">
 link</a> (2)</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/magnetic-distrib/wmm2020.exe">
 link</a> (1390)</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/magnetic-distrib/wmm2020.zip">
 link</a> (2)</center>
<tr>
 <td>igrf11
 <td><center>13</center>
 <td><center>1900--2015</center>
 <td><center>25</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/magnetic-distrib/igrf11.tar.bz2">
 link</a> (7)</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/magnetic-distrib/igrf11.exe">
 link</a> (310)</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/magnetic-distrib/igrf11.zip">
 link</a> (8)</center>
<tr>
 <td>igrf12
 <td><center>13</center>
 <td><center>1900--2020</center>
 <td><center>26</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/magnetic-distrib/igrf12.tar.bz2">
 link</a> (7)</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/magnetic-distrib/igrf12.exe">
 link</a> (310)</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/magnetic-distrib/igrf12.zip">
 link</a> (8)</center>
<tr>
 <td>igrf13
 <td><center>13</center>
 <td><center>1900--2025</center>
 <td><center>28</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/magnetic-distrib/igrf13.tar.bz2">
 link</a> (7)</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/magnetic-distrib/igrf13.exe">
 link</a> (1420)</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/magnetic-distrib/igrf13.zip">
 link</a> (8)</center>
<tr>
 <td>emm2010
 <td><center>739</center>
 <td><center>2010--2015</center>
 <td><center>4400</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/magnetic-distrib/emm2010.tar.bz2">
 link</a> (3700)</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/magnetic-distrib/emm2010.exe">
 link</a> (3000)</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/magnetic-distrib/emm2010.zip">
 link</a> (4100)</center>
<tr>
 <td>emm2015
 <td><center>729</center>
 <td><center>2000--2020</center>
 <td><center>4300</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/magnetic-distrib/emm2015.tar.bz2">
 link</a> (660)</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/magnetic-distrib/emm2015.exe">
 link</a> (990)</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/magnetic-distrib/emm2015.zip">
 link</a> (1030)</center>
<tr>
 <td>emm2017
 <td><center>790</center>
 <td><center>2000--2022</center>
 <td><center>5050</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/magnetic-distrib/emm2017.tar.bz2">
 link</a> (1740)</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/magnetic-distrib/emm2017.exe">
 link</a> (1700)</center>
 <td><center>
 <a href="https://sourceforge.net/projects/geographiclib/files/magnetic-distrib/emm2017.zip">
 link</a> (2750)</center>
</table>
</center>
The "size" column is the size of the uncompressed data.  <b>N.B.</b>, the
wmm2015 model is <b>deprecated</b>; use wmm2015v2 instead.

For Linux and Unix systems, GeographicLib provides a shell script
geographiclib-get-magnetic (typically installed in /usr/local/sbin)
which automates the process of downloading and installing the magnetic
models.  For example
\verbatim
   geographiclib-get-magnetic all  # install all available models
   geographiclib-get-magnetic -h   # for help
\endverbatim
This script should be run as a user with write access to the
installation directory, which is typically
/usr/local/share/GeographicLib (this can be overridden with the -p
flag), and the data will then be placed in the "magnetic" subdirectory.

Windows users should download and run the Windows installers.  These
will prompt for an installation directory with the default being
\verbatim
   C:/ProgramData/GeographicLib
\endverbatim
(which you probably should not change) and the data is installed in the
"magnetic" sub-directory.  (The second directory name is an alternate name
that Windows 7 uses for the "Application Data" directory.)

Otherwise download \e either the tar.bz2 file \e or the zip file (they
have the same contents).  To unpack these, run, for example
\verbatim
   mkdir -p /usr/local/share/GeographicLib
   tar xofjC wmm2020.tar.bz2 /usr/local/share/GeographicLib
   tar xofjC emm2010.tar.bz2 /usr/local/share/GeographicLib
   etc.
\endverbatim
and, again, the data will be placed in the "magnetic" subdirectory.

However you install the magnetic models, all the datasets should be
installed in the same directory.  MagneticModel and
<a href="MagneticField.1.html">MagneticField</a> uses a compile time
default to locate the datasets.  This is
- /usr/local/share/GeographicLib/magnetic, for non-Windows systems
- C:/ProgramData/GeographicLib/magnetic, for Windows systems
.
consistent with the examples above.  This may be overridden at run-time
by defining the GEOGRAPHICLIB_MAGNETIC_PATH or the GEOGRAPHIC_DATA
environment variables; see
MagneticModel::DefaultMagneticPath() for details.
Finally, the path may be set using the optional second argument to the
MagneticModel constructor or with the "-d" flag to
<a href="MagneticField.1.html">MagneticField</a>.  Supplying the "-h"
flag to
<a href="MagneticField.1.html">MagneticField</a> reports the default
path for magnetic models for that utility.  The "-v" flag causes
MagneticField to report the full path name of the data file it uses.

\section magneticformat The format of the magnetic model files

The constructor for MagneticModel reads a file called
NAME.wmm which specifies various properties for the magnetic model.  It
then opens a binary file NAME.wmm.cof to obtain the coefficients of the
spherical harmonic sum.

The first line of the .wmm file must consist of "WMMF-v" where WMMF
stands for "World Magnetic Model Format" and v is the version number of
the format (currently "2").

The rest of the File is read a line at a time.  A # character and
everything after it are discarded.  If the result is just white space it
is discarded.  The remaining lines are of the form "KEY WHITESPACE
VALUE".  In general, the KEY and the VALUE are case-sensitive.

MagneticModel only pays attention to the following
keywords
 - keywords that affect the field calculation, namely:
   - <b>Radius</b> (required), the normalizing radius of the model in
     meters.
   - <b>NumModels</b> (default 1), the number of models.  WMM2020
     consists of a single model giving the magnetic field and its time
     variation at 2020.  IGRF12 consists of 24 models for 1900 thru 2015
     at 5 year intervals.  The time variation is given only for the last
     model to allow extrapolation beyond 2015.  For dates prior to 2015,
     linear interpolation is used.
   - <b>NumConstants</b> (default 0), the number of time-independent
     terms; this can be 0 or 1.  This keyword was introduced in format
     version 2 (GeographicLib version 1.43) to support the EMM2015 and
     later models.  This model includes long wavelength time-varying
     components of degree 15.  This is supplemented by a short
     wavelength time-independent component with much higher degree.
   - <b>Epoch</b> (required), the time origin (in fractional years) for
     the first model.
   - <b>DeltaEpoch</b> (default 1), the interval between models in years
     (only relevant for NumModels &gt; 1).
   - <b>Normalization</b> (default schmidt), the normalization used for
     the associated Legendre functions (schmidt or full).
   - <b>ID</b> (required), 8 printable characters which serve as a
     signature for the .wmm.cof file (they must appear as the first 8
     bytes of this file).
 - keywords that store data that the user can query:
   - <b>Name</b>, the name of the model.
   - <b>Description</b>, a more descriptive name of the model.
   - <b>ReleaseDate</b>, when the model was created.
   - <b>MinTime</b>, the minimum date at which the model should be used.
   - <b>MaxTime</b>, the maximum date at which the model should be used.
   - <b>MinHeight</b>, the minimum height above the ellipsoid for which
     the model should be used.
   - <b>MaxHeight</b>, the maximum height above the ellipsoid for which
     the model should be used.
   .
   MagneticModel does not enforce the restrictions
   implied by last four quantities.  However,
   <a href="MagneticField.1.html">MagneticField</a> issues a warning if
   these limits are exceeded.
 - keywords that are examined to verify that their values are valid:
   - <b>Type</b> (default linear), the type of the model.  "linear"
     means that the time variation is piece-wise linear (either using
     interpolation between the field at two dates or using the field and
     its first derivative with respect to time).  This is the only type
     of model supported at present.
   - <b>ByteOrder</b> (default little), the order of bytes in the
     .wmm.cof file.  Only little endian is supported at present.
 .
Other keywords are ignored.

The coefficient file NAME.wmm.cof is a binary file in little endian
order.  The first 8 bytes of this file must match the ID given in
NAME.wmm.  This is followed by NumModels + 1 sets of spherical harmonic
coefficients.  The first NumModels of these model the magnetic field at
Epoch + \e i * DeltaEpoch for 0 &le; \e i &lt; NumModels.  The last set of
coefficients model the rate of change of the magnetic field at Epoch +
(NumModels &minus; 1) * DeltaEpoch.  The format for each set of coefficients
is:
 - \e N, the maximum degree of the sum stored as a 4-byte signed integer.
   This must satisfy \e N &ge; &minus;1.
 - \e M, the maximum order of the sum stored as a 4-byte signed integer.
   This must satisfy \e N &ge; \e M &ge; &minus;1.
 - <i>C</i><sub><i>nm</i></sub>, the coefficients of the cosine
   coefficients of the sum in column (i.e., \e m) major order.  There
   are (\e M + 1) (2\e N &minus; \e M + 2) / 2 elements which are stored
   as IEEE doubles (8 bytes).  For example for \e N = \e M = 3, there
   are 10 coefficients arranged as
   <i>C</i><sub>00</sub>,
   <i>C</i><sub>10</sub>,
   <i>C</i><sub>20</sub>,
   <i>C</i><sub>30</sub>,
   <i>C</i><sub>11</sub>,
   <i>C</i><sub>21</sub>,
   <i>C</i><sub>31</sub>,
   <i>C</i><sub>22</sub>,
   <i>C</i><sub>32</sub>,
   <i>C</i><sub>33</sub>.
 - <i>S</i><sub><i>nm</i></sub>, the coefficients of the sine
   coefficients of the sum in column (i.e., \e m) major order starting
   at \e m = 1.  There are \e M (2\e N &minus; \e M + 1) / 2 elements
   which are stored as IEEE doubles (8 bytes).  For example for \e N =
   \e M = 3, there are 6 coefficients arranged as
   <i>S</i><sub>11</sub>,
   <i>S</i><sub>21</sub>,
   <i>S</i><sub>31</sub>,
   <i>S</i><sub>22</sub>,
   <i>S</i><sub>32</sub>,
   <i>S</i><sub>33</sub>.
 .
Although the coefficient file is in little endian order, GeographicLib
can read it on big endian machines.  It can only be read on machines
which store doubles in IEEE format.

As an illustration, here is igrf11.wmm:
\verbatim
WMMF-1
# A World Magnetic Model (Format 1) file.  For documentation on the
# format of this file see
# https://geographiclib.sourceforge.io/C++/doc/magnetic.html#magneticformat
Name            igrf11
Description     International Geomagnetic Reference Field 11th Generation
URL             https://ngdc.noaa.gov/IAGA/vmod/igrf.html
Publisher       National Oceanic and Atmospheric Administration
ReleaseDate     2009-12-15
DataCutOff      2009-10-01
ConversionDate  2011-11-04
DataVersion     1
Radius          6371200
NumModels       23
Epoch           1900
DeltaEpoch      5
MinTime         1900
MaxTime         2015
MinHeight       -1000
MaxHeight       600000

# The coefficients are stored in a file obtained by appending ".cof" to
# the name of this file.  The coefficients were obtained from IGRF11.COF
# in the geomag70 distribution.
ID              IGRF11-A
\endverbatim

<center>
Back to \ref normalgravity.  Forward to \ref geodesic.  Up to \ref contents.
</center>

**********************************************************************/
/**
\page geodesic Geodesics on an ellipsoid of revolution

<center>
Back to \ref magnetic.  Forward to \ref nearest.  Up to \ref contents.
</center>

Geodesic and GeodesicLine provide accurate
solutions to the direct and inverse geodesic problems.  The
<a href="GeodSolve.1.html">GeodSolve</a> utility provides an interface
to these classes.  AzimuthalEquidistant implements the
azimuthal equidistant projection in terms of geodesics.
CassiniSoldner implements a transverse cylindrical
equidistant projection in terms of geodesics.  The
<a href="GeodesicProj.1.html">GeodesicProj</a> utility provides an
interface to these projections.

The algorithms used by Geodesic and GeodesicLine are based on a Taylor
expansion of the geodesic integrals valid when the flattening \e f is
small.  GeodesicExact and GeodesicLineExact evaluate the integrals
exactly (in terms of incomplete elliptic integrals).  For the WGS84
ellipsoid, the series solutions are about 2--3 times faster and 2--3
times more accurate (because it's easier to control round-off errors
with series solutions); thus Geodesic and GeodesicLine are recommended
for most geodetic applications.  However, in applications where the
absolute value of \e f is greater than about 0.02, the exact classes
should be used.

Go to
 - \ref testgeod
 - \ref geodseries
 - \ref geodellip
 - \ref meridian
 - \ref geodshort
 .
For some background information on geodesics on triaxial ellipsoids, see
\ref triaxial.

References:
 - F. W. Bessel,
   <a href="https://doi.org/10.1002/asna.201011352">The calculation
   of longitude and latitude from geodesic measurements (1825)</a>,
   Astron. Nachr. 331(8), 852--861 (2010);
   translated by C. F. F. Karney and R. E. Deakin; preprint:
   <a href="https://arxiv.org/abs/0908.1824">arXiv:0908.1824</a>.
 - F. R. Helmert,
   <a href="https://doi.org/10.5281/zenodo.32050">
   Mathematical and Physical Theories of Higher Geodesy, Part 1 (1880)</a>,
   Aeronautical Chart and Information Center (St. Louis, 1964),
   Chaps. 5--7.
 - J. Danielsen,
   The area under the geodesic,
   Survey Review 30(232), 61--66 (1989).
   DOI: <a href="https://doi.org/10.1179/003962689791474267">
   10.1179/003962689791474267</a>
 - C. F. F. Karney,
   <a href="https://doi.org/10.1007/s00190-012-0578-z">
   Algorithms for geodesics</a>,
   J. Geodesy 87(1), 43--55 (2013);
   DOI: <a href="https://doi.org/10.1007/s00190-012-0578-z">
   10.1007/s00190-012-0578-z</a>;
   addenda: <a href="https://geographiclib.sourceforge.io/geod-addenda.html">
   geod-addenda.html</a>;
   resource page:
   <a href="https://geographiclib.sourceforge.io/geod.html"> geod.html</a>.
 - A collection of some papers on geodesics is available at
   https://geographiclib.sourceforge.io/geodesic-papers/biblio.html
 - The wikipedia page,
   <a href="https://en.wikipedia.org/wiki/Geodesics_on_an_ellipsoid">
   Geodesics on an ellipsoid</a>.

\section testgeod Test data for geodesics

A test set a geodesics is available at
 - <a href="https://sourceforge.net/projects/geographiclib/files/testdata/GeodTest.dat.gz">
   GeodTest.dat.gz</a>
 - C. F. F. Karney, <i>Test set for geodesics</i> (2010), <br>
   DOI: <a href="https://doi.org/10.5281/zenodo.32156">
   10.5281/zenodo.32156</a>.
 .
This is about 39 MB (compressed).  This consists of a set of geodesics
for the WGS84 ellipsoid.  A subset of this (consisting of 1/50 of the
members &mdash; about 690 kB, compressed) is available at
 - <a href="https://sourceforge.net/projects/geographiclib/files/testdata/GeodTest-short.dat.gz">
   GeodTest-short.dat.gz</a>

Each line of the test set gives 10 space delimited numbers
 - latitude at point 1, \e lat1 (degrees, exact)
 - longitude at point 1, \e lon1 (degrees, always 0)
 - azimuth at point 1, \e azi1 (clockwise from north in degrees, exact)
 - latitude at point 2, \e lat2 (degrees, accurate to
   10<sup>&minus;18</sup> deg)
 - longitude at point 2, \e lon2 (degrees, accurate to
   10<sup>&minus;18</sup> deg)
 - azimuth at point 2, \e azi2 (degrees, accurate to
   10<sup>&minus;18</sup> deg)
 - geodesic distance from point 1 to point 2, \e s12 (meters, exact)
 - arc distance on the auxiliary sphere, \e a12 (degrees, accurate to
   10<sup>&minus;18</sup> deg)
 - reduced length of the geodesic, \e m12 (meters, accurate to 0.1&nbsp;pm)
 - the area under the geodesic, \e S12 (m<sup>2</sup>, accurate to
   1&nbsp;mm<sup>2</sup>)
 .
These are computed using as direct geodesic calculations with the given
\e lat1, \e lon1, \e azi1, and \e s12.  The distance \e s12 always
corresponds to an arc length \e a12 &le; 180&deg;, so the given
geodesics give the shortest paths from point 1 to point 2.  For
simplicity and without loss of generality, \e lat1 is chosen in
[0&deg;, 90&deg;], \e lon1 is taken to be zero, \e azi1 is
chosen in [0&deg;, 180&deg;].  Furthermore, \e lat1 and \e
azi1 are taken to be multiples of 10<sup>&minus;12</sup> deg and \e s12
is a multiple of 0.1&nbsp;&mu;m in [0&nbsp;m, 20003931.4586254&nbsp;m].
This results in \e lon2 in [0&deg;, 180&deg;] and \e azi2 in [0&deg;,
180&deg;].

The direct calculation uses an expansion of the geodesic equations
accurate to <i>f</i><sup>30</sup> (approximately 1 part in 10<sup>50</sup>)
and is computed with with
<a href="https://en.wikipedia.org/wiki/Maxima_(software)">Maxima</a>'s
bfloats and fpprec set to 100 (so the errors in the data are probably
1/2 of the values quoted above).

The contents of the file are as follows:
 - 100000 entries randomly distributed
 - 50000 entries which are nearly antipodal
 - 50000 entries with short distances
 - 50000 entries with one end near a pole
 - 50000 entries with both ends near opposite poles
 - 50000 entries which are nearly meridional
 - 50000 entries which are nearly equatorial
 - 50000 entries running between vertices (\e azi1 = \e azi2 = 90&deg;)
 - 50000 entries ending close to vertices
 .
(a total of 500000 entries).  The values for \e s12 for the geodesics
running between vertices are truncated to a multiple of 0.1&nbsp;pm and this
is used to determine point 2.

This data can be fed to the <a href="GeodSolve.1.html">GeodSolve</a>
utility as follows
 - Direct from point 1:
\verbatim
  gunzip -c GeodTest.dat.gz | cut -d' ' -f1,2,3,7 | ./GeodSolve
\endverbatim
   This should yield columns 4, 5, 6, and 9 of the test set.
 - Direct from point 2:
\verbatim
  gunzip -c GeodTest.dat.gz | cut -d' ' -f4,5,6,7 |
  sed "s/ \([^ ]*$\)/ -\1/" | ./GeodSolve
\endverbatim
   (The sed command negates the distance.)  This should yield columns 1,
   2, and 3, and the negative of column 9 of the test set.
 - Inverse between points 1 and 2:
\verbatim
  gunzip -c GeodTest.dat.gz | cut -d' ' -f1,2,4,5 | ./GeodSolve -i
\endverbatim
   This should yield columns 3, 6, 7, and 9 of the test set.
 .
Add, e.g., "-p 6", to the call to GeodSolve to change the precision of
the output.  Adding "-f" causes GeodSolve to print 12 fields specifying
the geodesic; these include the 10 fields in the test set plus the
geodesic scales \e M12 and \e M21 which are inserted between \e m12 and
\e S12.

Code for computing arbitrarily accurate geodesics in maxima is available
in <a href="geodesic.mac"> geodesic.mac</a> (this depends on
<a href="ellint.mac"> ellint.mac</a> and uses the series computed by
<a href="geod.mac"> geod.mac</a>).  This solve both the direct and
inverse geodesic problems and offers the ability to solve the problems
either using series expansions (similar to Geodesic) or in terms of
elliptic integrals (similar to GeodesicExact).

\section geodseries Expansions for geodesics

We give here the series expansions for the various geodesic integrals
valid to order <i>f</i><sup>10</sup>.  In this release of the code, we
use a 6th-order expansions.  This is sufficient to maintain accuracy for
doubles for the SRMmax ellipsoid (\e a = 6400&nbsp;km, \e f = 1/150).
However, the preprocessor macro GEOGRAPHICLIB_GEODESIC_ORDER can be used
to select an order from 3 thru 8.  (If using long doubles, with a 64-bit
fraction, the default order is 7.)  The series expanded to order
<i>f</i><sup>30</sup> are given in <a href="geodseries30.html">
geodseries30.html</a>.

In the formulas below ^ indicates exponentiation (<i>f</i>^3 =
<i>f</i><sup>3</sup>) and / indicates real division (3/5 = 0.6).  The
equations need to be converted to Horner form, but are here left in
expanded form so that they can be easily truncated to lower order.
These expansions were obtained using the Maxima code,
<a href="geod.mac">geod.mac</a>.

In the expansions below, we have
 - \f$ \alpha \f$ is the azimuth
 - \f$ \alpha_0 \f$ is the azimuth at the equator crossing
 - \f$ \lambda \f$ is the longitude measured from the equator crossing
 - \f$ \sigma \f$ is the spherical arc length
 - \f$ \omega = \tan^{-1}(\sin\alpha_0\tan\sigma) \f$ is the spherical longitude
 - \f$ a \f$ is the equatorial radius
 - \f$ b \f$ is the polar semi-axis
 - \f$ f \f$ is the flattening
 - \f$ e^2 = f(2 - f) \f$
 - \f$ e'^2 = e^2/(1-e^2) \f$
 - \f$ k^2 = e'^2 \cos^2\alpha_0 = 4 \epsilon / (1 - \epsilon)^2 \f$
 - \f$ n = f / (2 - f) \f$
 - \f$ c^2 = a^2/2 + b^2/2 (\tanh^{-1}e)/e \f$
 - \e ep2 = \f$ e'^2 \f$
 - \e k2 = \f$ k^2 \f$
 - \e eps = \f$ \epsilon = k^2 / (\sqrt{1 + k^2} + 1)^2\f$

The formula for distance is
\f[
 \frac sb = I_1(\sigma)
\f]
where
\f[
\begin{align}
  I_1(\sigma) &= A_1\bigl(\sigma + B_1(\sigma)\bigr) \\
  B_1(\sigma) &= \sum_{j=1} C_{1j} \sin 2j\sigma
\end{align}
\f]
and
\verbatim
A1 = (1 + 1/4 * eps^2
        + 1/64 * eps^4
        + 1/256 * eps^6
        + 25/16384 * eps^8
        + 49/65536 * eps^10) / (1 - eps);
\endverbatim
\verbatim
C1[1] = - 1/2 * eps
        + 3/16 * eps^3
        - 1/32 * eps^5
        + 19/2048 * eps^7
        - 3/4096 * eps^9;
C1[2] = - 1/16 * eps^2
        + 1/32 * eps^4
        - 9/2048 * eps^6
        + 7/4096 * eps^8
        + 1/65536 * eps^10;
C1[3] = - 1/48 * eps^3
        + 3/256 * eps^5
        - 3/2048 * eps^7
        + 17/24576 * eps^9;
C1[4] = - 5/512 * eps^4
        + 3/512 * eps^6
        - 11/16384 * eps^8
        + 3/8192 * eps^10;
C1[5] = - 7/1280 * eps^5
        + 7/2048 * eps^7
        - 3/8192 * eps^9;
C1[6] = - 7/2048 * eps^6
        + 9/4096 * eps^8
        - 117/524288 * eps^10;
C1[7] = - 33/14336 * eps^7
        + 99/65536 * eps^9;
C1[8] = - 429/262144 * eps^8
        + 143/131072 * eps^10;
C1[9] = - 715/589824 * eps^9;
C1[10] = - 2431/2621440 * eps^10;
\endverbatim

The function \f$ \tau(\sigma) = s/(b A_1) = \sigma + B_1(\sigma) \f$
may be inverted by series reversion giving
\f[
  \sigma(\tau) = \tau + \sum_{j=1} C'_{1j} \sin 2j\sigma
\f]
where
\verbatim
C1'[1] = + 1/2 * eps
         - 9/32 * eps^3
         + 205/1536 * eps^5
         - 4879/73728 * eps^7
         + 9039/327680 * eps^9;
C1'[2] = + 5/16 * eps^2
         - 37/96 * eps^4
         + 1335/4096 * eps^6
         - 86171/368640 * eps^8
         + 4119073/28311552 * eps^10;
C1'[3] = + 29/96 * eps^3
         - 75/128 * eps^5
         + 2901/4096 * eps^7
         - 443327/655360 * eps^9;
C1'[4] = + 539/1536 * eps^4
         - 2391/2560 * eps^6
         + 1082857/737280 * eps^8
         - 2722891/1548288 * eps^10;
C1'[5] = + 3467/7680 * eps^5
         - 28223/18432 * eps^7
         + 1361343/458752 * eps^9;
C1'[6] = + 38081/61440 * eps^6
         - 733437/286720 * eps^8
         + 10820079/1835008 * eps^10;
C1'[7] = + 459485/516096 * eps^7
         - 709743/163840 * eps^9;
C1'[8] = + 109167851/82575360 * eps^8
         - 550835669/74317824 * eps^10;
C1'[9] = + 83141299/41287680 * eps^9;
C1'[10] = + 9303339907/2972712960 * eps^10;
\endverbatim

The reduced length is given by
\f[
\begin{align}
  \frac mb &= \sqrt{1 + k^2 \sin^2\sigma_2} \cos\sigma_1 \sin\sigma_2 \\
  &\quad {}-\sqrt{1 + k^2 \sin^2\sigma_1} \sin\sigma_1 \cos\sigma_2 \\
  &\quad {}-\cos\sigma_1 \cos\sigma_2 \bigl(J(\sigma_2) - J(\sigma_1)\bigr)
\end{align}
\f]
where
\f[
\begin{align}
 J(\sigma) &= I_1(\sigma) - I_2(\sigma) \\
 I_2(\sigma) &= A_2\bigl(\sigma + B_2(\sigma)\bigr) \\
 B_2(\sigma) &= \sum_{j=1} C_{2j} \sin 2j\sigma
\end{align}
\f]
\verbatim
A2 = (1 - 3/4 * eps^2
        - 7/64 * eps^4
        - 11/256 * eps^6
        - 375/16384 * eps^8
        - 931/65536 * eps^10) / (1 + eps);
\endverbatim
\verbatim
C2[1] = + 1/2 * eps
        + 1/16 * eps^3
        + 1/32 * eps^5
        + 41/2048 * eps^7
        + 59/4096 * eps^9;
C2[2] = + 3/16 * eps^2
        + 1/32 * eps^4
        + 35/2048 * eps^6
        + 47/4096 * eps^8
        + 557/65536 * eps^10;
C2[3] = + 5/48 * eps^3
        + 5/256 * eps^5
        + 23/2048 * eps^7
        + 191/24576 * eps^9;
C2[4] = + 35/512 * eps^4
        + 7/512 * eps^6
        + 133/16384 * eps^8
        + 47/8192 * eps^10;
C2[5] = + 63/1280 * eps^5
        + 21/2048 * eps^7
        + 51/8192 * eps^9;
C2[6] = + 77/2048 * eps^6
        + 33/4096 * eps^8
        + 2607/524288 * eps^10;
C2[7] = + 429/14336 * eps^7
        + 429/65536 * eps^9;
C2[8] = + 6435/262144 * eps^8
        + 715/131072 * eps^10;
C2[9] = + 12155/589824 * eps^9;
C2[10] = + 46189/2621440 * eps^10;
\endverbatim

The longitude is given in terms of the spherical longitude by
\f[
\lambda = \omega - f \sin\alpha_0 I_3(\sigma)
\f]
where
\f[
\begin{align}
 I_3(\sigma) &= A_3\bigl(\sigma + B_3(\sigma)\bigr) \\
 B_3(\sigma) &= \sum_{j=1} C_{3j} \sin 2j\sigma
\end{align}
\f]
and
\verbatim
A3 = 1 - (1/2 - 1/2*n) * eps
       - (1/4 + 1/8*n - 3/8*n^2) * eps^2
       - (1/16 + 3/16*n + 1/16*n^2 - 5/16*n^3) * eps^3
       - (3/64 + 1/32*n + 5/32*n^2 + 5/128*n^3 - 35/128*n^4) * eps^4
       - (3/128 + 5/128*n + 5/256*n^2 + 35/256*n^3 + 7/256*n^4) * eps^5
       - (5/256 + 15/1024*n + 35/1024*n^2 + 7/512*n^3) * eps^6
       - (25/2048 + 35/2048*n + 21/2048*n^2) * eps^7
       - (175/16384 + 35/4096*n) * eps^8
       - 245/32768 * eps^9;
\endverbatim
\verbatim
C3[1] = + (1/4 - 1/4*n) * eps
        + (1/8 - 1/8*n^2) * eps^2
        + (3/64 + 3/64*n - 1/64*n^2 - 5/64*n^3) * eps^3
        + (5/128 + 1/64*n + 1/64*n^2 - 1/64*n^3 - 7/128*n^4) * eps^4
        + (3/128 + 11/512*n + 3/512*n^2 + 1/256*n^3 - 7/512*n^4) * eps^5
        + (21/1024 + 5/512*n + 13/1024*n^2 + 1/512*n^3) * eps^6
        + (243/16384 + 189/16384*n + 83/16384*n^2) * eps^7
        + (435/32768 + 109/16384*n) * eps^8
        + 345/32768 * eps^9;
C3[2] = + (1/16 - 3/32*n + 1/32*n^2) * eps^2
        + (3/64 - 1/32*n - 3/64*n^2 + 1/32*n^3) * eps^3
        + (3/128 + 1/128*n - 9/256*n^2 - 3/128*n^3 + 7/256*n^4) * eps^4
        + (5/256 + 1/256*n - 1/128*n^2 - 7/256*n^3 - 3/256*n^4) * eps^5
        + (27/2048 + 69/8192*n - 39/8192*n^2 - 47/4096*n^3) * eps^6
        + (187/16384 + 39/8192*n + 31/16384*n^2) * eps^7
        + (287/32768 + 47/8192*n) * eps^8
        + 255/32768 * eps^9;
C3[3] = + (5/192 - 3/64*n + 5/192*n^2 - 1/192*n^3) * eps^3
        + (3/128 - 5/192*n - 1/64*n^2 + 5/192*n^3 - 1/128*n^4) * eps^4
        + (7/512 - 1/384*n - 77/3072*n^2 + 5/3072*n^3 + 65/3072*n^4) * eps^5
        + (3/256 - 1/1024*n - 71/6144*n^2 - 47/3072*n^3) * eps^6
        + (139/16384 + 143/49152*n - 383/49152*n^2) * eps^7
        + (243/32768 + 95/49152*n) * eps^8
        + 581/98304 * eps^9;
C3[4] = + (7/512 - 7/256*n + 5/256*n^2 - 7/1024*n^3 + 1/1024*n^4) * eps^4
        + (7/512 - 5/256*n - 7/2048*n^2 + 9/512*n^3 - 21/2048*n^4) * eps^5
        + (9/1024 - 43/8192*n - 129/8192*n^2 + 39/4096*n^3) * eps^6
        + (127/16384 - 23/8192*n - 165/16384*n^2) * eps^7
        + (193/32768 + 3/8192*n) * eps^8
        + 171/32768 * eps^9;
C3[5] = + (21/2560 - 9/512*n + 15/1024*n^2 - 7/1024*n^3 + 9/5120*n^4) * eps^5
        + (9/1024 - 15/1024*n + 3/2048*n^2 + 57/5120*n^3) * eps^6
        + (99/16384 - 91/16384*n - 781/81920*n^2) * eps^7
        + (179/32768 - 55/16384*n) * eps^8
        + 141/32768 * eps^9;
C3[6] = + (11/2048 - 99/8192*n + 275/24576*n^2 - 77/12288*n^3) * eps^6
        + (99/16384 - 275/24576*n + 55/16384*n^2) * eps^7
        + (143/32768 - 253/49152*n) * eps^8
        + 33/8192 * eps^9;
C3[7] = + (429/114688 - 143/16384*n + 143/16384*n^2) * eps^7
        + (143/32768 - 143/16384*n) * eps^8
        + 429/131072 * eps^9;
C3[8] = + (715/262144 - 429/65536*n) * eps^8
        + 429/131072 * eps^9;
C3[9] = + 2431/1179648 * eps^9;
\endverbatim

The formula for area between the geodesic and the equator is given in
Sec. 6 of <a href="https://doi.org/10.1007/s00190-012-0578-z">
Algorithms for geodesics</a> in terms of \e S,
\f[
S = c^2 \alpha + e^2 a^2 \cos\alpha_0 \sin\alpha_0 I_4(\sigma)
\f]
where
\f[
I_4(\sigma) = \sum_{j=0} C_{4j} \cos(2j+1)\sigma
\f]
In the paper, this was expanded in \f$ e'^2 \f$ and \f$ k^2 \f$.
However, the series converges faster for eccentric ellipsoids if the
expansion is in \f$ n \f$ and \f$ \epsilon \f$.  The series to order
\f$ f^{10} \f$ becomes
\verbatim
C4[0] = + (2/3 - 4/15*n + 8/105*n^2 + 4/315*n^3 + 16/3465*n^4 + 20/9009*n^5 + 8/6435*n^6 + 28/36465*n^7 + 32/62985*n^8 + 4/11305*n^9)
        - (1/5 - 16/35*n + 32/105*n^2 - 16/385*n^3 - 64/15015*n^4 - 16/15015*n^5 - 32/85085*n^6 - 112/692835*n^7 - 128/1616615*n^8) * eps
        - (2/105 + 32/315*n - 1088/3465*n^2 + 1184/5005*n^3 - 128/3465*n^4 - 3232/765765*n^5 - 1856/1616615*n^6 - 6304/14549535*n^7) * eps^2
        + (11/315 - 368/3465*n - 32/6435*n^2 + 976/4095*n^3 - 154048/765765*n^4 + 368/11115*n^5 + 5216/1322685*n^6) * eps^3
        + (4/1155 + 1088/45045*n - 128/1287*n^2 + 64/3927*n^3 + 2877184/14549535*n^4 - 370112/2078505*n^5) * eps^4
        + (97/15015 - 464/45045*n + 4192/153153*n^2 - 88240/969969*n^3 + 31168/1322685*n^4) * eps^5
        + (10/9009 + 4192/765765*n - 188096/14549535*n^2 + 23392/855855*n^3) * eps^6
        + (193/85085 - 6832/2078505*n + 106976/14549535*n^2) * eps^7
        + (632/1322685 + 3456/1616615*n) * eps^8
        + 107/101745 * eps^9;
C4[1] = + (1/45 - 16/315*n + 32/945*n^2 - 16/3465*n^3 - 64/135135*n^4 - 16/135135*n^5 - 32/765765*n^6 - 112/6235515*n^7 - 128/14549535*n^8) * eps
        - (2/105 - 64/945*n + 128/1485*n^2 - 1984/45045*n^3 + 256/45045*n^4 + 64/109395*n^5 + 128/855855*n^6 + 2368/43648605*n^7) * eps^2
        - (1/105 - 16/2079*n - 5792/135135*n^2 + 3568/45045*n^3 - 103744/2297295*n^4 + 264464/43648605*n^5 + 544/855855*n^6) * eps^3
        + (4/1155 - 2944/135135*n + 256/9009*n^2 + 17536/765765*n^3 - 3053056/43648605*n^4 + 1923968/43648605*n^5) * eps^4
        + (1/9009 + 16/19305*n - 2656/153153*n^2 + 65072/2078505*n^3 + 526912/43648605*n^4) * eps^5
        + (10/9009 - 1472/459459*n + 106112/43648605*n^2 - 204352/14549535*n^3) * eps^6
        + (349/2297295 + 28144/43648605*n - 32288/8729721*n^2) * eps^7
        + (632/1322685 - 44288/43648605*n) * eps^8
        + 43/479655 * eps^9;
C4[2] = + (4/525 - 32/1575*n + 64/3465*n^2 - 32/5005*n^3 + 128/225225*n^4 + 32/765765*n^5 + 64/8083075*n^6 + 32/14549535*n^7) * eps^2
        - (8/1575 - 128/5775*n + 256/6825*n^2 - 6784/225225*n^3 + 4608/425425*n^4 - 128/124355*n^5 - 5888/72747675*n^6) * eps^3
        - (8/1925 - 1856/225225*n - 128/17325*n^2 + 42176/1276275*n^3 - 2434816/72747675*n^4 + 195136/14549535*n^5) * eps^4
        + (8/10725 - 128/17325*n + 64256/3828825*n^2 - 128/25935*n^3 - 266752/10392525*n^4) * eps^5
        - (4/25025 + 928/3828825*n + 292288/72747675*n^2 - 106528/6613425*n^3) * eps^6
        + (464/1276275 - 17152/10392525*n + 83456/72747675*n^2) * eps^7
        + (1168/72747675 + 128/1865325*n) * eps^8
        + 208/1119195 * eps^9;
C4[3] = + (8/2205 - 256/24255*n + 512/45045*n^2 - 256/45045*n^3 + 1024/765765*n^4 - 256/2909907*n^5 - 512/101846745*n^6) * eps^3
        - (16/8085 - 1024/105105*n + 2048/105105*n^2 - 1024/51051*n^3 + 4096/373065*n^4 - 1024/357357*n^5) * eps^4
        - (136/63063 - 256/45045*n + 512/1072071*n^2 + 494336/33948915*n^3 - 44032/1996995*n^4) * eps^5
        + (64/315315 - 16384/5360355*n + 966656/101846745*n^2 - 868352/101846745*n^3) * eps^6
        - (16/97461 + 14848/101846745*n + 74752/101846745*n^2) * eps^7
        + (5024/33948915 - 96256/101846745*n) * eps^8
        - 1744/101846745 * eps^9;
C4[4] = + (64/31185 - 512/81081*n + 1024/135135*n^2 - 512/109395*n^3 + 2048/1247103*n^4 - 2560/8729721*n^5) * eps^4
        - (128/135135 - 2048/405405*n + 77824/6891885*n^2 - 198656/14549535*n^3 + 8192/855855*n^4) * eps^5
        - (512/405405 - 2048/530145*n + 299008/130945815*n^2 + 280576/43648605*n^3) * eps^6
        + (128/2297295 - 2048/1438965*n + 241664/43648605*n^2) * eps^7
        - (17536/130945815 + 1024/43648605*n) * eps^8
        + 2944/43648605 * eps^9;
C4[5] = + (128/99099 - 2048/495495*n + 4096/765765*n^2 - 6144/1616615*n^3 + 8192/4849845*n^4) * eps^5
        - (256/495495 - 8192/2807805*n + 376832/53348295*n^2 - 8192/855855*n^3) * eps^6
        - (6784/8423415 - 432128/160044885*n + 397312/160044885*n^2) * eps^7
        + (512/53348295 - 16384/22863555*n) * eps^8
        - 16768/160044885 * eps^9;
C4[6] = + (512/585585 - 4096/1422135*n + 8192/2078505*n^2 - 4096/1322685*n^3) * eps^6
        - (1024/3318315 - 16384/9006855*n + 98304/21015995*n^2) * eps^7
        - (103424/189143955 - 8192/4203199*n) * eps^8
        - 1024/189143955 * eps^9;
C4[7] = + (1024/1640925 - 65536/31177575*n + 131072/43648605*n^2) * eps^7
        - (2048/10392525 - 262144/218243025*n) * eps^8
        - 84992/218243025 * eps^9;
C4[8] = + (16384/35334585 - 131072/82447365*n) * eps^8
        - 32768/247342095 * eps^9;
C4[9] = + 32768/92147055 * eps^9;
\endverbatim

\section geodellip Geodesics in terms of elliptic integrals

GeodesicExact and GeodesicLineExact solve the geodesic problem using
elliptic integrals.  The formulation of geodesic in terms of incomplete
elliptic integrals is given in
 - C. F. F. Karney,
   <a href="https://arxiv.org/abs/1102.1215v1">Geodesics
   on an ellipsoid of revolution</a>,
   Feb. 2011; preprint
   <a href="https://arxiv.org/abs/1102.1215v1">arxiv:1102.1215v1</a>.
 .
It is most convenient to use the form derived for a prolate ellipsoid in
Appendix D.  For an oblate ellipsoid this results in elliptic integrals
with an imaginary modulus.  However, the integrals themselves are real
and the algorithms used to compute the elliptic integrals handles the
case of an imaginary modulus using real arithmetic.

The key relations used by GeographicLib are
\f[
 \begin{align}
  \frac sb &= E(\sigma, ik), \\
  \lambda &= (1 - f) \sin\alpha_0  G(\sigma, \cos^2\alpha_0, ik) \\
          &= \chi
           - \frac{e'^2}{\sqrt{1+e'^2}}\sin\alpha_0 H(\sigma, -e'^2, ik), \\
  J(\sigma) &= k^2 D(\sigma, ik),
 \end{align}
\f]
where \f$ \chi \f$ is a modified spherical longitude given by
\f[
\tan\chi = \sqrt{\frac{1+e'^2}{1+k^2\sin^2\sigma}}\tan\omega,
\f]
and
\f[
 \begin{align}
 D(\phi,k) &= \int_0^\phi
 \frac{\sin^2\theta}{\sqrt{1 - k^2\sin^2\theta}}\,d\theta\\
 &=\frac{F(\phi, k) - E(\phi, k)}{k^2},\\
 G(\phi,\alpha^2,k) &= \int_0^\phi
 \frac{\sqrt{1 - k^2\sin^2\theta}}{1 - \alpha^2\sin^2\theta}\,d\theta\\
 &=\frac{k^2}{\alpha^2}F(\phi, k)
 +\biggl(1-\frac{k^2}{\alpha^2}\biggr)\Pi(\phi, \alpha^2, k),\\
 H(\phi, \alpha^2, k)
 &= \int_0^\phi
   \frac{\cos^2\theta}{(1-\alpha^2\sin^2\theta)\sqrt{1-k^2\sin^2\theta}}
   \,d\theta \\
 &=
 \frac1{\alpha^2} F(\phi, k) +
      \biggl(1 - \frac1{\alpha^2}\biggr) \Pi(\phi, \alpha^2, k),
 \end{align}
\f]
and \f$F(\phi, k)\f$, \f$E(\phi, k)\f$, \f$D(\phi, k)\f$, and
\f$\Pi(\phi, \alpha^2, k)\f$, are incomplete elliptic integrals (see
https://dlmf.nist.gov/19.2.ii).  The formula for \f$ s \f$ and the
first expression for \f$ \lambda \f$ are given by Legendre (1811) and
are the most common representation of geodesics in terms of elliptic
integrals.  The second (equivalent) expression for \f$ \lambda \f$,
which was given by Cayley (1870), is useful in that the elliptic
integral is relegated to a small correction term.  This form allows
the longitude to be computed more accurately and is used in
GeographicLib.  (The equivalence of the two expressions for \f$
\lambda \f$ follows from https://dlmf.nist.gov/19.7.E8.)

Nominally, GeodesicExact and GeodesicLineExact will give "exact" results
for any value of the flattening.  However, the geographic latitude is a
distorted measure of distance from the equator with very eccentric
ellipsoids and this introducing an irreducible representational error in
the algorithms in this case.  It is therefore recommended to restrict
the use of these classes to <i>b</i>/\e a &isin; [0.01, 100] or \e f
&isin; [&minus;99, 0.99].  GeodesicExact uses an discrete sine transform
fit for the area \e S12.  The number of terms used is adjusted to give
accurate results for this same range of flattenings, \e f &isin;
[&minus;99, 0.99].

Thomas (1952) and Rollins (2010) use a different independent variable
for geodesics, \f$\theta\f$ instead of \f$\sigma\f$, where \f$
\tan\theta = \sqrt{1 + k^2} \tan\sigma \f$.  The corresponding
expressions for \f$ s \f$ and \f$ \lambda \f$ are given here for
completeness:
\f[
\begin{align}
\frac sb &= \sqrt{1-k'^2} \Pi(\theta, k'^2, k'), \\
\lambda &= (1-f) \sqrt{1-k'^2} \sin\alpha_0 \Pi(\theta, k'^2/e^2, k'),
\end{align}
\f]
where \f$ k' = k/\sqrt{1 + k^2} \f$.  The expression for \f$ s \f$
can be written in terms of elliptic integrals of the second kind and
Cayley's technique can be used to subtract out the leading order
behavior of \f$ \lambda \f$ to give
\f[
\begin{align}
\frac sb &=\frac1{\sqrt{1-k'^2}}
  \biggl( E(\theta, k') -
    \frac{k'^2 \sin\theta \cos\theta}{\sqrt{1-k'^2\sin^2\theta}} \biggr), \\
\lambda &= \psi + (1-f) \sqrt{1-k'^2} \sin\alpha_0
\bigl( F(\theta, k') - \Pi(\theta, e^2, k') \bigr),
\end{align}
\f]
where
\f[
\begin{align}
\tan\psi &= \sqrt{\frac{1+k^2\sin^2\sigma}{1+e'^2}}\tan\omega \\
         &= \sqrt{\frac{1-e^2}{1+k^2\cos^2\theta}}\sin\alpha_0\tan\theta.
\end{align}
\f]
The tangents of the three "longitude-like" angles are in geometric
progression, \f$ \tan\chi/\tan\omega = \tan\omega/\tan\psi \f$.

\section meridian Parameters for the meridian

The formulas for \f$ s \f$ given in the previous section are the same as
those for the distance along a meridian for an ellipsoid with equatorial
radius \f$ a \sqrt{1 - e^2 \sin^2\alpha_0} \f$ and polar semi-axis \f$ b
\f$.  Here is a list of possible ways of expressing the meridian
distance in terms of elliptic integrals using the notation:
- \f$ a \f$, equatorial axis,
- \f$ b \f$, polar axis,
- \f$ e = \sqrt{(a^2 - b^2)/a^2} \f$, eccentricity,
- \f$ e' = \sqrt{(a^2 - b^2)/b^2} \f$, second eccentricity,
- \f$ \phi = \mathrm{am}(u, e) \f$, the geographic latitude,
- \f$ \phi' = \mathrm{am}(v', ie') = \pi/2 - \phi \f$,
  the geographic colatitude,
- \f$ \beta = \mathrm{am}(v, ie') \f$, the parametric latitude
  (\f$ \tan^2\beta = (1 - e^2) \tan^2\phi \f$),
- \f$ \beta' = \mathrm{am}(u', e) = \pi/2 - \beta \f$,
  the parametric colatitude,
- \f$ M \f$, the length of a quarter meridian (equator to pole),
- \f$ y \f$, the distance along the meridian (measured from the equator).
- \f$ y' = M -y \f$, the distance along the meridian (measured from the pole).
.
The eccentricities \f$ (e, e') \f$ are real (resp. imaginary) for
oblate (resp. prolate) ellipsoids.  The elliptic variables \f$(u,
u')\f$ and \f$(v, v')\f$ are defined by
- \f$ u = F(\phi, e) ,\quad u' = F(\beta', e) \f$
- \f$ v = F(\beta, ie') ,\quad v' = F(\phi', ie') \f$,
.
and are linearly related by
- \f$ u + u' = K(e) ,\quad v + v' = K(ie') \f$
- \f$ v = \sqrt{1-e^2} u ,\quad u = \sqrt{1+e'^2} v \f$.
.
The cartesian coordinates for the meridian \f$ (x, z) \f$ are given by
\f[
\begin{align}
  x &= a \cos\beta = a \cos\phi / \sqrt{1 - e^2 \sin^2\phi} \\
    &= a \sin\beta' = (a^2/b) \sin\phi' / \sqrt{1 + e'^2 \sin^2\phi'} \\
    &= a \,\mathrm{cn}(v, ie) = a \,\mathrm{cd}(u, e) \\
    &= a \,\mathrm{sn}(u', e) = (a^2/b) \,\mathrm{sd}(v', ie'),
\end{align}
\f]
\f[
\begin{align}
  z &= b \sin\beta = (b^2/a) \sin\phi / \sqrt{1 - e^2 \sin^2\phi} \\
    &= b \cos\beta' = b \cos\phi' / \sqrt{1 + e'^2 \sin^2\phi'} \\
    &= b \,\mathrm{sn}(v, ie) = (b^2/a) \,\mathrm{sd}(u, e)  \\
    &= b \,\mathrm{cn}(u', e) = b \,\mathrm{cd}(v', ie').
\end{align}
\f]
The distance along the meridian can be expressed variously as
\f[
\begin{align}
  y  &=   b   \int \sqrt{1 + e'^2  \sin^2\beta}\, d\beta
     =   b E(\beta, ie') \\
     &= \frac{b^2}a \int \frac1{(1 - e^2 \sin^2\phi)^{3/2}}\, d\phi
      = \frac{b^2}a \Pi(\phi, e^2, e) \\
     &=   a \biggl(E(\phi, e) -
        \frac{e^2\sin\phi \cos\phi}{\sqrt{1 - e^2\sin^2\phi}}\biggr) \\
     &=  b \int \mathrm{dn}^2(v, ie')\, dv
      = \frac{b^2}a \int \mathrm{nd}^2(u, e)\, du
      = \cal E(v, ie'),
\end{align}
\f]
\f[
\begin{align}
  y' &=   a   \int \sqrt{1 - e^2  \sin^2\beta'}\, d\beta'
     =   a E(\beta', e) \\
     &= \frac{a^2}b \int \frac1{(1 + e'^2 \sin^2\phi')^{3/2}}\, d\phi'
      = \frac{a^2}b \Pi(\phi', -e'^2, ie') \\
     &=  b \biggl(E(\phi', ie') +
        \frac{e'^2\sin\phi' \cos\phi'}{\sqrt{1 + e'^2\sin^2\phi'}}\biggr) \\
     &=  a \int \mathrm{dn}^2(u', e)\, du'
      = \frac{a^2}b \int \mathrm{nd}^2(v', ie')\, dv'
      = \cal E(u', e),
\end{align}
\f]
with the quarter meridian distance given by
\f[
  M  = aE(e) = bE(ie') = (b^2/a)\Pi(e^2,e) = (a^2/b)\Pi(-e'^2,ie').
\f]
(Here \f$ E, F, \Pi \f$ are elliptic integrals defined in
https://dlmf.nist.gov/19.2.ii.  \f$ \cal E, \mathrm{am}, \mathrm{sn},
\mathrm{cn}, \mathrm{sd}, \mathrm{cd}, \mathrm{dn}, \mathrm{nd} \f$ are
Jacobi elliptic functions defined in https://dlmf.nist.gov/22.2 and
https://dlmf.nist.gov/22.16.)

There are several considerations in the choice of independent variable
for evaluate the meridian distance
- The use of an imaginary modulus (namely, \f$ ie' \f$, above) is of no
  practical concern.  The integrals are real in this case and modern
  methods (GeographicLib uses the method given in
  https://dlmf.nist.gov/19.36.i) for computing integrals handles this
  case using just real arithmetic.
- If the "natural" origin is the equator, choose one of \f$ \phi, \beta,
  u, v \f$ (this might be preferred in geodesy).  If it's the pole,
  choose one of the complementary quantities \f$ \phi', \beta', u', v'
  \f$ (this might be preferred by mathematicians).
- Applying these formulas to the geodesic problems, \f$ \beta \f$
  becomes the arc length, \f$ \sigma \f$, on the auxiliary sphere.  This
  is the traditional method of solution used by Legendre (1806), Oriani
  (1806), Bessel (1825), Helmert (1880), Rainsford (1955), Thomas
  (1970), Vincenty (1975), Rapp (1993), and so on.  Many of the
  solutions in terms of elliptic functions use one of the elliptic
  variables (\f$ u \f$ or \f$ v \f$), see, for example, Jacobi (1855),
  Halphen (1888), Forsyth (1896).  In the context of geodesics \f$
  \phi \f$ becomes Thomas' variable \f$ \theta \f$; this is used by
  Thomas (1952) and Rollins (2010) in their formulation of the
  geodesic problem (see the previous section).
- For highly eccentric ellipsoids the variation of the meridian with
  respect to \f$ \beta \f$ is considerably "better behaved" than other
  choices (see the figure below).  The choice of \f$ \phi \f$ is
  probably a poor one in this case.
.
GeographicLib uses the geodesic generalization of
\f$ y = b E(\beta, ie') \f$, namely \f$ s = b E(\sigma, ik) \f$.  See
\ref geodellip.

\image html meridian-measures.png "Comparison of meridian measures"

\section geodshort Short geodesics

Here we describe Bowring's method for solving the inverse geodesic
problem in the limit of short geodesics and contrast it with the great
circle solution using Bessel's auxiliary sphere.  References:
 - B. R. Bowring, The Direct and Inverse Problems for Short Geodesic
   Lines on the Ellipsoid, Surveying and Mapping 41(2), 135--141 (1981).
 - R. H. Rapp,
   <a href="https://hdl.handle.net/1811/24333">
   Geometric Geodesy, Part I</a>, Ohio State Univ. (1991), Sec. 6.5.

Bowring considers the conformal mapping of the ellipsoid to a sphere of
radius \f$ R \f$ such that circles of latitude and meridians are
preserved (and hence the azimuth of a line is preserved).  Let \f$
(\phi, \lambda) \f$ and \f$ (\phi', \lambda') \f$ be the latitude and
longitude on the ellipsoid and sphere respectively.  Define isometric
latitudes for the sphere and the ellipsoid as
\f[
\begin{align}
  \psi' &= \sinh^{-1} \tan \phi', \\
  \psi &= \sinh^{-1} \tan \phi - e \tanh^{-1}(e \sin\phi).
\end{align}
\f]
The most general conformal mapping satisfying Bowring's conditions is
\f[
\psi' = A \psi + K, \quad \lambda' = A \lambda,
\f]
where \f$ A \f$ and \f$ K \f$ are constants.  (In fact a constant can be
added to the equation for \f$ \lambda' \f$, but this does affect the
analysis.)  The scale of this mapping is
\f[
m(\phi) = \frac{AR}{\nu}\frac{\cos\phi'}{\cos\phi},
\f]
where \f$ \nu = a/\sqrt{1 - e^2\sin^2\phi} \f$ is the transverse radius
of curvature.  (Note that in Bowring's Eq. (10), \f$ \phi \f$ should be
replaced by \f$ \phi' \f$.)  The mapping from the ellipsoid to the sphere
depends on three parameters \f$ R, A, K \f$.  These will be selected to
satisfy certain conditions at some representative latitude \f$ \phi_0
\f$.  Two possible choices are given below.

\subsection bowring Bowring's method

Bowring (1981) requires that
\f[
m(\phi_0) = 1,\quad
\left.\frac{dm(\phi)}{d\phi}\right|_{\phi=\phi_0} = 0,\quad
\left.\frac{d^2m(\phi)}{d\phi^2}\right|_{\phi=\phi_0} = 0,
\f]
i.e, \f$m\approx 1\f$ in the vicinity of \f$\phi = \phi_0\f$.
This gives
\f[
\begin{align}
R &= \frac{\sqrt{1 + e'^2}}{B^2} a, \\
A &= \sqrt{1 + e'^2 \cos^4\phi_0}, \\
\tan\phi'_0 &= \frac1B \tan\phi_0,
\end{align}
\f]
where \f$ e' = e/\sqrt{1-e^2} \f$ is the second eccentricity, \f$ B =
\sqrt{1+e'^2\cos^2\phi_0} \f$, and \f$ K \f$ is defined implicitly by
the equation for \f$\phi'_0\f$.  The radius \f$ R \f$ is the (Gaussian)
mean radius of curvature of the ellipsoid at \f$\phi_0\f$ (so near
\f$\phi_0\f$ the ellipsoid can be deformed to fit the sphere snugly).
The third derivative of \f$ m \f$ is given by
\f[
\left.\frac{d^3m(\phi)}{d\phi^3}\right|_{\phi=\phi_0} =
\frac{-2e'^2\sin2\phi_0}{B^4}.
\f]

The method for solving the inverse problem between two nearby points \f$
(\phi_1, \lambda_1) \f$ and \f$ (\phi_2, \lambda_2) \f$ is as follows:
Set \f$\phi_0 = (\phi_1 + \phi_2)/2\f$.  Compute \f$ R, A, \phi'_0 \f$,
and hence find \f$ (\phi'_1, \lambda'_1) \f$ and \f$ (\phi'_2,
\lambda'_2) \f$.  Finally, solve for the great circle on a sphere of
radius \f$ R \f$; the resulting distance and azimuths are good
approximations for the corresponding quantities for the ellipsoidal
geodesic.

Consistent with the accuracy of this method, we can compute
\f$\phi'_1\f$ and \f$\phi'_2\f$ using a Taylor expansion about
\f$\phi_0\f$.  This also avoids numerical errors that arise from
subtracting nearly equal quantities when using the equation for
\f$\phi'\f$ directly.  Write \f$\Delta \phi = \phi - \phi_0\f$ and
\f$\Delta \phi' = \phi' - \phi'_0\f$; then we have
\f[
\Delta\phi' \approx
\frac{\Delta\phi}B \biggl[1 +
\frac{\Delta\phi}{B^2}\frac{e'^2}2
  \biggl(3\sin\phi_0\cos\phi_0 +
   \frac{\Delta\phi}{B^2}
   \bigl(B^2 - \sin^2\phi_0(2 - 3 e'^2 \cos^2\phi_0)\bigr)\biggr)\biggr],
\f]
where the error is \f$O(f\Delta\phi^4)\f$.
This is essentially Bowring's method.  Significant differences between
this result, "Bowring (improved)", compared to Bowring's paper, "Bowring
(original)", are:
 - Bowring elects to use \f$\phi_0 = \phi_1\f$.  This simplifies the
   calculations somewhat but increases the error by about a factor of
   4.
 - Bowring's expression for \f$ \Delta\phi' \f$ is only accurate in the
   limit \f$ e' \rightarrow 0 \f$.
 .
In fact, arguably, the highest order \f$O(f\Delta\phi^3)\f$ terms should
be dropped altogether.  Their inclusion does result in a better estimate
for the distance.  However, if your goal is to generate both accurate
distances \e and accurate azimuths, then \f$\Delta\phi\f$ needs to be
restricted sufficiently to allow these terms to be dropped to give the
"Bowring (truncated)" method.

With highly eccentric ellipsoids, the parametric latitude \f$ \beta \f$
is a better behaved independent variable to use.  In this case, \f$
\phi_0 \f$ is naturally defined using \f$\beta_0 = (\beta_1 +
\beta_2)/2\f$ and in terms of \f$\Delta\beta = \beta - \beta_0\f$, we
have
\f[
\Delta\phi' \approx
\frac{\Delta\beta}{B'} \biggl[1 +
\frac{\Delta\beta}{B'^2}\frac{e'^2}2
  \biggl(\sin\beta_0\cos\beta_0 +
   \frac{\Delta\beta}{3B'^2}
    \bigl( \cos^2\beta_0 - \sin^2\beta_0 B'^2\bigr)
\biggr)\biggr],
\f]
where \f$B' = \sqrt{1+e'^2\sin^2\beta_0} = \sqrt{1+e'^2}/B\f$, and the
error once again is \f$O(f\Delta\phi^4)\f$.  This is the &quot;Bowring
(using \f$\beta\f$)&quot; method.

\subsection auxsphere Bessel's auxiliary sphere

GeographicLib's uses the auxiliary sphere method of Legendre, Bessel,
and Helmert.  For short geodesics, this is equivalent to picking
\f$ R, A, K \f$ so that
\f[
m(\phi_0) = 1,\quad
\left.\frac{dm(\phi)}{d\phi}\right|_{\phi=\phi_0} = 0,\quad
\tan\phi'_0 = (1 - f) \tan\phi_0.
\f]
Bowring's requirement that the second derivative of \f$m\f$ vanish has
been replaced by the last relation which states that \f$\phi'_0 =
\beta_0\f$, the parametric latitude corresponding to \f$\phi_0\f$.  This
gives
\f[
\begin{align}
R &= B'(1-f)a, \\
A &= \frac1{B'(1-f)}, \\
\left.\frac{d^2m(\phi)}{d\phi^2}\right|_{\phi=\phi_0} &=
-e^2B'^2\sin^2\phi_0.
\end{align}
\f]

Similar to Bowring's method, we can compute \f$\phi'_1\f$ and
\f$\phi'_2\f$ using a Taylor expansion about \f$\beta_0\f$.  This results
in the simple expression
\f[
\Delta\phi' \approx \Delta\beta,
\f]
where the error is \f$O(f\Delta\beta^2)\f$.

\subsection shorterr Estimating the accuracy

In assessing the accuracy of these methods we use two metrics:
 - The absolute error in the distance.
 - The consistency of the predicted azimuths.  Imagine starting
   ellipsoidal geodesics at \f$ (\phi_1, \lambda_1) \f$ and \f$ (\phi_2,
   \lambda_2) \f$ with the predicted azimuths.  What is the distance
   between them when they are extended a distance \f$ a \f$ beyond the
   second point?
 .
(The second metric is much more stringent.)  We may now compare the
methods by asking for a bound to the length of a geodesic which ensures
that the one or other of the errors fall below 1&nbsp;mm (an "engineering"
definition of accurate) or 1&nbsp;nm (1 nanometer, about the round-off
limit).

<center>
<table>
<caption>Maximum distance that can be used in various methods for
computing short geodesics while keeping the errors within prescribed
bounds</caption>
<tr>
 <th rowspan="2">method
 <th colspan="2"><center>distance metric</center></th>
 <th colspan="2"><center>azimuth metric</center></th>
<tr>
 <th>1 mm error</th>
 <th>1 nm error</th>
 <th>1 mm error</th>
 <th>1 nm error</th>
<tr>
 <td>Bowring (original)
 <td><center>87 km</center>
 <td><center>870 m</center>
 <td><center>35 km</center>
 <td><center>350 m</center>
<tr>
 <td>Bowring (improved)
 <td><center>180 km</center>
 <td><center>1.8 km</center>
 <td><center>58 km</center>
 <td><center>580 m</center>
<tr>
 <td>Bowring (truncated)
 <td><center>52 km</center>
 <td><center>520 m</center>
 <td><center>52 km</center>
 <td><center>520 m</center>
<tr>
 <td>Bowring (using \f$\beta\f$)
 <td><center>380 km</center>
 <td><center>24 km</center>
 <td><center>60 km</center>
 <td><center>600 m</center>
<tr>
 <td>Bessel's aux. sphere
 <td><center>42 km</center>
 <td><center>420 m</center>
 <td><center>1.7 km</center>
 <td><center>1.7 m</center>
</table>
</center>

For example, if you're only interested in measuring distances and an
accuracy of 1&nbsp;mm is sufficient, then Bowring's improved method can
be used for distances up to 180&nbsp;km.  On the other hand,
GeographicLib uses Bessel's auxiliary sphere and we require both the
distance and the azimuth to be accurate, so the great circle
approximation can only be used for distances less than 1.7&nbsp;m.  The
reason that GeographicLib does not use Bowring's method is that the
information necessary for auxiliary sphere method is already available
as part of the general solution and, as much as possible, we allow all
geodesics to be computed by the general method.

<center>
Back to \ref magnetic.  Forward to \ref nearest.  Up to \ref contents.
</center>

**********************************************************************/
/**
\page nearest Finding nearest neighbors

<center>
Back to \ref geodesic.  Forward to \ref  triaxial.  Up to \ref contents.
</center>

The problem of finding the maritime boundary defined by the "median
line" is discussed in Section 14 of
 - C. F. F. Karney,
   <a href="https://arxiv.org/abs/1102.1215v1">Geodesics
   on an ellipsoid of revolution</a>,
   Feb. 2011; preprint
   <a href="https://arxiv.org/abs/1102.1215v1">arxiv:1102.1215v1</a>.
 .
Figure 14 shows the median line which is equidistant from Britain and
mainland Europe.  Determining the median line involves finding, for any
given \e P, the closest points on the coast of Britain and on the coast
of mainland Europe.  The operation of finding the closest in a set of
points is usually referred to as the <i>nearest neighbor</i> problem and
the NearestNeighbor class implements an efficient algorithm for solving
it.

The NearestNeighbor class implements nearest-neighbor calculations using
the vantage-point tree described by
- J. K. Uhlmann,
  <a href="https://doi.org/10.1016/0020-0190(91)90074-r">
  Satisfying general proximity/similarity queries with metric trees</a>,
  Information Processing Letters 40 175--179 (1991).
- P. N. Yianilos,
  <a href="https://dl.acm.org/doi/10.5555/313559.313789">
  Data structures and algorithms for nearest neighbor search in general
  metric spaces</a>, Proc. 4th ACM-SIAM Symposium on Discrete Algorithms,
  (SIAM, 1993). pp. 311--321.

Given a set of points \e x, \e y, \e z, &hellip;, in some space and a
distance function \e d satisfying the metric conditions,
\f[
\begin{align}
 d(x,y) &\ge 0,\\
 d(x,y) &= 0, \ \text{iff $x = y$},\\
 d(x,y) &= d(y,x),\\
 d(x,z) &\le d(x,y) + d(y,z),
\end{align}
\f]
the vantage-point (VP) tree provides an efficient way of determining
nearest neighbors.  The geodesic distance (implemented by the Geodesic
class) satisfies these metric conditions, while the great ellipse
distance and the rhumb line distance <i>do not</i> (they do not satisfy
the last condition, the triangle inequality).  Typically the cost of
constructing a VP tree of \e N points is \e N log \e N, while the cost
of a query is log \e N.  Thus a VP tree should be used in situations
where \e N is large and at least log \e N queries are to be made.  The
condition, \e N is large, should be taken to mean that \f$ N \gg 2^D
\f$, where \e D is the dimensionality of the space.

- This implementation includes Yianilos' upper and lower bounds for the
  inside and outside sets.  This helps limit the number of searches
  (compared to just using the median distance).
- Rather than do a depth-first or breath-first search on the tree, the
  nodes to be processed are put on a priority queue with the nodes most
  likely to contain close points processed first.  Frequently, this allows
  nodes lower down on the priority queue to be skipped.
- This technique also allows non-exhaustive searchs to be performed (to
  answer questions such as "are there any points within 1km of the query
  point?).
- When building the tree, the first vantage point is (arbitrarily)
  chosen as the middle element of the set.  Thereafter, the points
  furthest from the parent vantage point in both the inside and outside
  sets are selected as the children's vantage points.  This information
  is already available from the computation of the upper and lower
  bounds of the children.  This choice seems to lead to a reasonably
  optimized tree.
- The leaf nodes can contain a bucket of points (instead of just a vantage
  point).
- Coincident points are allowed in the set; these are treated as distinct
  points.

The figure below shows the construction of the VP tree for the points
making up the coastlines of Britain and Ireland (about 5000 points shown
in blue).  The set of points is recursively split into 2 equal "inside"
and "outside" subsets based on the distance from a "vantage point".  The
boundaries between the inside and outside sets are shown as green
circular arcs (arcs of geodesic circles).  At each stage, the newly
added vantage points are shown as red dots and the vantage points for
the next stage are shown as red plus signs.  The data is shown in the
Cassini-Soldner projection with a central meridian of 5&deg;W.

\image html vptree.gif "Vantage-point tree"

<center>
Back to \ref geodesic.  Forward to \ref triaxial.  Up to \ref contents.
</center>

**********************************************************************/
/**
\page triaxial Geodesics on a triaxial ellipsoid

<center>
Back to \ref nearest.  Forward to \ref jacobi.  Up to \ref contents.
</center>

Jacobi (1839) showed that the problem of geodesics on a triaxial
ellipsoid (with 3 unequal axes) can be reduced to quadrature.  Despite
this, the detailed behavior of the geodesics is not very well known.  In
this section, I briefly give Jacobi's solution and illustrate the
behavior of the geodesics and outline an algorithm for the solution of
the inverse problem.

See also
 - The wikipedia page,
   <a href="https://en.wikipedia.org/wiki/Geodesics_on_an_ellipsoid#Geodesics_on_a_triaxial_ellipsoid">
   Geodesics on  a triaxial ellipsoid</a>.

Go to
 - \ref triaxial-coords
 - \ref triaxial-jacobi
 - \ref triaxial-survey
 - \ref triaxial-stab
 - \ref triaxial-inverse

<b>NOTES</b>
 -# A triaxial ellipsoid approximates the earth only slightly better
    than an ellipsoid of revolution.  If you are really considering
    measuring distances on the earth using a triaxial ellipsoid, you
    should also be worrying about the shape of the geoid, which
    essentially makes the geodesic problem a hopeless mess; see, for
    example, <a href="https://arxiv.org/abs/1112.3231"> Waters
    (2011)</a>.
 -# There is nothing new in this section.  It is just an exercise in
    exploring Jacobi's solution.  My interest here is in generating long
    geodesics with the correct long-time behavior.  Arnold gives a
    nice qualitative description of the solution in <i>Mathematical
    Methods of Classical Mechanics</i> (2nd edition, Springer, 1989),
    pp. 264--266.
 -# Possible reasons this problem might, nevertheless, be of interest
    are:
    - It is the first example of a dynamical system which has a
      non-trivial constant of motion.  As such, Jacobi's paper generated
      a lot of excitement and was followed by many papers elaborating
      his solution.  In particular, the unstable behavior of one of the
      closed geodesics of the ellipsoid, is an early example of a system
      with a positive Lyapunov exponent (one of the essential
      ingredients for chaotic behavior in dynamical systems).
    - Knowledge of ellipsoidal coordinates (used by Jacobi) might be
      useful in other areas of geodesy.
    - Geodesics which pass through the pole on an ellipsoid of revolution
      represent a degenerate class (they are all closed and all pass
      through the opposite pole).  It is of interest to see how this
      degeneracy is broken with a surface with a more general shape.
    - Similarly, it is of interest to see how the Mercator projection of
      the ellipsoid generalizes; this is another problem addressed by
      Jacobi.
 -# My interest in this problem was piqued by Jean-Marc Baillard.  I put
    him onto Jacobi's solution without having looked at it in detail
    myself; and he quickly implemented the solution for an HP-41
    calculator(!) which is posted
    <a href="http://hp41programs.yolasite.com/geod3axial.php"> here</a>.
 -# I do not give full citations of the papers here.  You can find these
    in the
    <a href="https://geographiclib.sourceforge.io/geodesic-papers/biblio.html">
    Geodesic Bibliography</a>; this includes links to online
    versions of the papers.
 -# An alternative to exploring geodesics using Jacobi's solution is to
    integrate the equations for the geodesics directly.  This is the
    approach taken by
    <a href="http://www.math.harvard.edu/~knill/caustic/">
    Oliver Knill and Michael Teodorescu</a>.  However it is difficult to
    ensure that the long time behavior is correctly modeled with such an
    approach.
 -# At this point, I have no plans to add the solution of triaxial
    geodesic problem to GeographicLib.
 -# If you only want to learn about geodesics on a biaxial ellipsoid (an
    ellipsoid of revolution), then see \ref geodesic or the paper
    - C. F. F. Karney,
      <a href="https://doi.org/10.1007/s00190-012-0578-z">
      Algorithms for geodesics</a>,
      J. Geodesy 87(1), 43--55 (2013);
      DOI: <a href="https://doi.org/10.1007/s00190-012-0578-z">
      10.1007/s00190-012-0578-z</a>;
      addenda: <a href="https://geographiclib.sourceforge.io/geod-addenda.html">
      geod-addenda.html</a>.

\section triaxial-coords Triaxial coordinate systems

Consider the ellipsoid defined by
\f[
  f = \frac{X^2}{a^2} + \frac{Y^2}{b^2} + \frac{Z^2}{c^2} = 1,
\f]
where, without loss of generality, \f$ a \ge b \ge c \gt 0\f$.  A
point on the surface is specified by a latitude and longitude.  The \e
geographical latitude and longitude \f$(\phi, \lambda)\f$ are defined by
\f[
 \frac{\nabla f}{\left| \nabla f\right|} = \left(
\begin{array}{c} \cos\phi \cos\lambda \\ \cos\phi \sin\lambda \\ \sin\phi
\end{array}\right).
\f]
The \e parametric latitude and longitude \f$(\phi', \lambda')\f$ are
defined by
\f[
\begin{align}
 X &= a \cos\phi' \cos\lambda', \\
 Y &= b \cos\phi' \sin\lambda', \\
 Z &= c \sin\phi'.
\end{align}
\f]
Jacobi employed the \e ellipsoidal latitude and longitude \f$(\beta,
\omega)\f$ defined by
\f[
\begin{align}
  X &= a \cos\omega
      \frac{\sqrt{a^2 - b^2\sin^2\beta - c^2\cos^2\beta}}
           {\sqrt{a^2 - c^2}}, \\
  Y &= b \cos\beta \sin\omega, \\
  Z &= c \sin\beta
      \frac{\sqrt{a^2\sin^2\omega + b^2\cos^2\omega - c^2}}
           {\sqrt{a^2 - c^2}}.
\end{align}
\f]
Grid lines of constant \f$\beta\f$ and \f$\omega\f$ are given in Fig. 1.

<center>
<img src="https://upload.wikimedia.org/wikipedia/commons/b/bd/Triaxial_ellipsoid_coordinate_system.svg"
width=419 height=397 alt="Geodesic grid on a triaxial ellipsoid">
Fig. 1
</center>\n
Fig. 1:
The ellipsoidal grid.  The blue (resp. green) lines are lines of constant
\f$\beta\f$ (resp. \f$\omega\f$); the grid spacing is 10&deg;.  Also
shown in red are two of the principal sections of the ellipsoid, defined
by \f$x = 0\f$ and \f$z = 0\f$.  The third principal section, \f$y =
0\f$, is covered by the lines \f$\beta = \pm 90^\circ\f$ and \f$\omega =
90^\circ \pm 90^\circ\f$.  These lines meet at four umbilical points (two
of which are visible in this figure) where the principal radii of
curvature are equal.  The parameters of the ellipsoid are \f$a =
1.01\f$, \f$b = 1\f$, \f$c = 0.8\f$, and it is viewed in an orthographic
projection from a point above \f$\phi = 40^\circ\f$, \f$\lambda =
30^\circ\f$.  These parameters were chosen to accentuate the ellipsoidal
effects on geodesics (relative to those on the earth) while still
allowing the connection to an ellipsoid of revolution to be made.

The grid lines of the ellipsoid coordinates are "lines of curvature" on
the ellipsoid, i.e., they are parallel to the direction of principal
curvature (Monge, 1796).  They are also intersections of the ellipsoid
with confocal systems of hyperboloids of one and two sheets (Dupin,
1813).  Finally they are geodesic ellipses and hyperbolas defined using
two adjacent umbilical points.  For example, the lines of constant
\f$\beta\f$ in Fig. 1 can be generated with the familiar string
construction for ellipses with the ends of the string pinned to the two
umbilical points.

The element of length on the ellipsoid in ellipsoidal coordinates is
given by
\f[
\begin{align}
\frac{ds^2}{(a^2-b^2)\sin^2\omega+(b^2-c^2)\cos^2\beta} &=
\frac{b^2\sin^2\beta+c^2\cos^2\beta}
     {a^2-b^2\sin^2\beta-c^2\cos^2\beta}
 d\beta^2 \\
&\qquad+
\frac{a^2\sin^2\omega+b^2\cos^2\omega}
     {a^2\sin^2\omega+b^2\cos^2\omega-c^2}
 d\omega^2.
\end{align}
\f]

The torus \f$(\omega, \beta) \in [-\pi,\pi] \times [-\pi,\pi]\f$ covers
the ellipsoid twice.  In order to facilitate passing to the limit of an
oblate ellipsoid, we may regard as the principal sheet \f$[-\pi,\pi]
\times [-\frac12\pi,\frac12\pi]\f$ and insert branch cuts at
\f$\beta=\pm\frac12\pi\f$.  The rule for switching sheets is
\f[
\begin{align}
\omega & \rightarrow -\omega,\\
\beta & \rightarrow \pi-\beta,\\
\alpha & \rightarrow \pi+\alpha,
\end{align}
\f]
where \f$\alpha\f$ is the heading of a path, relative to a line of
constant \f$\omega\f$.

In the limit \f$b\rightarrow a\f$ (resp. \f$b\rightarrow c\f$), the
umbilic points converge on the \f$z\f$ (resp. \f$x\f$) axis and an
oblate (resp. prolate) ellipsoid is obtained with \f$\beta\f$
(resp. \f$\omega\f$) becoming the standard parametric latitude and
\f$\omega\f$ (resp. \f$\beta\f$) becoming the standard longitude.  The
sphere is a non-uniform limit, with the position of the umbilic points
depending on the ratio \f$(a-b)/(b-c)\f$.

Inter-conversions between the three different latitudes and longitudes
and the cartesian coordinates are simple algebraic exercises.

\section triaxial-jacobi Jacobi's solution

Solving the geodesic problem for an ellipsoid of revolution is, from the
mathematical point of view, trivial; because of symmetry, geodesics have
a constant of the motion (analogous to the angular momentum) which was
found by Clairaut (1733).  By 1806 (with the work of Legendre, Oriani,
et al.), there was a complete understanding of the qualitative behavior
of geodesics on an ellipsoid of revolution.

On the other hand, geodesics on a triaxial ellipsoid have no obvious
constant of the motion and thus represented a challenging "unsolved"
problem in the first half of the nineteenth century.  Jacobi discovered
that the geodesic equations are separable if they are expressed in
ellipsoidal coordinates.  You can get an idea of the importance Jacobi
attached to his discovery from the
<a href="https://books.google.com/books?id=_09tAAAAMAAJ&pg=PA385">
letter</a> he wrote to his friend and neighbor Bessel:
<blockquote> The day before yesterday, I reduced to quadrature the
problem of geodesic lines on an <i>ellipsoid with three unequal
axes</i>.  They are the simplest formulas in the world, Abelian
integrals, which become the well known elliptic integrals if 2 axes are
set equal.\n
K&ouml;nigsberg, 28th Dec. '38.
</blockquote>

On the same day he wrote a similar letter to the editor of Compte Rendus
and his result was published in J. Crelle in (1839) with a French
translation (from German) appearing in J. Liouville in (1841).

Here is the solution, exactly as given by Jacobi
<a href="https://books.google.com/books?id=Rh8GAAAAYAAJ&pg=PA268"> here</a>
(with minor changes in notation):
\f[
\begin{align}
\delta &= \int \frac
{\sqrt{b^2\sin^2\beta + c^2\cos^2\beta}\,d\beta}
{\sqrt{a^2 - b^2\sin^2\beta - c^2\cos^2\beta}
 \sqrt{(b^2-c^2)\cos^2\beta - \gamma}}\\
&\quad -
\int \frac
{\sqrt{a^2\sin^2\omega + b^2\cos^2\omega}\,d\omega}
{\sqrt{a^2\sin^2\omega + b^2\cos^2\omega - c^2}
 \sqrt{(a^2-b^2)\sin^2\omega + \gamma}}
\end{align}
\f]
As Jacobi notes &quot;a function of the angle \f$\beta\f$ equals a
function of the angle \f$\omega\f$.  These two functions are just
Abelian integrals&hellip;&quot; Two constants \f$\delta\f$ and \f$\gamma\f$
appear in the solution.  Typically \f$\delta\f$ is zero if the lower
limits of the integrals are taken to be the starting point of the geodesic
and the direction of the geodesics is determined by \f$\gamma\f$.
However for geodesics that start at an umbilical points, we have \f$\gamma
= 0\f$ and \f$\delta\f$ determines the direction at the umbilical point.
Incidentally the constant \f$\gamma\f$ may be expressed as
\f[
\gamma = (b^2-c^2)\cos^2\beta\sin^2\alpha-(a^2-b^2)\sin^2\omega\cos^2\alpha
\f]
where \f$\alpha\f$ is the angle the geodesic makes with lines of
constant \f$\omega\f$. In the limit \f$b\rightarrow a\f$, this reduces
to \f$\cos\beta\sin\alpha = \text{const.}\f$, the familiar Clairaut
relation.  A nice derivation of Jacobi's result is given by Darboux
(1894) <a href="https://books.google.com/books?id=hGMSAAAAIAAJ&pg=PA9">
&sect;&sect;583--584</a> where he gives the solution found by Liouville
(1846) for general quadratic surfaces.  In this formulation, the
distance along the geodesic, \f$s\f$, is also found using
\f[
\begin{align}
\frac{ds}{(b^2-c^2)\cos^2\beta + (a^2-b^2)\sin^2\omega}
&= \frac
{\sqrt{b^2\sin^2\beta + c^2\cos^2\beta}\,d\beta}
{\sqrt{a^2 - b^2\sin^2\beta - c^2\cos^2\beta}
 \sqrt{(b^2-c^2)\cos^2\beta - \gamma}}\\
&= \frac
{\sqrt{a^2\sin^2\omega + b^2\cos^2\omega}\,d\omega}
{\sqrt{a^2\sin^2\omega + b^2\cos^2\omega - c^2}
 \sqrt{(a^2-b^2)\sin^2\omega + \gamma}}
\end{align}
\f]
An alternative expression for the distance is
\f[
\begin{align}
ds
&= \frac
{\sqrt{b^2\sin^2\beta + c^2\cos^2\beta}
 \sqrt{(b^2-c^2)\cos^2\beta - \gamma}\,d\beta}
{\sqrt{a^2 - b^2\sin^2\beta - c^2\cos^2\beta}}\\
&\quad {}+ \frac
{\sqrt{a^2\sin^2\omega + b^2\cos^2\omega}
 \sqrt{(a^2-b^2)\sin^2\omega + \gamma}\,d\omega}
{\sqrt{a^2\sin^2\omega + b^2\cos^2\omega - c^2}}
\end{align}
\f]

Jacobi's solution is a convenient way to compute geodesics on an
ellipsoid.  Care must be taken with the signs of the square roots (which
are determined by the initial azimuth of the geodesic).  Also if
\f$\gamma \gt 0\f$ (resp. \f$\gamma \lt 0\f$), then the \f$\beta\f$
(resp. \f$\omega\f$) integrand diverges.  The integrand can be
transformed into a finite one by a change of variable, e.g.,
\f$\sin\beta = \sin\sigma \sqrt{1 - \gamma/(b^2-c^2)}\f$.  The resulting
integrals are periodic, so the behavior of an arbitrarily long geodesic
is entirely captured by tabulating the integrals over a single period.

The situation is more complicated if \f$\gamma = 0\f$ (corresponding to
umbilical geodesics).  Both integrands have simple poles at the umbilical
points.  However, this behavior may be subtracted from the integrands to
yield (for example) the sum of a term involving
\f$\tanh^{-1}\sin\beta\f$ and a finite integral.  Since both integrals
contain similar logarithmic singularities they can be equated (thus
fixing the ratio \f$\cos\beta/\sin\omega\f$ at the umbilical point) and
connection formulas can be found which allow the geodesic to be followed
through the umbilical point.  The study of umbilical geodesics was of
special interest to a group of Irish mathematicians in the 1840's and
1850's, including Michael and William Roberts (twins!), Hart, Graves,
and Salmon.

\section triaxial-survey Survey of triaxial geodesics

Before delving into the nature of geodesics on a triaxial geodesic, it
is worth reviewing geodesics on an ellipsoid of revolution.  There are
two classes of simple closed geodesics (i.e., geodesics which close on
themselves without intersection): the equator and all the meridians.
All other geodesics oscillate between two equal and opposite circles of
latitude; but after completing a full oscillation in latitude these fall
slightly short (for an oblate ellipsoid) of completing a full circuit in
longitude.

Turning to the triaxial case, we find that there are only 3 simple
closed geodesics, the three principal sections of the ellipsoid given by
\f$x = 0\f$, \f$y = 0\f$, and \f$z = 0\f$.  To survey the other
geodesics, it is convenient to consider geodesics which intersect the
middle principal section, \f$y = 0\f$, at right angles.  Such geodesics
are shown in Figs. 2--6, where I use the same ellipsoid parameters as in
Fig. 1 and the same viewing direction.  In addition, the three principal
ellipses are shown in red in each of these figures.

If the starting point is \f$\beta_1 \in (-90^\circ, 90^\circ)\f$,
\f$\omega_1 = 0\f$, and \f$\alpha_1 = 90^\circ\f$, then the geodesic
encircles the ellipsoid in a "circumpolar" sense.  The geodesic
oscillates north and south of the equator; on each oscillation it
completes slightly less that a full circuit around the ellipsoid
resulting in the geodesic filling the area bounded by the two latitude
lines \f$\beta = \pm \beta_1\f$.  Two examples are given in
Figs. 2 and 3.  Figure 2 shows practically the same behavior as for an
oblate ellipsoid of revolution (because \f$a \approx b\f$).  However, if
the starting point is at a higher latitude (Fig. 3) the distortions
resulting from \f$a \ne b\f$ are evident.

<center>
<img src="https://upload.wikimedia.org/wikipedia/commons/a/a9/Circumpolar_geodesic_on_a_triaxial_ellipsoid_case_A.svg"
width=419 height=397 alt="Example of a circumpolar geodesic on a
triaxial ellipsoid">
Fig. 2
</center>\n
Fig. 2:
Example of a circumpolar geodesic on a triaxial ellipsoid.  The starting
point of this geodesic is \f$\beta_1 = 45.1^\circ\f$, \f$\omega_1 =
0^\circ\f$, and \f$\alpha_1 = 90^\circ\f$.

<center>
<img src="https://upload.wikimedia.org/wikipedia/commons/f/f3/Circumpolar_geodesic_on_a_triaxial_ellipsoid_case_B.svg"
width=419 height=397 alt="Another example of a circumpolar geodesic on a
triaxial ellipsoid">
Fig. 3
</center>\n
Fig. 3:
Another example of a circumpolar geodesic on a triaxial ellipsoid.  The
starting point of this geodesic is \f$\beta_1 = 87.48^\circ\f$, \f$\omega_1 =
0^\circ\f$, and \f$\alpha_1 = 90^\circ\f$.

If the starting point is \f$\beta_1 = 90^\circ\f$, \f$\omega_1 \in
(0^\circ, 180^\circ)\f$, and \f$\alpha_1 = 180^\circ\f$, then the geodesic
encircles the ellipsoid in a "transpolar" sense.  The geodesic
oscillates east and west of the ellipse \f$x = 0\f$; on each oscillation
it completes slightly more that a full circuit around the ellipsoid
resulting in the geodesic filling the area bounded by the two longitude
lines \f$\omega = \omega_1\f$ and \f$\omega = 180^\circ - \omega_1\f$.
If \f$a = b\f$, all meridians are geodesics; the effect of \f$a \ne b\f$
causes such geodesics to oscillate east and west.  Two examples are
given in Figs. 4 and 5.

<center>
<img src="https://upload.wikimedia.org/wikipedia/commons/8/83/Transpolar_geodesic_on_a_triaxial_ellipsoid_case_A.svg"
width=419 height=397 alt="Example of a transpolar geodesic on a
triaxial ellipsoid">
Fig. 4
</center>\n
Fig. 4:
Example of a transpolar geodesic on a triaxial ellipsoid.  The
starting point of this geodesic is \f$\beta_1 = 90^\circ\f$, \f$\omega_1 =
39.9^\circ\f$, and \f$\alpha_1 = 180^\circ\f$.

<center>
<img src="https://upload.wikimedia.org/wikipedia/commons/9/9c/Transpolar_geodesic_on_a_triaxial_ellipsoid_case_B.svg"
width=419 height=397 alt="Another example of a transpolar geodesic on a
triaxial ellipsoid">
Fig. 5
</center>\n
Fig. 5:
Another example of a transpolar geodesic on a triaxial ellipsoid.  The
starting point of this geodesic is \f$\beta_1 = 90^\circ\f$, \f$\omega_1 =
9.966^\circ\f$, and \f$\alpha_1 = 180^\circ\f$.

If the starting point is \f$\beta_1 = 90^\circ\f$, \f$\omega_1 =
0^\circ\f$ (an umbilical point), and \f$\alpha_1 = 135^\circ\f$ (the
geodesic leaves the ellipse \f$y = 0\f$ at right angles), then the
geodesic repeatedly intersects the opposite umbilical point and returns to
its starting point.  However on each circuit the angle at which it
intersects \f$y = 0\f$ becomes closer to \f$0^\circ\f$ or
\f$180^\circ\f$ so that asymptotically the geodesic lies on the ellipse
\f$y = 0\f$.  This is shown in Fig. 6.  Note that a single geodesic does
not fill an area on the ellipsoid.

<center>
<img src="https://upload.wikimedia.org/wikipedia/commons/e/e5/Unstable_umbilical_geodesic_on_a_triaxial_ellipsoid.svg"
width=419 height=397 alt="Example of an umbilical geodesic on a
triaxial ellipsoid">
Fig. 6
</center>\n
Fig. 6:
Example of an umbilical geodesic on a triaxial ellipsoid.  The
starting point of this geodesic is \f$\beta_1 = 90^\circ\f$, \f$\omega_1 =
0^\circ\f$, and \f$\alpha_1 = 135^\circ\f$ and the geodesics is followed
forwards and backwards until it lies close to the plane \f$y = 0\f$ in
both directions.

Umbilical geodesics enjoy several interesting properties.
 - Through any point on the ellipsoid, there are two umbilical geodesics.
 - The geodesic distance between opposite umbilical points is the same
   regardless of the initial direction of the geodesic.
 - Whereas the closed geodesics on the ellipses \f$x = 0\f$ and \f$z =
   0\f$ are stable (an geodesic initially close to and nearly parallel to
   the ellipse remains close to the ellipse), the closed geodesic on the
   ellipse \f$y = 0\f$, which goes through all 4 umbilical points, is \e
   unstable.  If it is perturbed, it will swing out of the plane \f$y =
   0\f$ and flip around before returning to close to the plane.  (This
   behavior may repeat depending on the nature of the initial
   perturbation.).

\section triaxial-stab The stability of closed geodesics

The stability of the three simple closed geodesics can be determined by
examining the properties of Jacobi's solution.  In particular the
unstable behavior of umbilical geodesics was shown by Hart (1849).
However an alternative approach is to use the equations that Gauss
(1828) gives for a perturbed geodesic
\f[
\frac {d^2m}{ds^2} + Km = 0
\f]
where \f$m\f$ is the distance of perturbed geodesic from a reference
geodesic and \f$K\f$ is the Gaussian curvature of the surface.  If the
reference geodesic is closed, then this is a linear homogeneous
differential equation with periodic coefficients.  In fact it's a
special case of Hill's equation which can be treated using Floquet
theory, see <a href="https://dlmf.nist.gov/28.29">DLMF, &sect;28.29</a>.
Using the notation of &sect;3 of
<a href="https://doi.org/10.1007/s00190-012-0578-z"> Algorithms for
geodesics</a>, the stability is determined by computing the reduced
length \f$m_{12}\f$ and the geodesic scales \f$M_{12}, M_{21}\f$ over
half the perimeter of the ellipse and determining the eigenvalues
\f$\lambda_{1,2}\f$ of
\f[
{\cal M} = \left(\begin{array}{cc}
M_{12} & m_{12}\\
-\frac{1 - M_{12}M_{21}}{m_{12}} & M_{21}
\end{array}\right).
\f]
Because \f$\mathrm{det}\,{\cal M} = 1\f$, the eigenvalues are determined
by \f$\mathrm{tr}\,{\cal M}\f$.  In particular if
\f$\left|\mathrm{tr}\,{\cal M}\right| < 2\f$, we have
\f$\left|\lambda_{1,2}\right| = 1\f$ and the solution is stable; if
\f$\left|\mathrm{tr}\,{\cal M}\right| > 2\f$, one of
\f$\left|\lambda_{1,2}\right|\f$ is larger than unity and the solution
is (exponentially) unstable.  In the transition case,
\f$\left|\mathrm{tr}\,{\cal M}\right| = 2\f$, the solution is stable
provided that the off-diagonal elements of \f${\cal M}\f$ are zero;
otherwise the solution is linearly unstable.

The exponential instability of the geodesic on the ellipse \f$y = 0\f$
is confirmed by this analysis and results from the resonance between the
natural frequency of the equation for \f$m\f$ and the driving frequency
when \f$b\f$ lies in \f$(c, a)\f$.  If \f$b\f$ is equal to either of the
other axes (and the triaxial ellipsoid degenerates to an ellipsoid of
revolution), then the solution is linearly unstable.  (For example, a
geodesic is which is close to a meridian on an oblate ellipsoid, slowly
moves away from that meridian.)

\section triaxial-inverse The inverse problem

In order to solve the inverse geodesic problem, it helps to have an
understanding of the properties of all the geodesics emanating from a
single point \f$(\beta_1, \omega_1)\f$.
 - If the point is an umbilical point, all the lines meet at the
   opposite umbilical point.
 - Otherwise, the first envelope of the geodesics is a 4-pointed
   astroid.  The cusps of the astroid lie on either \f$\beta = -
   \beta_1\f$ or \f$\omega = \omega_1 + \pi\f$; see
   <a href="https://doi.org/10.1080/10586458.2003.10504515"> Sinclair
   (2003)</a>.
 - All geodesics intersect (or, in the case of \f$\alpha_1 = 0\f$ or
   \f$\pi\f$, touch) the line \f$\omega = \omega_1 + \pi\f$.
 - All geodesics intersect (or, in the case of \f$\alpha_1 =
   \pm\pi/2\f$, touch) the line \f$\beta = -\beta_1\f$.
 - Two geodesics with azimuths \f$\pm\alpha_1\f$ first intersect on
   \f$\omega = \omega_1 + \pi\f$ and their lengths to the point of
   intersection are equal.
 - Two geodesics with azimuths \f$\alpha_1\f$ and \f$\pi-\alpha_1\f$
   first intersect on \f$\beta = -\beta_1\f$ and their lengths to the
   point of intersection are equal.
 .
(These assertions follow directly from the equations for the geodesics;
some of them are somewhat surprising given the asymmetries of the
ellipsoid.)  Consider now terminating the geodesics from \f$(\beta_1,
\omega_1)\f$ at the point where they first intersect (or touch) the line
\f$\beta = -\beta_1\f$.  To focus the discussion, take \f$\beta_1 \le
0\f$.
 - The geodesics completely fill the portion of the ellipsoid satisfying
   \f$\beta \le -\beta_1\f$.
 - None of geodesics intersect any other geodesics.
 - Any initial portion of these geodesics is a shortest path.
 - Each geodesic intersects the line \f$\beta = \beta_2\f$, where
   \f$\beta_1 < \beta_2 < -\beta_1\f$, exactly once.
 - For a given \f$\beta_2\f$, this defines a continuous monotonic
   mapping of the circle of azimuths \f$\alpha_1\f$ to the circle of
   longitudes \f$\omega_2\f$.
 - If \f$\beta_2 = \pm \beta_1\f$, then the previous two assertions need
   to be modified similarly to the case for an ellipsoid of revolution.

These properties show that the inverse problem can be solved using
techniques similar to those employed for an ellipsoid of revolution (see
&sect;4 of
<a href="https://doi.org/10.1007/s00190-012-0578-z"> Algorithms for
geodesics</a>).
 - If the points are opposite umbilical points, an arbitrary
   \f$\alpha_1\f$ may be chosen.
 - If the points are neighboring umbilical points, the shortest path
   lies on the ellipse \f$y = 0\f$.
 - If only one point is an umbilicial point, the azimuth at the
   non-umbilical point is found using the generalization of Clairaut's
   equation (given above) with \f$\gamma = 0\f$.
 - Treat the cases where the geodesic might follow a line of constant
   \f$\beta\f$.  There are two such cases: (a)&nbsp;the points lie on
   the ellipse \f$z = 0\f$ on a general ellipsoid and (b)&nbsp;the
   points lie on an ellipse whose major axis is the \f$x\f$ axis on a
   prolate ellipsoid (\f$a = b > c\f$).  Determine the reduced length
   \f$m_{12}\f$ for the geodesic which is the shorter path along the
   ellipse.  If \f$m_{12} \ge 0\f$, then this is the shortest path on
   the ellipsoid; otherwise proceed to the general case (next).
 - Swap the points, if necessary, so that the first point is the one
   closest to a pole.  Estimate \f$\alpha_1\f$ (by some means) and solve
   the \e hybrid problem, i.e., determine the longitude \f$\omega_2\f$
   corresponding to the first intersection of the geodesic with \f$\beta
   = \beta_2\f$.  Adjust \f$\alpha_1\f$ so that the value of
   \f$\omega_2\f$ matches the given \f$\omega_2\f$ (there is a single
   root).  If a sufficiently close solution can be found, Newton's
   method can be employed since the necessary derivative can be
   expressed in terms of the reduced length \f$m_{12}\f$.

The shortest path found by this method is unique unless:
 - The length of the geodesic vanishes \f$s_{12}=0\f$, in which case any
   constant can be added to the azimuths.
 - The points are opposite umbilical points.  In this case,
   \f$\alpha_1\f$ can take on any value and \f$\alpha_2\f$ needs to be
   adjusted to maintain the value of \f$\tan\alpha_1 / \tan\alpha_2\f$.
   Note that \f$\alpha\f$ increases by \f$\pm 90^\circ\f$ as the
   geodesic passes through an umbilical point, depending on whether the
   geodesic is considered as passing to the right or left of the point.
   Here \f$\alpha_2\f$ is the \e forward azimuth at the second umbilical
   point, i.e., its azimuth immediately \e after passage through the
   umbilical point.
 - \f$\beta_1 + \beta_2 = 0\f$ and \f$\cos\alpha_1\f$ and
   \f$\cos\alpha_2\f$ have opposite signs.  In this case, there another
   shortest geodesic with azimuths \f$\pi - \alpha_1\f$ and
   \f$\pi - \alpha_2\f$.

\section triaxial-conformal Jacobi's conformal projection

This material is now on its own page; see \ref jacobi.

<center>
Back to \ref nearest.  Forward to \ref jacobi.  Up to \ref contents.
</center>

**********************************************************************/
/**
\page jacobi Jacobi's conformal projection

<center>
Back to \ref triaxial.  Forward to \ref rhumb.  Up to \ref contents.
</center>

In addition to solving the geodesic problem for the triaxial ellipsoid,
Jacobi (1839) briefly mentions the problem of the conformal projection
of ellipsoid.  He covers this in greater detail in
<a href="https://books.google.com/books?id=ryEOAAAAQAAJ&pg=PA212">
<i>Vorlesungen &uuml;ber Dynamik</i>, &sect;28</a>, which is now
available in an <a href="https://www.worldcat.org/oclc/440645889">
English translation: Lectures on Dynamics</a>
(<a href="https://geographiclib.sourceforge.io/jacobi-errata.html">
errata</a>).

\section jacobi-conformal Conformal projection

It is convenient to rotate Jacobi's ellipsoidal coordinate system so
that \f$(X,Y,Z)\f$ are respectively the middle, large, and small axes of
the ellipsoid.  The longitude \f$\omega\f$ is shifted so that
\f$(\beta=0,\omega=0)\f$ is the point \f$(b,0,0)\f$.  The coordinates
are thus defined by
\f[
\begin{align}
  X &= b \cos\beta \cos\omega, \\
  Y &= a \sin\omega
      \frac{\sqrt{a^2 - b^2\sin^2\beta - c^2\cos^2\beta}}
           {\sqrt{a^2 - c^2}}, \\
  Z &= c \sin\beta
      \frac{\sqrt{a^2\cos^2\omega + b^2\sin^2\omega - c^2}}
           {\sqrt{a^2 - c^2}}.
\end{align}
\f]
After this change, the large principal ellipse is the equator,
\f$\beta=0\f$, while the small principal ellipse is the prime meridian,
\f$\omega=0\f$.  The four umbilic points, \f$\left|\omega\right| =
\left|\beta\right| = \frac12\pi\f$, lie on middle principal ellipse in
the plane \f$X=0\f$.

Jacobi gives the following conformal mapping of the triaxial ellipsoid
onto a plane
\f[
\begin{align}
x &= \frac{\sqrt{a^2-c^2}}b  \int \frac
{\sqrt{a^2 \cos^2\omega + b^2 \sin^2\omega}}
{\sqrt{a^2 \cos^2\omega + b^2 \sin^2\omega - c^2}}\, d\omega, \\
y &= \frac{\sqrt{a^2-c^2}}b \int \frac
{\sqrt{b^2 \sin^2\beta + c^2 \cos^2\beta}}
{\sqrt{a^2 - b^2 \sin^2\beta - c^2 \cos^2\beta}}\, d\beta.
\end{align}
\f]
The scale of the projection is
\f[
k = \frac{\sqrt{a^2-c^2}}
{b\sqrt{a^2 \cos^2\omega + b^2 (\sin^2\omega-\sin^2\beta) - c^2 \cos^2\beta}}.
\f]
I have scaled the Jacobi's projection by a constant factor,
\f[
\frac{\sqrt{a^2-c^2}}{2b},
\f]
so that it reduces to the familiar formulas in the case of an oblate ellipsoid.

\section jacobi-elliptic The projection in terms of elliptic integrals

The projection may be expressed in terms of elliptic integrals,
\f[
\begin{align}
x&=(1+e_a^2)\,\Pi(\omega',-e_a^2, \cos\nu),\\
y&=(1-e_c^2)\,\Pi(\beta' , e_c^2, \sin\nu),\\
k&=\frac{\sqrt{e_a^2+e_c^2}}{b\sqrt{e_a^2\cos^2\omega+e_c^2\cos^2\beta}},
\end{align}
\f]
where
\f[
\begin{align}
e_a &= \frac{\sqrt{a^2-b^2}}b, \qquad
e_c = \frac{\sqrt{b^2-c^2}}b, \\
\tan\omega' &= \frac ba \tan\omega = \frac 1{\sqrt{1+e_a^2}} \tan\omega, \\
\tan\beta' &= \frac bc \tan\beta = \frac 1{\sqrt{1-e_c^2}} \tan\beta, \\
\tan\nu &= \frac ac \frac{\sqrt{b^2-c^2}}{\sqrt{a^2-b^2}}
=\frac{e_c}{e_a}\frac{\sqrt{1+e_a^2}}{\sqrt{1-e_c^2}},
\end{align}
\f]
\f$\nu\f$ is the geographic latitude of the umbilic point at \f$\beta =
\omega = \frac12\pi\f$ (the angle a normal at the umbilic point makes
with the equatorial plane), and \f$\Pi(\phi,\alpha^2,k)\f$ is the
elliptic integral of the third kind, https://dlmf.nist.gov/19.2.E7.  This
allows the projection to be numerically computed using
EllipticFunction::Pi(real phi) const.

Nyrtsov, et al.,
 - M. V. Nyrtsov, M. E. Flies, M. M. Borisov, P. J. Stooke,
   <a href="https://doi.org/10.1007/978-3-642-32618-9_17">
   Jacobi conformal projection of the triaxial ellipsoid: new projection
   for mapping of small celestial bodies,</a> in
   <i>Cartography from Pole to Pole</i>
   (Springer, 2014), pp. 235--246.
 .
also expressed the projection in terms of elliptic integrals.
However, their expressions don't allow the limits of ellipsoids of
revolution to be readily recovered.  The relations
https://dlmf.nist.gov/19.7.E5 can be used to put their results in the
form given here.

\section jacobi-properties Properties of the projection

\f$x\f$ (resp. \f$y\f$) depends on \f$\omega\f$ (resp. \f$\beta\f$)
alone, so that latitude-longitude grid maps to straight lines in the
projection.  In this sense, the Jacobi projection is the natural
generalization of the Mercator projection for the triaxial ellipsoid.
(See below for the limit \f$a\rightarrow b\f$, which makes this
connection explicit.)

In the general case (all the axes are different), the scale diverges
only at the umbilic points.  The behavior of these singularities is
illustrated by the complex function
\f[
f(z;e) = \cosh^{-1}(z/e) - \log(2/e).
\f]
For \f$e > 0\f$, this function has two square root singularities at
\f$\pm e\f$, corresponding to the two northern umbilic points.
Plotting contours of its real (resp. imaginary) part gives the
behavior of the lines of constant latitude (resp. longitude) near the
north pole in Fig. 1.  If we pass to the limit \f$e\rightarrow 0\f$,
then \f$ f(z;e)\rightarrow\log z\f$, and the two branch points merge
yielding a stronger (logarithmic) singularity at \f$z = 0\f$,
concentric circles of latitude, and radial lines of longitude.

Again in the general case, the extents of \f$x\f$ and \f$y\f$ are
finite,
\f[
\begin{align}
x\bigl(\tfrac12\pi\bigr) &=(1+e_a^2)\,\Pi(-e_a^2, \cos\nu),\\
y\bigl(\tfrac12\pi\bigr) &=(1-e_c^2)\,\Pi(e_c^2, \sin\nu),
\end{align}
\f]
where \f$\Pi(\alpha^2,k)\f$ is the complete elliptic integral of the
third kind, https://dlmf.nist.gov/19.2.E8.

In particular, if we substitute values appropriate for the earth,
\f[
\begin{align}
a&=(6378137+35)\,\mathrm m,\\
b&=(6378137-35)\,\mathrm m,\\
c&=6356752\,\mathrm m,\\
\end{align}
\f]
we have
\f[
\begin{align}
x\bigl({\textstyle\frac12}\pi\bigr) &= 1.5720928 = \hphantom{0}90.07428^\circ,\\
y\bigl({\textstyle\frac12}\pi\bigr) &= 4.2465810 = 243.31117^\circ.\\
\end{align}
\f]

The projection may be inverted (to give \f$\omega\f$ in terms of \f$x\f$
and \f$\beta\f$ in terms of \f$y\f$) by using Newton's method to find
the root of, for example, \f$x(\omega) - x_0 = 0\f$.  The derivative of
the elliptic integral is, of course, just given by its defining
relation.

If rhumb lines are defined as curves with a constant bearing relative
to the ellipsoid coordinates, then these are straight lines in the
Jacobi projection.  A rhumb line which passes over an umbilic point
immediately retraces its path.  A rhumb line which crosses the line
joining the two northerly umbilic points starts traveling south with
a reversed heading (e.g., a NE heading becomes a SW heading).  This
behavior is preserved in the limit \f$a\rightarrow b\f$ (although the
longitude becomes indeterminate in this limit).

\section jacobi-limiting Limiting cases

<b>Oblate ellipsoid</b>, \f$a\rightarrow b\f$.  The coordinate system
is
\f[
\begin{align}
  X &= b \cos\beta \cos\omega, \\
  Y &= b \cos\beta \sin\omega, \\
  Z &= c \sin\beta.
\end{align}
\f]
Thus \f$\beta\f$ (resp. \f$\beta'\f$) is the parametric
(resp. geographic) latitude and \f$\omega=\omega'\f$ is the longitude;
the quantity \f$e_c\f$ is the eccentricity of the ellipsoid.
Using https://dlmf.nist.gov/19.6.E12 and https://dlmf.nist.gov/19.2.E19
the projection reduces to the normal Mercator projection for an oblate
ellipsoid,
\f[
\begin{align}
x &= \omega,\\
y &= \sinh^{-1}\tan\beta'
- e_c \tanh^{-1}(e_c\sin\beta'),\\
k &= \frac1{b\cos\beta}.
\end{align}
\f]

<b>Prolate ellipsoid</b>, \f$c\rightarrow b\f$. The coordinate system
is
\f[
\begin{align}
  X &= b \cos\omega \cos\beta, \\
  Y &= a \sin\omega, \\
  Z &= b \cos\omega \sin\beta.
\end{align}
\f]
Thus \f$\omega\f$ (resp. \f$\omega'\f$) now plays the role of the
parametric (resp. geographic) latitude and while \f$\beta=\beta'\f$ is
the longitude.  Using https://dlmf.nist.gov/19.6.E12
https://dlmf.nist.gov/19.2.E19 and https://dlmf.nist.gov/19.2.E18 the
projection reduces to similar expressions with the roles of \f$\beta\f$
and \f$\omega\f$ switched,
\f[
\begin{align}
x &= \sinh^{-1}\tan\omega'
+ e_a
  \tan^{-1}(e_a\sin\omega'),\\
y &= \beta,\\
k &= \frac1{b\cos\omega}.
\end{align}
\f]

<b>Sphere</b>, \f$a\rightarrow b\f$ and \f$c\rightarrow b\f$.  This is a
non-uniform limit depending on the parameter \f$\nu\f$,
\f[
\begin{align}
  X &= b \cos\omega \cos\beta, \\
  Y &= b \sin\omega \sqrt{1 - \sin^2\nu\sin^2\beta}, \\
  Z &= b \sin\beta \sqrt{1 - \cos^2\nu\sin^2\omega}.
\end{align}
\f]
Using https://dlmf.nist.gov/19.6.E13 the projection can be put in terms
of the elliptic integral of the first kind, https://dlmf.nist.gov/19.2.E4
\f[
\begin{align}
x &= F(\omega, \cos\nu), \\
y &= F(\beta, \sin\nu), \\
k &= \frac1{b \sqrt{\cos^2\nu\cos^2\omega + \sin^2\nu\cos^2\beta}}.
\end{align}
\f]
Obtaining the limit of a sphere via an oblate (resp. prolate) ellipsoid
corresponds to setting \f$\nu = \frac12\pi\f$ (resp. \f$\nu
=0\f$).  In these limits, the elliptic integral reduces to elementary
functions https://dlmf.nist.gov/19.6.E7 and https://dlmf.nist.gov/19.6.E8
\f[
\begin{align}
F(\phi, 0) &= \phi, \\
F(\phi, 1) &= \mathop{\mathrm{gd}}\nolimits^{-1}\phi = \sinh^{-1}\tan\phi.
\end{align}
\f]
The spherical limit gives the projection found by &Eacute;. Guyou in
 - <a href="https://books.google.com/books?id=saBDAQAAIAAJ&pg=PA308">
   Sur un nouveau syst&egrave;me de projection de la sph&egrave;re</a>,
   Comptes Rendus 102(6), 308--310 (1886).
 - <a href="https://books.google.com/books?id=VjU8AQAAMAAJ&pg=PA16">
   Nouveau syst&egrave;me de projection de la sph&egrave;re:
   g&eacute;n&eacute;ralisation de la projection de Mercator</a>,
   Annales Hydrographiques (2nd series) 9, 16--35 (1887).
 .
who apparently derived it without realizing that it is just a special
case of the projection Jacobi had given some 40 years earlier.  Guyou's
name is usually associated with the particular choice,
\f$\nu=\frac14\pi\f$, in which case the hemisphere
\f$\left|\omega\right|\le\frac12\pi\f$ is mapped into a square.
However, by varying \f$\nu\in[0,\frac12\pi]\f$ the hemisphere can be
mapped into a rectangle with any aspect ratio, \f$K(\cos\nu) :
K(\sin\nu)\f$, where \f$K(k)\f$ is the complete elliptic integral of
the first find, https://dlmf.nist.gov/19.2.E8.

\section jacobi-sphere Conformal mapping of an ellipsoid to a sphere

An essential tool in deriving conformal projections of an ellipsoid of
revolution is the conformal mapping of the ellipsoid onto a sphere.
This allows conformal projections of the sphere to be generalized to the
case of an ellipsoid of revolution.  This conformal mapping is obtained
by using the ellipsoidal Mercator projection to map the ellipsoid to
the plane and then using the spherical Mercator projection to map the
plane onto the sphere.

A similar construction is possible for a triaxial ellipsoid.  Map each
octant of the ellipsoid onto a rectangle using the Jacobi projection.
The aspect ratio of this rectangle is
\f[
(1+e_a^2)\,\Pi(-e_a^2, \cos\nu) :
(1-e_c^2)\,\Pi( e_c^2, \sin\nu).
\f]
Find the value of \f$\nu'\f$ such that this ratio equals
\f[
K(\cos\nu') : K(\sin\nu').
\f]
Map the rectangle onto the equivalent octant of the sphere using Guyou's
projection with parameter \f$\nu = \nu'\f$.  This reduces to the
standard construction in the limit of an ellipsoid of revolution.

\section jacobi-implementation An implementation of the projection

The JacobiConformal class provides an implementation of the Jacobi
conformal projection is given here.  <b>NOTE:</b> This is just sample
code.  It is not part of GeographicLib itself.

<center>
Back to \ref triaxial.  Forward to \ref rhumb.  Up to \ref contents.
</center>

**********************************************************************/
/**
\page rhumb Rhumb lines

<center>
Back to \ref jacobi.  Forward to \ref greatellipse.  Up to \ref contents.
</center>

The Rhumb and RhumbLine classes together with the
<a href="RhumbSolve.1.html">RhumbSolve</a> utility perform rhumb line
calculations.  A rhumb line (also called a loxodrome) is a line of
constant azimuth on the surface of the ellipsoid.  It is important for
historical reasons because sailing with a constant compass heading is
simpler than sailing the shorter geodesic course; see Osborne (2013).
The formulation of the problem on an ellipsoid is covered by Smart
(1946) and Carlton-Wippern (1992).  Computational approaches are given
by Williams (1950) and Bennett (1996).  My interest in this problem was
piqued by Botnev and Ustinov (2014) who discuss various techniques to
improve the accuracy of rhumb line calculations.  The items of interest
here are:
 - Review of accurate formulas for the auxiliary latitudes.
 - The calculation of the area under a rhumb line.
 - Using divided differences to compute \f$\mu_{12}/\psi_{12}\f$
   maintaining full accuracy.  Two cases are treated:
   - If \f$f\f$ is small, Kr&uuml;ger's series for the transverse
     Mercator projection relate \f$\chi\f$ and \f$\mu\f$.
   - For arbitrary \f$f\f$, the divided difference formula for
     incomplete elliptic integrals of the second relates \f$\beta\f$ and
     \f$\mu\f$.
   .
 - Extending Clenshaw summation to compute the divided difference of a
   trigonometric sum.

Go to
 - \ref rhumbform
 - \ref rhumblat
 - \ref rhumbarea
 - \ref divideddiffs
 - \ref dividedclenshaw

References:
 - G. G. Bennett,
   <a href="https://doi.org/10.1017/S0373463300013151">
   Practical Rhumb Line Calculations on the Spheroid</a>
   J. Navigation 49(1), 112--119 (1996).
 - F. W. Bessel,
   <a href="https://doi.org/10.1002/asna.201011352">The calculation
   of longitude and latitude from geodesic measurements (1825)</a>,
   Astron. Nachr. 331(8), 852--861 (2010);
   translated by C. F. F. Karney and R. E. Deakin; preprint:
   <a href="https://arxiv.org/abs/0908.1824">arXiv:0908.1824</a>.
 - V.A. Botnev, S.M. Ustinov,
   <a href="http://ntv.spbstu.ru/fulltext/T3.198.2014_05.PDF">
   Metody resheniya pryamoy i obratnoy geodezicheskikh zadach s vysokoy
   tochnost'yu
   </a> (Methods for direct and inverse geodesic problems
   solving with high precision), St. Petersburg State Polytechnical
   University Journal 3(198), 49--58 (2014).
 - K. C. Carlton-Wippern
   <a href="https://doi.org/10.1017/S0373463300010791">
   On Loxodromic Navigation</a>,
   J. Navigation 45(2), 292--297 (1992).
 - K. E. Engsager and K. Poder,
   <a href="http://icaci.org/files/documents/ICC_proceedings/ICC2007/documents/doc/THEME 2/oral 1/2.1.2 A HIGHLY ACCURATE WORLD WIDE ALGORITHM FOR THE TRANSVE.doc">
   A highly accurate world wide algorithm for the
   transverse Mercator mapping (almost)</a>,
   Proc. XXIII Intl. Cartographic Conf. (ICC2007), Moscow (2007).
 - F. R. Helmert,
   <a href="https://doi.org/10.5281/zenodo.32050">
   Mathematical and Physical Theories of Higher Geodesy, Part 1 (1880)</a>,
   Aeronautical Chart and Information Center (St. Louis, 1964),
   Chaps. 5--7.
 - W. M. Kahan and R. J. Fateman,
   <a href="https://www.cs.berkeley.edu/~fateman/papers/divdiff.pdf">
   Symbolic computation of divided differences</a>,
   SIGSAM Bull. 33(2), 7--28 (1999)
   DOI: <a href="https://doi.org/10.1145/334714.334716">
   10.1145/334714.334716</a>.
 - C. F. F. Karney,
   <a href="https://doi.org/10.1007/s00190-011-0445-3">
   Transverse Mercator with an accuracy of a few nanometers</a>,
   J. Geodesy 85(8), 475--485 (Aug. 2011);
   addenda: <a href="https://geographiclib.sourceforge.io/tm-addenda.html">
   tm-addenda.html</a>;
   preprint:
   <a href="https://arxiv.org/abs/1002.1417"> arXiv:1002.1417</a>;
   resource page:
   <a href="https://geographiclib.sourceforge.io/tm.html"> tm.html</a>.
 - C. F. F. Karney,
   <a href="https://doi.org/10.1007/s00190-012-0578-z">
   Algorithms for geodesics</a>,
   J. Geodesy 87(1), 43--55 (2013);
   DOI: <a href="https://doi.org/10.1007/s00190-012-0578-z">
   10.1007/s00190-012-0578-z</a>;
   addenda: <a href="https://geographiclib.sourceforge.io/geod-addenda.html">
   geod-addenda.html</a>;
   resource page:
   <a href="https://geographiclib.sourceforge.io/geod.html"> geod.html</a>.
 - L. Kr&uuml;ger,
   <a href="https://doi.org/10.2312/GFZ.b103-krueger28"> Konforme
   Abbildung des Erdellipsoids in der Ebene</a> (Conformal mapping of
   the ellipsoidal earth to the plane), Royal Prussian Geodetic Institute,
   New Series 52, 172 pp. (1912).
 - P. Osborne,
   <a href="https://doi.org/10.5281/zenodo.35392">
   The Mercator Projections</a> (2013), &sect;2.5 and &sect;6.5;
   DOI: <a href="https://doi.org/10.5281/zenodo.35392">
   10.5281/zenodo.35392</a>.
 - W. M. Smart
   <a href="https://doi.org/10.1093/mnras/106.2.124">
   On a Problem in Navigation</a>
   MNRAS 106(2), 124--127 (1946).
 - J. E. D. Williams
   <a href="https://doi.org/10.1017/S0373463300045549">
   Loxodromic Distances on the Terrestrial Spheroid Journal</a>,
   J. Navigation 3(2), 133--140 (1950)

\section rhumbform Formulation of the rhumb line problem

The rhumb line can formulated in terms of three types of latitude, the
isometric latitude \f$\psi\f$ (this serves to define the course of rhumb
lines), the rectifying latitude \f$\mu\f$ (needed for computing
distances along rhumb lines), and the parametric latitude \f$\beta\f$
(needed for dealing with rhumb lines that run along a parallel).  These
are defined in terms of the geographical latitude \f$\phi\f$ by
\f[
\begin{align}
 \frac{d\psi}{d\phi} &= \frac{\rho}R, \\
 \frac{d\mu}{d\phi}  &= \frac{\pi}{2M}\rho, \\
\end{align}
\f]
where \f$\rho\f$ is the meridional radius of curvature, \f$R =
a\cos\beta\f$ is the radius of a circle of latitude, and \f$M\f$ is the
length of a quarter meridian (from the equator to the pole).

Rhumb lines are straight in the Mercator projection, which maps a point
with geographical coordinates \f$ (\phi,\lambda) \f$ on the ellipsoid to
a point \f$ (\psi,\lambda) \f$.  The azimuth \f$\alpha_{12}\f$ of a
rhumb line from \f$ (\phi_1,\lambda_1) \f$ to \f$ (\phi_2,\lambda_2) \f$
is thus given by
\f[
\tan\alpha_{12} = \frac{\lambda_{12}}{\psi_{12}},
\f]
where the quadrant of \f$\alpha_{12}\f$ is determined by the signs of
the numerator and denominator of the right-hand side, \f$\lambda_{12} =
\lambda_2 - \lambda_1\f$, \f$\psi_{12} = \psi_2 - \psi_1\f$, and,
typically, \f$\lambda_{12}\f$ is reduced to the range \f$[-\pi,\pi]\f$
(thus giving the course of the <i>shortest</i> rhumb line).

The distance is given by
\f[
\begin{align}
s_{12} &= \frac {2M}{\pi} \mu_{12} \sec\alpha_{12} \\
  &= \frac {2M}{\pi}
\frac{\mu_{12}}{\psi_{12}} \sqrt{\lambda_{12}^2 + \psi_{12}^2},
\end{align}
\f]
where \f$\mu_{12} = \mu_2 - \mu_1\f$.  This relation is indeterminate if
\f$\phi_1 = \phi_2\f$, so we take the limits
\f$\psi_{12}\rightarrow0\f$ and \f$\mu_{12}\rightarrow0\f$ and apply
L'H&ocirc;pital's rule to yield
\f[
\begin{align}
 s_{12} &= \frac {2M}{\pi} \frac{d\mu}{d\psi} \left|\lambda_{12}\right|\\
&=a \cos\beta \left|\lambda_{12}\right|,
\end{align}
\f]
where the last relation is given by the defining equations for
\f$\psi\f$ and \f$\mu\f$.

This provides a complete solution for rhumb lines.  This formulation
entirely encapsulates the ellipsoidal shape of the earth via the
\ref auxlat; thus in the Rhumb class, the ellipsoidal
generalization is handled by the Ellipsoid class where the auxiliary
latitudes are defined.

\section rhumblat Determining the auxiliary latitudes

Here we brief develop the necessary formulas for the \ref auxlat.

<b>Isometric latitude:</b> The equation for \f$\psi\f$ can be integrated
to give
\f[
\psi = \sinh^{-1}\tan\phi - e\tanh^{-1}(e \sin\phi),
\f]
where \f$e = \sqrt{f(2-f)}\f$ is the eccentricity of the ellipsoid.  To
invert this equation (to give \f$\phi\f$ in terms of \f$\psi\f$), it is
convenient to introduce the variables \f$\tau=\tan\phi\f$ and \f$\tau' =
\tan\chi = \sinh\psi\f$ (\f$\chi\f$ is the conformal latitude) which are
related by (Karney, 2011)
\f[
\begin{align}
\tau' &= \tau \sqrt{1 + \sigma^2} - \sigma \sqrt{1 + \tau^2}, \\
\sigma &= \sinh\bigl(e \tanh^{-1}(e \tau/\sqrt{1 + \tau^2}) \bigr).
\end{align}
\f]
The equation for \f$\tau'\f$ can be inverted to give \f$\tau\f$ in terms
of \f$\tau'\f$ using Newton's method with \f$\tau_0 = \tau'/(1-e^2)\f$
as a starting guess; and, of course, \f$\phi\f$ and \f$\psi\f$ are then
given by
\f[
\begin{align}
\phi &= \tan^{-1}\tau,\\
\psi &= \sinh^{-1}\tau'.
\end{align}
\f]
This allows conversions to and from \f$\psi\f$ to be carried out for any
value of the flattening \f$f\f$; these conversions are implemented by
Ellipsoid::IsometricLatitude and Ellipsoid::InverseIsometricLatitude.
(For prolate ellipsoids, \f$f\f$ is negative and \f$e\f$ is imaginary,
and the equation for \f$\sigma\f$ needs to be recast in terms of the
tangent function.)

For small values of \f$f\f$, Engsager and Poder (2007) express
\f$\chi\f$ as a trigonometric series in \f$\phi\f$.  This series can be
reverted to give a series for \f$\phi\f$ in terms of \f$\chi\f$.  Both
series can be efficiently evaluated using Clenshaw summation and this
provides a fast non-iterative way of making the conversions.

<b>Parametric latitude:</b> This is given by
\f[
\tan\beta = (1-f)\tan\phi,
\f]
which allows rapid and accurate conversions; these conversions are
implemented by Ellipsoid::ParametricLatitude and
Ellipsoid::InverseParametricLatitude.

<b>Rectifying latitude:</b> Solving for distances on the meridian is
naturally carried out in terms of the parametric latitude instead of the
geographical latitude.  This leads to a simpler elliptic integral which
is easier to evaluate and to invert.  Helmert (1880, &sect;5.11) notes
that the series for meridian distance in terms of \f$\beta\f$ converge
faster than the corresponding ones in terms of \f$\phi\f$.

In terms of \f$\beta\f$, the rectifying latitude  is given by
\f[
\mu = \frac{\pi}{2E(ie')} E(\beta,ie'),
\f]
where \f$e'=e/\sqrt{1-e^2}\f$ is the second eccentricity and \f$E(k)\f$
and \f$E(\phi,k)\f$ are the complete and incomplete elliptic integrals
of the second kind; see https://dlmf.nist.gov/19.2.ii.  These can be
evaluated accurately for arbitrary flattening using the method given in
https://dlmf.nist.gov/19.36.i.  To find \f$\beta\f$ in terms of
\f$\mu\f$, Newton's method can be used (noting that \f$dE(\phi,k)/d\phi
= \sqrt{1 - k^2\sin^2\phi}\f$); for a starting guess use \f$\beta_0 =
\mu\f$ (or use the first terms in the reverted series; see below).
These conversions are implemented by Ellipsoid::RectifyingLatitude and
Ellipsoid::InverseRectifyingLatitude.

If the flattening is small, \f$\mu\f$ can be expressed as a series in
various ways.  The most economical series is in terms of the third
flattening \f$ n = f/(2-f)\f$ and was found by Bessel (1825); see
Eq. 5.5.7 of Helmert (1880).  Helmert (1880, Eq. 5.6.8) also gives the
reverted series (finding \f$\beta\f$ given \f$\mu\f$).  These series are
used by the Geodesic class where the coefficients are \f$C_{1j}\f$ and
\f$C_{1j}'\f$; see Eqs. (18) and (21) of Karney (2013) and \ref
geodseries.  (Make the replacements, \f$\sigma\rightarrow\beta\f$,
\f$\tau\rightarrow\mu\f$, \f$\epsilon\rightarrow n\f$, to convert the
notation of the geodesic problem to the problem at hand.)  The series
are evaluated by Clenshaw summation.

These relations allow inter-conversions between the various latitudes
\f$\phi\f$, \f$\psi\f$, \f$\chi\f$, \f$\beta\f$, \f$\mu\f$ to be carried
out simply and accurately.  The approaches using series apply only if
\f$f\f$ is small.  The others apply for arbitrary values of \f$f\f$.

\section rhumbarea The area under a rhumb line

The area between a rhumb line and the equator is given by
\f[
S_{12} = \int_{\lambda_1}^{\lambda_2} c^2 \sin\xi \,d\lambda,
\f]
where \f$c\f$ is the authalic radius and \f$\xi\f$ is the authalic
latitude.  Express \f$\sin\xi\f$ in terms of the conformal latitude
\f$\chi\f$ and expand as a series in the third flattening \f$n\f$.  This
can be expressed in terms of powers of \f$\sin\chi\f$.  Substitute
\f$\sin\chi=\tanh\psi\f$, where \f$\psi\f$ is the isometric latitude.
For a rhumb line we have
\f[
\begin{align}
\psi &= m(\lambda-\lambda_0),\\
m &= \frac{\psi_2 - \psi_1}{\lambda_2 - \lambda_1}.
\end{align}
\f]
Performing the integral over \f$\lambda\f$ gives
\f[
S_{12} = c^2 \lambda_{12} \left<\sin\xi\right>_{12},
\f]
where \f$\left<\sin\xi\right>_{12}\f$ is the mean value of \f$\sin\xi\f$
given by
\f[
\begin{align}
\left<\sin\xi\right>_{12} &= \frac{S(\chi_2) - S(\chi_1)}{\psi_2 - \psi_1},\\
S(\chi) &= \log\sec\chi + \sum_{l=1} R_l\cos(2l\chi),
\end{align}
\f]
\f$\log\f$ is the natural logarithm, and \f$R_l = O(n^l)\f$ is given as
a series in \f$n\f$ below.  In the spherical limit, the sum vanishes.

Note the simple way that longitude enters into the expression for the
area.  In the limit \f$\chi_2 \rightarrow \chi_1\f$, we can apply
l'H&ocirc;pital's rule
\f[
\left<\sin\xi\right>_{12}
\rightarrow \frac{dS(\chi_1)/d\chi_1}{\sec\chi_1}
=\sin\xi_1,
\f]
as expected.

In order to maintain accuracy, particularly for rhumb lines which nearly
follow a parallel, evaluate \f$\left<\sin\xi\right>_{12}\f$ using
divided differences (see the \ref divideddiffs "next section") and use
Clenshaw summation to evaluate the sum (see the \ref dividedclenshaw
"last section").

Here is the series expansion accurate to 10th order, found by the Maxima
script <a href="rhumbarea.mac">rhumbarea.mac</a>:

\verbatim
R[1] = - 1/3 * n
       + 22/45 * n^2
       - 356/945 * n^3
       + 1772/14175 * n^4
       + 41662/467775 * n^5
       - 114456994/638512875 * n^6
       + 258618446/1915538625 * n^7
       - 1053168268/37574026875 * n^8
       - 9127715873002/194896477400625 * n^9
       + 33380126058386/656284056553125 * n^10;
R[2] = - 2/15 * n^2
       + 106/315 * n^3
       - 1747/4725 * n^4
       + 18118/155925 * n^5
       + 51304574/212837625 * n^6
       - 248174686/638512875 * n^7
       + 2800191349/14801889375 * n^8
       + 10890707749202/64965492466875 * n^9
       - 3594078400868794/10719306257034375 * n^10;
R[3] = - 31/315 * n^3
       + 104/315 * n^4
       - 23011/51975 * n^5
       + 1554472/14189175 * n^6
       + 114450437/212837625 * n^7
       - 8934064508/10854718875 * n^8
       + 4913033737121/21655164155625 * n^9
       + 591251098891888/714620417135625 * n^10;
R[4] = - 41/420 * n^4
       + 274/693 * n^5
       - 1228489/2027025 * n^6
       + 3861434/42567525 * n^7
       + 1788295991/1550674125 * n^8
       - 215233237178/123743795175 * n^9
       + 95577582133463/714620417135625 * n^10;
R[5] = - 668/5775 * n^5
       + 1092376/2027025 * n^6
       - 3966679/4343625 * n^7
       + 359094172/10854718875 * n^8
       + 7597613999411/3093594879375 * n^9
       - 378396252233936/102088631019375 * n^10;
R[6] = - 313076/2027025 * n^6
       + 4892722/6081075 * n^7
       - 1234918799/834978375 * n^8
       - 74958999806/618718975875 * n^9
       + 48696857431916/9280784638125 * n^10;
R[7] = - 3189007/14189175 * n^7
       + 930092876/723647925 * n^8
       - 522477774212/206239658625 * n^9
       - 2163049830386/4331032831125 * n^10;
R[8] = - 673429061/1929727800 * n^8
       + 16523158892/7638505875 * n^9
       - 85076917909/18749059875 * n^10;
R[9] = - 39191022457/68746552875 * n^9
       + 260863656866/68746552875 * n^10;
R[10] = - 22228737368/22915517625 * n^10;
\endverbatim

\section divideddiffs Use of divided differences

Despite our ability to compute latitudes accurately, the way that
distances enter into the solution involves the ratio
\f$\mu_{12}/\psi_{12}\f$; the numerical calculation of this term is
subject to catastrophic round-off errors when \f$\phi_1\f$ and
\f$\phi_2\f$ are close.  A simple solution, suggested by Bennett (1996),
is to extend the treatment of rhumb lines along parallels to this case,
i.e., to replace the ratio by the derivative evaluated at the midpoint.
However this is not entirely satisfactory: you have to pick the
transition point where the derivative takes over from the ratio of
differences; and, near this transition point, many bits of accuracy will
be lost (I estimate that about 1/3 of bits are lost, leading to errors
on the order of 0.1&nbsp;mm for double precision).  Note too that this
problem crops up even in the spherical limit.

It turns out that we can do substantially better than this and maintain
full double precision accuracy.  Indeed Botnev and Ustinov provide
formulas which allow \f$\mu_{12}/\psi_{12}\f$ to be computed accurately
(but the formula for \f$\mu_{12}\f$ applies only for small flattening).

Here I give a more systematic treatment using the algebra of <i>divided
differences</i>.  Many readers will be already using this technique,
e.g., when writing, for example,
\f[
  \frac{\sin(2x) - \sin(2y)}{x-y} = 2\frac{\sin(x-y)}{x-y}\cos(x+y).
\f]
However, Kahan and Fateman (1999) provide an extensive set of rules
which allow the technique to be applied to a wide range of problems.
(The classes LambertConformalConic and AlbersEqualArea use this
technique to improve the accuracy.)

To illustrate the technique, consider the relation between \f$\psi\f$
and \f$\chi\f$,
\f[
  \psi = \sinh^{-1}\tan\chi.
\f]
The divided difference operator is defined by
\f[
 \Delta[f](x,y) =
\begin{cases}
df(x)/dx, & \text{if $x=y$,} \\
\displaystyle\frac{f(x)-f(y)}{x-y}, & \text{otherwise.}
\end{cases}
\f]
Many of the rules for differentiation apply to divided differences; in
particular, we can use the chain rule to write
\f[
\frac{\psi_1 - \psi_2}{\chi_1 - \chi_2}
= \Delta[\sinh^{-1}](\tan\chi_1,\tan\chi_2) \times
  \Delta[\tan](\chi_1,\chi_2).
\f]

Kahan and Fateman catalog the divided difference formulas for all the
elementary functions.  In this case, we need
\f[
\begin{align}
\Delta[\tan](x,y) &= \frac{\tan(x-y)}{x-y} (1 + \tan x\tan y),\\
\Delta[\sinh^{-1}](x,y) &= \frac1{x-y}
\sinh^{-1}\biggl(\frac{(x-y)(x+y)}{x\sqrt{1+y^2}+y\sqrt{1+x^2}}\biggr).
\end{align}
\f]
The crucial point in these formulas is that the right hand sides can be
evaluated accurately even if \f$x-y\f$ is small.  (I've only included
the basic results here; Kahan and Fateman supplement these with the
derivatives if \f$x-y\f$ vanishes or the direct ratios if \f$x-y\f$ is
not small.)

To complete the computation of \f$\mu_{12}/\psi_{12}\f$, we now need
\f$(\mu_1 - \mu_2)/(\chi_1 - \chi_2)\f$, i.e., we need the divided
difference of the function that converts the conformal latitude to
rectifying latitude.  We could go through the chain of relations between
these quantities.  However, we can take a short cut and recognize that
this function was given as a trigonometric series by Kr&uuml;ger (1912)
in his development of the transverse Mercator projection.  This is also
given by Eqs. (5) and (35) of Karney (2011) with the replacements
\f$\zeta \rightarrow \mu\f$ and \f$\zeta' \rightarrow \chi\f$.  The
coefficients appearing in this series and the reverted series Eqs. (6)
and (36) are already computed by the TransverseMercator class.  The
series can be readily converted to divided difference form (see the
example at the beginning of this section) and Clenshaw summation can be
used to evaluate it (see below).

The approach of using the series for the transverse Mercator projection
limits the applicability of the method to \f$\left|f\right|\lt 0.01\f$.
If we want to extend the method to arbitrary flattening we need to
compute \f$\Delta[E](x,y;k)\f$.  The necessary relation is the "addition
theorem" for the incomplete elliptic integral of the second kind given
in https://dlmf.nist.gov/19.11.E2.  This can be converted in the
following divided difference formula
\f[
\Delta[E](x,y;k)
=\begin{cases}
\sqrt{1 - k^2\sin^2x}, & \text{if $x=y$,} \\
\displaystyle \frac{E(x,k)-E(y,k)}{x-y}, & \text{if $xy \le0$,}\\
\displaystyle
\biggl(\frac{E(z,k)}{\sin z} - k^2 \sin x \sin y\biggr)\frac{\sin z}{x-y},
&\text{otherwise,}
\end{cases}
\f]
where the angle \f$z\f$ is given by
\f[
\begin{align}
\sin z &= \frac{2t}{1 + t^2},\quad\cos z = \frac{(1-t)(1+t)}{1 + t^2},\\
t &=
\frac{(x-y)\Delta[\sin](x,y)}
{\sin x\sqrt{1 - k^2\sin^2y} + \sin y\sqrt{1 - k^2\sin^2x}}
\frac{\sin x + \sin y}{\cos x + \cos y}.
\end{align}
\f]
We also need to apply the divided difference formulas to the conversions
from \f$\phi\f$ to \f$\beta\f$ and \f$\phi\f$ to \f$\psi\f$; but these
all involve elementary functions and the techniques given in Kahan and
Fateman can be used.

The end result is that the Rhumb class allows the computation of all
rhumb lines for any flattening with full double precision accuracy (the
maximum error is about 10 nanometers).  I've kept the implementation
simple, which results in rather a lot of repeated evaluations of
quantities.  However, in this case, the need for clarity trumps the
desire for speed.

\section dividedclenshaw Clenshaw evaluation of differenced sums

The use of
<a href="https://en.wikipedia.org/wiki/Clenshaw_algorithm#Meridian_arc_length_on_the_ellipsoid">
Clenshaw summation</a> for summing series of the form,
\f[
g(x) = \sum_{k=1}^N c_k \sin kx,
\f]
is well established.  However when computing divided differences, we are
interested in evaluating
\f[
g(x)-g(y) = \sum_{k=1}^N 2 c_k
\sin\bigl({\textstyle\frac12}k(x-y)\bigr)
\cos\bigl({\textstyle\frac12}k(x+y)\bigr).
\f]
Clenshaw summation can be used in this case if we simultaneously compute
\f$g(x)+g(y)\f$ and perform a matrix summation,
\f[
\mathsf G(x,y) = \begin{bmatrix}
(g(x) + g(y)) / 2\\
(g(x) - g(y)) / (x - y)
\end{bmatrix} = \sum_{k=1}^N c_k \mathsf F_k(x,y),
\f]
where
\f[
\mathsf F_k(x,y)=
\begin{bmatrix}
     \cos kd \sin kp\\
     {\displaystyle\frac{\sin kd}d} \cos kp
\end{bmatrix},
\f]
\f$d=\frac12(x-y)\f$, \f$p=\frac12(x+y)\f$, and, in the limit
\f$d\rightarrow0\f$, \f$(\sin kd)/d \rightarrow k\f$.  The first element
of \f$\mathsf G(x,y)\f$ is the average value of \f$g\f$ and the second
element, the divided difference, is the average slope.  \f$\mathsf
F_k(x,y)\f$ satisfies the recurrence relation
\f[
  \mathsf F_{k+1}(x,y) =
  \mathsf A(x,y) \mathsf F_k(x,y) - \mathsf F_{k-1}(x,y),
\f]
where
\f[
 \mathsf A(x,y) = 2\begin{bmatrix}
  \cos d \cos p & -d\sin d \sin p \\
- {\displaystyle\frac{\sin d}d} \sin p &   \cos d \cos p
\end{bmatrix},
\f]
and \f$\lim_{d\rightarrow0}(\sin d)/d = 1\f$.  The standard Clenshaw
algorithm can now be applied to yield
\f[
\begin{align}
\mathsf B_{N+1} &= \mathsf B_{N+2} = \mathsf 0, \\
\mathsf B_k &= \mathsf A \mathsf B_{k+1} - \mathsf B_{k+2} +
c_k \mathsf I, \qquad\text{for $N\ge k \ge 1$},\\
\mathsf G(x,y) &= \mathsf B_1 \mathsf F_1(x,y),
\end{align}
\f]
where \f$\mathsf B_k\f$ are \f$2\times2\f$ matrices.  The divided
difference \f$\Delta[g](x,y)\f$ is given by the second element of
\f$\mathsf G(x,y)\f$.

The same basic recursion applies to the more general case,
\f[
g(x) = \sum_{k=0}^N c_k \sin\bigl( (k+k_0)x + x_0\bigr).
\f]
For example, the sum area arising in the computation of geodesic areas,
\f[
\sum_{k=0}^N c_k\cos\bigl((2k+1)\sigma)
\f]
is given by \f$x=2\sigma\f$, \f$x_0=\frac12\pi\f$, \f$k_0=\frac12\f$.
Again we consider the matrix sum,
\f[
\mathsf G(x,y) = \begin{bmatrix}
(g(x) + g(y)) / 2\\
(g(x) - g(y)) / (x - y)
\end{bmatrix} = \sum_{k=0}^N c_k \mathsf F_k(x,y),
\f]
where
\f[
\mathsf F_k(x,y)=
\begin{bmatrix}
     \cos\bigl( (k+k_0)d \bigr) \sin \bigl( (k+k_0)p + x_0 \bigr)\\
     {\displaystyle\frac{\sin\bigl( (k+k_0)d \bigr)}d}
     \cos \bigl( (k+k_0)p + x_0 \bigr)
\end{bmatrix}.
\f]
The recursion for \f$\mathsf B_k\f$ is identical to the previous case;
in particular, the expression for \f$\mathsf A(x,y)\f$ remains
unchanged.  The result for the sum is
\f[
\mathsf G(x,y) =
  (c_0\mathsf I - \mathsf B_2) \mathsf F_0(x,y)
  + \mathsf B_1 \mathsf F_1(x,y).
\f]

<center>
Back to \ref jacobi.  Forward to \ref greatellipse.  Up to \ref contents.
</center>

**********************************************************************/
/**
\page greatellipse Great Ellipses

<center>
Back to \ref rhumb.  Forward to \ref transversemercator.  Up to \ref contents.
</center>

Great ellipses are sometimes proposed (Williams, 1996; Pallikaris &
Latsas, 2009) as alternatives to geodesics for the purposes of
navigation.  This is predicated on the assumption that solving the
geodesic problems is complex and costly.  These assumptions are no
longer true, and geodesics should normally be used in place of great
ellipses.  This is discussed in more detail in \ref gevsgeodesic.

Solutions of the great ellipse problems implemented for MATLAB and
Octave are provided by
 - gedoc: briefly describe the routines
 - gereckon: solve the direct great ellipse problem
 - gedistance: solve the inverse great ellipse problem
 .
At this time, there is no C++ support in GeographicLib for great
ellipses.

References:
 - P. D. Thomas,
   <a href="https://apps.dtic.mil/docs/citations/AD0627893">
   Mathematical Models for Navigation Systems</a>,
   TR-182 (U.S. Naval Oceanographic Office, 1965).
 - B. R. Bowring,
   <a href="https://doi.org/10.1007/BF02521760">
   The direct and inverse solutions for the great elliptic line on the
   reference ellipsoid</a>, Bull. Geod. 58, 101--108 (1984).
 - M. A. Earle,
   A vector solution for navigation on a great ellipse,
   J. Navigation 53(3), 473--481 (2000).
 - M. A. Earle,
   <a href="https://doi.org/10.1017/S037346330800475X">
   Vector solutions for azimuth</a>,
   J. Navigation 61(3), 537--545 (2008).
 - C. F. F. Karney,
   <a href="https://doi.org/10.1007/s00190-012-0578-z">
   Algorithms for geodesics</a>, J. Geodesy 87(1), 43--55 (2013);
   addenda: <a href="https://geographiclib.sourceforge.io/geod-addenda.html">
   geod-addenda.html</a>.
 - A. Pallikaris & G. Latsas,
   <a href="https://doi.org/10.1017/S0373463309005323">
   New algorithm for great elliptic sailing (GES)</a>,
   J. Navigation 62(3), 493--507 (2009).
 - A. Pallikaris, L. Tsoulos, & D. Paradissis,
   New meridian arc formulas for sailing calculations in navigational
   GIS, International Hydrographic Review, 24--34 (May 2009).
 - L. E. Sj&ouml;berg,
   <a href="https://doi.org/10.2478/v10156-011-0040-9">
   Solutions to the direct and inverse navigation problems on the great
   ellipse</a>, J. Geodetic Science 2(3), 200--205 (2012).
 - R. Williams,
   <a href="https://doi.org/10.1017/S0373463300013333">
   The Great Ellipse on the Surface of the Spheroid</a>,
   J. Navigation 49(2), 229--234 (1996).
 - T. Vincenty,
   <a href="https://www.ngs.noaa.gov/PUBS_LIB/inverse.pdf">
   Direct and Inverse Solutions of Geodesics on the Ellipsoid with
   Application of Nested Equations</a>,
   Survey Review 23(176), 88--93 (1975).
 - Wikipedia page, <a href="https://en.wikipedia.org/wiki/Great_ellipse">
   Great ellipse</a>.

Go to
 - \ref geformulation
 - \ref gearea
 - \ref gevsgeodesic

\section geformulation Solution of great ellipse problems

Adopt the usual notation for the ellipsoid: equatorial semi-axis
\f$a\f$, polar semi-axis \f$b\f$, flattening \f$f = (a-b)/a\f$,
eccentricity \f$e = \sqrt{f(2-f)}\f$, second eccentricity \f$e' =
e/(1-f)\f$, and third flattening \f$n=(a-b)/(a+b)=f/(2-f)\f$.

There are several ways in which an ellipsoid can be mapped into a sphere
converting the great ellipse into a great circle.  The simplest ones
entail scaling the ellipsoid in the \f$\hat z\f$ direction, the
direction of the axis of rotation, scaling the ellipsoid radially, or a
combination of the two.

One such combination (scaling by \f$a^2/b\f$ in the \f$\hat z\f$
direction, following by a radial scaling to the sphere) preserves the
geographical latitude \f$\phi\f$.  This enables a great ellipse to be
plotted on a chart merely by determining way points on the corresponding
great circle and transferring them directly on the chart.  In this
exercise the flattening of the ellipsoid can be <i>ignored</i>!

Bowring (1984), Williams (1996), Earle (2000, 2008) and Pallikaris &
Latsas (2009), scale the ellipsoid radially onto a sphere preserving the
geocentric latitude \f$\theta\f$.  More convenient than this is to scale
the ellipsoid along \f$\hat z\f$ onto the sphere, as is done by
Thomas (1965) and Sj&ouml;berg (2012), thus preserving the parametric
latitude \f$\beta\f$.  The advantage of this "parametric" mapping is
that Bessel's rapidly converging series for meridian arc in terms of
parametric latitude can be used (a possibility that is overlooked by
Sj&ouml;berg).

The full parametric mapping is:
 - The geographic latitude \f$\phi\f$ on the ellipsoid maps to the
   parametric latitude \f$\beta\f$ on the sphere,
   where
   \f[a\tan\beta = b\tan\phi.\f]
 - The longitude \f$\lambda\f$ is unchanged.
 - The azimuth \f$\alpha\f$ on the ellipsoid maps to an azimuth
   \f$\gamma\f$ on the sphere where
   \f[
   \begin{align}
   \tan\alpha &= \frac{\tan\gamma}{\sqrt{1-e^2\cos^2\beta}}, \\
   \tan\gamma &= \frac{\tan\alpha}{\sqrt{1+e'^2\cos^2\phi}},
   \end{align}
   \f]
   and the quadrants of \f$\alpha\f$ and \f$\gamma\f$ are the same.
 - Positions on the great circle of radius \f$a\f$ are parametrized by
   arc length \f$\sigma\f$ measured from the northward crossing of the
   equator.  The great ellipse has semi-axes \f$a\f$ and
   \f$b'=a\sqrt{1-e^2\cos^2\gamma_0}\f$, where \f$\gamma_0\f$ is the
   great-circle azimuth at the northward equator crossing, and
   \f$\sigma\f$ is the parametric angle on the ellipse.  [In contrast,
   the ellipse giving distances on a geodesic has semi-axes
   \f$b\sqrt{1+e'^2\cos^2\alpha_0}\f$ and \f$b\f$.]
 .
To determine the distance along the ellipse in terms of \f$\sigma\f$ and
vice versa, the series for distance \f$s\f$ and for \f$\tau\f$ given in
\ref geodseries can be used.  The direct and inverse great ellipse
problems are now simply solved by mapping the problem to the sphere,
solving the resulting great circle problem, and mapping this back onto
the ellipsoid.

\section gearea The area under a great ellipse

The area between the segment of a great ellipse and the equator can be
found by very similar methods to those used for geodesic areas; see
<a href="https://doi.org/10.1179/003962689791474267"> Danielsen
(1989)</a>.  The notation here is similar to that employed by
<a href="https://doi.org/10.1007/s00190-012-0578-z"> Karney
(2013)</a>.
\f[
\begin{align}
S_{12} &= S(\sigma_2) - S(\sigma_1) \\
S(\sigma) &= c^2\gamma + e^2 a^2 \cos\gamma_0 \sin\gamma_0 I_4(\sigma)\\
c^2 &= {\textstyle\frac12}\bigl(a^2 + b^2 (\tanh^{-1}e)/e\bigr)
\end{align}
\f]
\f[
I_4(\sigma) = - \sqrt{1+e'^2}\int
\frac{r(e'^2) - r(k^2\sin^2\sigma)}{e'^2 - k^2\sin^2\sigma}
\frac{\sin\sigma}2 \,d\sigma
\f]
\f[
\begin{align}
k &= e' \cos\gamma_0,\\
r(x) &= \sqrt{1+x} + (\sinh^{-1}\sqrt x)/\sqrt x.
\end{align}
\f]
Expand in terms of the third flattening of the ellipsoid, \f$n\f$, and the
third flattening of the great ellipse, \f$\epsilon=(a-b')/(a+b')\f$, by
substituting
\f[
\begin{align}
e'&=\frac{2\sqrt n}{1-n},\\
k&=\frac{1+n}{1-n} \frac{2\sqrt{\epsilon}}{1+\epsilon},
\end{align}
\f]
to give
\f[
I_4(\sigma) = \sum_{l = 0}^\infty G_{4l}\cos \bigl((2l+1)\sigma\bigr).
\f]
Compared to the area under a geodesic, we have
- \f$\gamma\f$ and \f$\gamma_0\f$ instead of \f$\alpha\f$ and
  \f$\alpha_0\f$.  In both cases, these are azimuths on the auxiliary
  sphere; however, for the geodesic, they are also the azimuths on the
  ellipsoid.
- \f$r(x) = \bigl(1+t(x)\bigr)/\sqrt{1+x}\f$ instead of \f$t(x)\f$ with
  a balancing factor of \f$\sqrt{1+e'^2}\f$ appearing in front of the
  integral.  These changes are because expressing
  \f$\sin\phi\,d\lambda\f$ in terms of \f$\sigma\f$ is a little more
  complicated with great ellipses.  (Don't worry about the addition of
  \f$1\f$ to \f$t(x)\f$; that's immaterial.)
- the factors involving \f$n\f$ in the expression for \f$k\f$ in terms
  of \f$\epsilon\f$.  This is because \f$k\f$ is defined in terms of
  \f$e'\f$, whereas it is \f$e\cos\gamma_0\f$ that plays the role of the
  eccentricity for the great ellipse.

Here is the series expansion accurate to 10th order, found by
<a href="gearea.mac">gearea.mac</a>:

\verbatim
G4[0] = + (1/6 + 7/30 * n + 8/105 * n^2 + 4/315 * n^3 + 16/3465 * n^4 + 20/9009 * n^5 + 8/6435 * n^6 + 28/36465 * n^7 + 32/62985 * n^8 + 4/11305 * n^9)
        - (3/40 + 12/35 * n + 589/840 * n^2 + 1063/1155 * n^3 + 14743/15015 * n^4 + 14899/15015 * n^5 + 254207/255255 * n^6 + 691127/692835 * n^7 + 1614023/1616615 * n^8) * eps
        + (67/280 + 7081/5040 * n + 5519/1320 * n^2 + 6417449/720720 * n^3 + 708713/45045 * n^4 + 2700154/109395 * n^5 + 519037063/14549535 * n^6 + 78681626/1616615 * n^7) * eps^2
        - (29597/40320 + 30013/5280 * n + 33759497/1441440 * n^2 + 100611307/1441440 * n^3 + 16639623457/98017920 * n^4 + 3789780779/10581480 * n^5 + 1027832503/1511640 * n^6) * eps^3
        + (357407/147840 + 19833349/823680 * n + 61890679/480480 * n^2 + 97030756063/196035840 * n^3 + 2853930388817/1862340480 * n^4 + 15123282583393/3724680960 * n^5) * eps^4
        - (13200233/1537536 + 306285589/2882880 * n + 26279482199/37340160 * n^2 + 3091446335399/931170240 * n^3 + 93089556575647/7449361920 * n^4) * eps^5
        + (107042267/3294720 + 253176989449/522762240 * n + 57210830762263/14898723840 * n^2 + 641067300459403/29797447680 * n^3) * eps^6
        - (51544067373/398295040 + 38586720036247/17027112960 * n + 104152290127363/4966241280 * n^2) * eps^7
        + (369575321823/687964160 + 1721481081751393/158919720960 * n) * eps^8
        - 10251814360817/4445306880 * eps^9;
G4[1] = + (1/120 + 4/105 * n + 589/7560 * n^2 + 1063/10395 * n^3 + 14743/135135 * n^4 + 14899/135135 * n^5 + 254207/2297295 * n^6 + 691127/6235515 * n^7 + 1614023/14549535 * n^8) * eps
        - (53/1680 + 847/4320 * n + 102941/166320 * n^2 + 1991747/1441440 * n^3 + 226409/90090 * n^4 + 3065752/765765 * n^5 + 24256057/4157010 * n^6 + 349229428/43648605 * n^7) * eps^2
        + (4633/40320 + 315851/332640 * n + 5948333/1441440 * n^2 + 11046565/864864 * n^3 + 9366910279/294053760 * n^4 + 23863367599/349188840 * n^5 + 45824943037/349188840 * n^6) * eps^3
        - (8021/18480 + 39452953/8648640 * n + 3433618/135135 * n^2 + 29548772933/294053760 * n^3 + 44355142973/139675536 * n^4 + 4771229132843/5587021440 * n^5) * eps^4
        + (2625577/1537536 + 5439457/247104 * n + 353552588953/2352430080 * n^2 + 405002114215/558702144 * n^3 + 61996934629789/22348085760 * n^4) * eps^5
        - (91909777/13178880 + 2017395395921/18819440640 * n + 51831652526149/59594895360 * n^2 + 1773086701957889/357569372160 * n^3) * eps^6
        + (35166639971/1194885120 + 26948019211109/51081338880 * n + 7934238355871/1596291840 * n^2) * eps^7
        - (131854991623/1031946240 + 312710596037369/119189790720 * n) * eps^8
        + 842282436291/1481768960 * eps^9;
G4[2] = + (1/560 + 29/2016 * n + 1027/18480 * n^2 + 203633/1441440 * n^3 + 124051/450450 * n^4 + 1738138/3828825 * n^5 + 98011493/145495350 * n^6 + 4527382/4849845 * n^7) * eps^2
        - (533/40320 + 14269/110880 * n + 908669/1441440 * n^2 + 15253627/7207200 * n^3 + 910103119/163363200 * n^4 + 2403810527/193993800 * n^5 + 746888717/30630600 * n^6) * eps^3
        + (2669/36960 + 2443153/2882880 * n + 1024791/200200 * n^2 + 10517570057/490089600 * n^3 + 164668999127/2327925600 * n^4 + 1826633124599/9311702400 * n^5) * eps^4
        - (5512967/15375360 + 28823749/5765760 * n + 31539382001/871270400 * n^2 + 1699098121381/9311702400 * n^3 + 287618085731/398361600 * n^4) * eps^5
        + (22684703/13178880 + 25126873327/896163840 * n + 10124249914577/42567782400 * n^2 + 836412216748957/595948953600 * n^3) * eps^6
        - (3259030001/398295040 + 2610375232847/17027112960 * n + 2121882247763/1418926080 * n^2) * eps^7
        + (13387413913/343982080 + 939097138279/1135140864 * n) * eps^8
        - 82722916855/444530688 * eps^9;
G4[3] = + (5/8064 + 23/3168 * n + 1715/41184 * n^2 + 76061/480480 * n^3 + 812779/1782144 * n^4 + 9661921/8953560 * n^5 + 40072069/18106088 * n^6) * eps^3
        - (409/59136 + 10211/109824 * n + 46381/73920 * n^2 + 124922951/43563520 * n^3 + 12524132449/1241560320 * n^4 + 30022391821/1022461440 * n^5) * eps^4
        + (22397/439296 + 302399/384384 * n + 461624513/74680320 * n^2 + 1375058687/41385344 * n^3 + 4805085120841/34763688960 * n^4) * eps^5
        - (14650421/46126080 + 17533571183/3136573440 * n + 1503945368767/29797447680 * n^2 + 43536234862451/139054755840 * n^3) * eps^6
        + (5074867067/2788065280 + 479752611137/13243310080 * n + 1228808683449/3310827520 * n^2) * eps^7
        - (12004715823/1203937280 + 17671119291563/79459860480 * n) * eps^8
        + 118372499107/2222653440 * eps^9;
G4[4] = + (7/25344 + 469/109824 * n + 13439/411840 * n^2 + 9282863/56010240 * n^3 + 37558503/59121920 * n^4 + 44204289461/22348085760 * n^5) * eps^4
        - (5453/1317888 + 58753/823680 * n + 138158857/224040960 * n^2 + 191056103/53209728 * n^3 + 712704605341/44696171520 * n^4) * eps^5
        + (28213/732160 + 331920271/448081920 * n + 2046013913/283785216 * n^2 + 11489035343/241274880 * n^3) * eps^6
        - (346326947/1194885120 + 11716182499/1891901440 * n + 860494893431/12770334720 * n^2) * eps^7
        + (750128501/386979840 + 425425087409/9287516160 * n) * eps^8
        - 80510858479/6667960320 * eps^9;
G4[5] = + (21/146432 + 23/8320 * n + 59859/2263040 * n^2 + 452691/2687360 * n^3 + 21458911/26557440 * n^4) * eps^5
        - (3959/1464320 + 516077/9052160 * n + 51814927/85995520 * n^2 + 15444083489/3611811840 * n^3) * eps^6
        + (1103391/36208640 + 120920041/171991040 * n + 18522863/2263040 * n^2) * eps^7
        - (92526613/343982080 + 24477436759/3611811840 * n) * eps^8
        + 1526273559/740884480 * eps^9;
G4[6] = + (11/133120 + 1331/696320 * n + 145541/6615040 * n^2 + 46863487/277831680 * n^3) * eps^6
        - (68079/36208640 + 621093/13230080 * n + 399883/680960 * n^2) * eps^7
        + (658669/26460160 + 186416197/277831680 * n) * eps^8
        - 748030679/2963537920 * eps^9;
G4[7] = + (143/2785280 + 11011/7938048 * n + 972829/52093440 * n^2) * eps^7
        - (434863/317521920 + 263678129/6667960320 * n) * eps^8
        + 185257501/8890613760 * eps^9;
G4[8] = + (715/21168128 + 27313/26148864 * n) * eps^8
        - 1838551/1778122752 * eps^9;
G4[9] = + 2431/104595456 * eps^9;
\endverbatim

\section gevsgeodesic Great ellipses vs geodesics

Some papers advocating the use of great ellipses for navigation exhibit
a prejudice against the use of geodesics.  These excerpts from
Pallikaris, Tsoulos, & Paradissis (2009) give the flavor
 - &hellip; it is required to adopt realistic accuracy standards in order not
   only to eliminate the significant errors of the spherical model but
   also to avoid the exaggerated and unrealistic requirements of sub
   meter accuracy.
 - Calculation of shortest sailings paths on the ellipsoid by a geodetic
   inverse method involve formulas that are much too complex.
 - Despite the fact that contemporary computers are fast enough to
   handle more complete geodetic formulas of sub meter accuracy, a basic
   principle for the design of navigational systems is the avoidance of
   unnecessary consumption of computing power.
 .
This prejudice was probably due to the fact that the most well-known
algorithms for geodesics, by Vincenty (1975), come with some
"asterisks":
 - no derivation was given (although they follow in a straightforward
   fashion from classic 19th century methods);
 - the accuracy is "only" 0.5&nbsp;mm or so, surely good enough for most
   applications, but still a reason for a user to worry and a spur to
   numerous studies "validating" the algorithms;
 - no indication is given for how to extend the series to improve the
   accuracy;
 - there was a belief in some quarters (erroneous!) that the Vincenty
   algorithms could not be used to compute waypoints;
 - the algorithm for the inverse problem fails to converge for some
   inputs.
 .
These problems meant that users were reluctant to bundle the algorithms
into a library and treat them as a part of the software infrastructure
(much as you might regard the computation of \f$\sin x\f$ as a given).
In particular, I regard the last issue, lack of convergence of the
inverse solution, as fatal.  Even though the problem only arises for
nearly antipodal points, it means all users of the library must have
some way to handle this problem.

For these reasons, substitution of a great ellipse for the geodesic
makes some sense.  The solution of the great ellipse is, in principle,
no more difficult than solving for the great circle and, for paths of
less then 10000&nbsp;km, the error in the distance is less than
13.5&nbsp;m.

Now (2014), however, the situation has reversed.  The algorithms given
by Karney (2013)&mdash;and used in GeographicLib since
2009&mdash;explicitly resolve the issues with Vincenty's algorithm.  The
geodesic problem is conveniently bundled into a library.  Users can call
this with an assurance of an accurate result much as they would when
evaluating \f$\sin x\f$.  On the other hand, great ellipses come with
their own set of asterisks:
 - To the extent that they are regarded as approximations to geodesics,
   the errors need to be quantified, the limits of allowable use
   documented, etc.
 - The user is now left with decisions on when to trust the results and
   to find alternative solutions if necessary.
 - Even though the great ellipse is no more that 13.5&nbsp;m longer than
   a 10000&nbsp;km geodesic, the path of the great ellipse can deviate
   from the geodesic by as much as 8.3&nbsp;km.  This disqualifies great
   ellipses from use in congested air corridors where the
   <a href="https://en.wikipedia.org/wiki/Strategic_lateral_offset_procedure">
   strategic lateral offset procedure</a> is in effect and in any
   situation which demands coordination in the routes of vessels.
 - Because geodesics obey the triangle inequality, while great ellipses
   do not, the solutions for realistic navigational problems, e.g.,
   determining the time of closest approach of two vessels, are often
   simpler in terms of geodesics.

To address some other of the objections in the quotes from Pallikaris et
al. given above:
 - "exaggerated and unrealistic requirements of sub meter accuracy": The
   geodesic algorithms allow full double precision accuracy at
   essentially no cost.  This is because Clenshaw summation allows
   additional terms to be added to the series without the need for
   addition trigonometric evaluations.  Full accuracy is important to
   maintain because it allows the results to be used reliably in more
   complex problems (e.g., in the determination of maritime boundaries).
 - "unnecessary consumption of computing power": The solution of the
   inverse geodesic problem takes 2.3&nbsp;&mu;s; multiple points on a
   geodesic can be computed at a rate of one point per 0.4&nbsp;&mu;s.
   The actual power consumed in these calculations is minuscule compared
   to the power needed to drive the display of a navigational computer.
 - "formulas that are much too complex": There's no question that the
   solution of the geodesic problem is more complex than for great
   ellipses.  However this complexity is only an issue when
   <b>implementing</b> the algorithms.  For the <b>end user</b>,
   navigational software employing geodesics is <b>less</b> complex
   compared with that employing great ellipses.  Here is what the user
   needs to know about the geodesic solution:
   <blockquote>"The shortest path is found."</blockquote>
   And here is the corresponding documentation for great ellipses:
   <blockquote>"A path which closely approximates the shortest path is
   found.  Provided that the distance is less than 10000&nbsp;km, the
   error in distance is no more than 14&nbsp;m and the deviation the
   route from that of the shortest path is no more than 9&nbsp;km.
   These bounds apply to the WGS84 ellipsoid.  The deviation of the path
   means that it should be used with caution when planning routes.  In
   addition, great ellipses do not obey the triangle inequality; this
   disqualifies them from use in some applications."</blockquote>

Having all the geodesic functions bundled up into a reliable "black box"
enables users to concentrate on how to solve problems using geodesics
(instead of figuring out how to solve for the geodesics).  A wide range
of problems (intersection of paths, the path for an interception, the
time of closest approach, median lines, tri-points, etc.) are all
amenable to simple and fast solutions in terms of geodesics.

<center>
Back to \ref rhumb.  Forward to \ref transversemercator.  Up to \ref contents.
</center>

**********************************************************************/
/**
\page transversemercator Transverse Mercator projection

<center>
Back to \ref greatellipse.  Forward to \ref geocentric.  Up to \ref contents.
</center>

TransverseMercator and TransverseMercatorExact provide accurate
implementations of the transverse Mercator projection.  The
<a href="TransverseMercatorProj.1.html">TransverseMercatorProj</a>
utility provides an interface to these classes.

Go to
 - \ref testmerc
 - \ref tmseries
 - \ref tmfigures

References:
 - L. Kr&uuml;ger,
   <a href="https://doi.org/10.2312/GFZ.b103-krueger28"> Konforme
   Abbildung des Erdellipsoids in der Ebene</a> (Conformal mapping of
   the ellipsoidal earth to the plane), Royal Prussian Geodetic Institute,
   New Series 52, 172 pp. (1912).
 - L. P. Lee,
   Conformal Projections Based on Elliptic Functions,
   (B. V. Gutsell, Toronto, 1976), 128pp.,
   ISBN: 0919870163
   (Also appeared as:
   Monograph 16, Suppl. No. 1 to Canadian Cartographer, Vol 13).
   Part V, pp. 67--101,
   <a href="https://doi.org/10.3138/X687-1574-4325-WM62">Conformal
   Projections Based On Jacobian Elliptic Functions</a>;
   <a href="https://archive.org/details/conformalproject0000leel/page/92">
   borrow from archive.org</a>.
 - C. F. F. Karney,
   <a href="https://doi.org/10.1007/s00190-011-0445-3">
   Transverse Mercator with an accuracy of a few nanometers</a>,
   J. Geodesy 85(8), 475--485 (Aug. 2011);
   addenda: <a href="https://geographiclib.sourceforge.io/tm-addenda.html">
   tm-addenda.html</a>;
   preprint:
   <a href="https://arxiv.org/abs/1002.1417"> arXiv:1002.1417</a>;
   resource page:
   <a href="https://geographiclib.sourceforge.io/tm.html"> tm.html</a>.
 .
The algorithm for TransverseMercator is based on
Kr&uuml;ger (1912); that for TransverseMercatorExact is
based on Lee (1976).

See <a href="https://geographiclib.sourceforge.io/tm-grid.kmz"
type="application/vnd.google-earth.kmz"> tm-grid.kmz</a>, for an
illustration of the exact transverse Mercator grid in Google Earth.

\section testmerc Test data for the transverse Mercator projection

A test set for the transverse Mercator projection is available at
 - <a href="https://sourceforge.net/projects/geographiclib/files/testdata/TMcoords.dat.gz">
   TMcoords.dat.gz</a>
 - C. F. F. Karney, <i>Test data for the transverse Mercator projection</i>
   (2009),  <br>
   DOI: <a href="https://doi.org/10.5281/zenodo.32470">
   10.5281/zenodo.32470</a>.
 .
This is about 17 MB (compressed).  This test set consists of a set of
geographic coordinates together with the corresponding transverse
Mercator coordinates.  The WGS84 ellipsoid is used, with central
meridian 0&deg;, central scale factor 0.9996 (the UTM value),
false easting = false northing = 0&nbsp;m.

Each line of the test set gives 6 space delimited numbers
 - latitude (degrees, exact)
 - longitude (degrees, exact &mdash; see below)
 - easting (meters, accurate to 0.1&nbsp;pm)
 - northing (meters, accurate to 0.1&nbsp;pm)
 - meridian convergence (degrees, accurate to 10<sup>&minus;18</sup> deg)
 - scale (accurate to 10<sup>&minus;20</sup>)
 .
The latitude and longitude are all multiples of 10<sup>&minus;12</sup>
deg and should be regarded as exact, except that longitude =
82.63627282416406551 should be interpreted as exactly (1 &minus; \e e)
90&deg;.  These results are computed using Lee's formulas with
<a href="https://en.wikipedia.org/wiki/Maxima_(software)">Maxima</a>'s
bfloats and fpprec set to 80 (so the errors in the data are probably 1/2
of the values quoted above).  The Maxima code,
<a href="tm.mac">tm.mac</a> and <a href="ellint.mac">ellint.mac</a>,
was used to prepare this data set is included in the distribution; you
will need to have Maxima installed to use this code.  The comments at
the top of <a href="tm.mac">tm.mac</a> illustrate how to run it.

The contents of the file are as follows:
 - 250000 entries randomly distributed in lat in [0&deg;, 90&deg;], lon
   in [0&deg;, 90&deg;];
 - 1000 entries randomly distributed on lat in [0&deg;, 90&deg;], lon =
   0&deg;;
 - 1000 entries randomly distributed on lat = 0&deg;, lon in [0&deg;,
   90&deg;];
 - 1000 entries randomly distributed on lat in [0&deg;, 90&deg;], lon =
   90&deg;;
 - 1000 entries close to lat = 90&deg; with lon in [0&deg;, 90&deg;];
 - 1000 entries close to lat = 0&deg;, lon = 0&deg; with lat &ge;
   0&deg;, lon &ge; 0&deg;;
 - 1000 entries close to lat = 0&deg;, lon = 90&deg; with lat &ge;
   0&deg;, lon &le; 90&deg;;
 - 2000 entries close to lat = 0&deg;, lon = (1 &minus; \e e) 90&deg;
   with lat &ge; 0&deg;;
 - 25000 entries randomly distributed in lat in [&minus;89&deg;,
   0&deg;], lon in [(1 &minus; \e e) 90&deg;, 90&deg;];
 - 1000 entries randomly distributed on lat in [&minus;89&deg;, 0&deg;],
   lon = 90&deg;;
 - 1000 entries randomly distributed on lat in [&minus;89&deg;, 0&deg;],
   lon = (1 &minus; \e e) 90&deg;;
 - 1000 entries close to lat = 0&deg;, lon = 90&deg; (lat &lt; 0&deg;, lon
   &le; 90&deg;);
 - 1000 entries close to lat = 0&deg;, lon = (1 &minus; \e e) 90&deg;
   (lat &lt; 0&deg;, lon &le; (1 &minus; \e e) 90&deg;);
 .
(a total of 287000 entries).  The entries for lat &lt; 0&deg; and
lon in [(1 &minus; \e e) 90&deg;, 90&deg;] use the "extended"
domain for the transverse Mercator projection explained in Sec. 5 of
Karney (2011).  The first
258000 entries have lat &ge; 0&deg; and are suitable for testing
implementations following the standard convention.

\section tmseries Series approximation for transverse Mercator

Kr&uuml;ger (1912) gives a 4th-order approximation to the transverse
Mercator projection.  This is accurate to about 200&nbsp;nm (200
nanometers) within the UTM domain.  Here we present the series
extended to 10th order.  By default, TransverseMercator
uses the 6th-order approximation. The preprocessor macro
GEOGRAPHICLIB_TRANSVERSEMERCATOR_ORDER can be used to select an order
from 4 thru 8.  The series expanded to order <i>n</i><sup>30</sup> are
given in <a href="tmseries30.html"> tmseries30.html</a>.

In the formulas below ^ indicates exponentiation (<i>n</i>^3 =
<i>n</i>*<i>n</i>*<i>n</i>) and / indicates real division (3/5 = 0.6).
The equations need to be converted to Horner form, but are here left in
expanded form so that they can be easily truncated to lower order in \e
n.  Some of the integers here are not representable as 32-bit integers
and will need to be included as 64-bit integers.

\e A in Kr&uuml;ger, p. 12, eq. (5)
\verbatim
  A = a/(n + 1) * (1 + 1/4 * n^2
                     + 1/64 * n^4
                     + 1/256 * n^6
                     + 25/16384 * n^8
                     + 49/65536 * n^10);
\endverbatim

&gamma; in Kr&uuml;ger, p. 21, eq. (41)
\verbatim
alpha[1] =   1/2 * n
           - 2/3 * n^2
           + 5/16 * n^3
           + 41/180 * n^4
           - 127/288 * n^5
           + 7891/37800 * n^6
           + 72161/387072 * n^7
           - 18975107/50803200 * n^8
           + 60193001/290304000 * n^9
           + 134592031/1026432000 * n^10;
alpha[2] =   13/48 * n^2
           - 3/5 * n^3
           + 557/1440 * n^4
           + 281/630 * n^5
           - 1983433/1935360 * n^6
           + 13769/28800 * n^7
           + 148003883/174182400 * n^8
           - 705286231/465696000 * n^9
           + 1703267974087/3218890752000 * n^10;
alpha[3] =   61/240 * n^3
           - 103/140 * n^4
           + 15061/26880 * n^5
           + 167603/181440 * n^6
           - 67102379/29030400 * n^7
           + 79682431/79833600 * n^8
           + 6304945039/2128896000 * n^9
           - 6601904925257/1307674368000 * n^10;
alpha[4] =   49561/161280 * n^4
           - 179/168 * n^5
           + 6601661/7257600 * n^6
           + 97445/49896 * n^7
           - 40176129013/7664025600 * n^8
           + 138471097/66528000 * n^9
           + 48087451385201/5230697472000 * n^10;
alpha[5] =   34729/80640 * n^5
           - 3418889/1995840 * n^6
           + 14644087/9123840 * n^7
           + 2605413599/622702080 * n^8
           - 31015475399/2583060480 * n^9
           + 5820486440369/1307674368000 * n^10;
alpha[6] =   212378941/319334400 * n^6
           - 30705481/10378368 * n^7
           + 175214326799/58118860800 * n^8
           + 870492877/96096000 * n^9
           - 1328004581729009/47823519744000 * n^10;
alpha[7] =   1522256789/1383782400 * n^7
           - 16759934899/3113510400 * n^8
           + 1315149374443/221405184000 * n^9
           + 71809987837451/3629463552000 * n^10;
alpha[8] =   1424729850961/743921418240 * n^8
           - 256783708069/25204608000 * n^9
           + 2468749292989891/203249958912000 * n^10;
alpha[9] =   21091646195357/6080126976000 * n^9
           - 67196182138355857/3379030566912000 * n^10;
alpha[10]=   77911515623232821/12014330904576000 * n^10;
\endverbatim

&beta; in Kr&uuml;ger, p. 18, eq. (26*)
\verbatim
 beta[1] =   1/2 * n
           - 2/3 * n^2
           + 37/96 * n^3
           - 1/360 * n^4
           - 81/512 * n^5
           + 96199/604800 * n^6
           - 5406467/38707200 * n^7
           + 7944359/67737600 * n^8
           - 7378753979/97542144000 * n^9
           + 25123531261/804722688000 * n^10;
 beta[2] =   1/48 * n^2
           + 1/15 * n^3
           - 437/1440 * n^4
           + 46/105 * n^5
           - 1118711/3870720 * n^6
           + 51841/1209600 * n^7
           + 24749483/348364800 * n^8
           - 115295683/1397088000 * n^9
           + 5487737251099/51502252032000 * n^10;
 beta[3] =   17/480 * n^3
           - 37/840 * n^4
           - 209/4480 * n^5
           + 5569/90720 * n^6
           + 9261899/58060800 * n^7
           - 6457463/17740800 * n^8
           + 2473691167/9289728000 * n^9
           - 852549456029/20922789888000 * n^10;
 beta[4] =   4397/161280 * n^4
           - 11/504 * n^5
           - 830251/7257600 * n^6
           + 466511/2494800 * n^7
           + 324154477/7664025600 * n^8
           - 937932223/3891888000 * n^9
           - 89112264211/5230697472000 * n^10;
 beta[5] =   4583/161280 * n^5
           - 108847/3991680 * n^6
           - 8005831/63866880 * n^7
           + 22894433/124540416 * n^8
           + 112731569449/557941063680 * n^9
           - 5391039814733/10461394944000 * n^10;
 beta[6] =   20648693/638668800 * n^6
           - 16363163/518918400 * n^7
           - 2204645983/12915302400 * n^8
           + 4543317553/18162144000 * n^9
           + 54894890298749/167382319104000 * n^10;
 beta[7] =   219941297/5535129600 * n^7
           - 497323811/12454041600 * n^8
           - 79431132943/332107776000 * n^9
           + 4346429528407/12703122432000 * n^10;
 beta[8] =   191773887257/3719607091200 * n^8
           - 17822319343/336825216000 * n^9
           - 497155444501631/1422749712384000 * n^10;
 beta[9] =   11025641854267/158083301376000 * n^9
           - 492293158444691/6758061133824000 * n^10;
 beta[10]=   7028504530429621/72085985427456000 * n^10;
\endverbatim

The high-order expansions for &alpha; and &beta; were produced by the
Maxima program <a href="tmseries.mac"> tmseries.mac</a> (included in the
distribution).  To run, start Maxima and enter
\verbatim
  load("tmseries.mac")$
\endverbatim
Further instructions are included at the top of the file.

\section tmfigures Figures from paper on transverse Mercator projection

This section gives color versions of the figures in
 - C. F. F. Karney,
   <a href="https://doi.org/10.1007/s00190-011-0445-3">
   Transverse Mercator with an accuracy of a few nanometers</a>,
   J. Geodesy 85(8), 475--485 (Aug. 2011);
   addenda: <a href="https://geographiclib.sourceforge.io/tm-addenda.html">
   tm-addenda.html</a>;
   preprint:
   <a href="https://arxiv.org/abs/1002.1417"> arXiv:1002.1417</a>;
   resource page:
   <a href="https://geographiclib.sourceforge.io/tm.html"> tm.html</a>.

\b NOTE:
The numbering of the figures matches that in the paper cited above.  The
figures in this section are relatively small in order to allow them to
be displayed quickly.  Vector versions of these figures are available in
<a href="https://geographiclib.sourceforge.io/tm-figs.pdf"
type="application/pdf"> tm-figs.pdf</a>.  This allows you to magnify the
figures to show the details more clearly.

\image html gauss-schreiber-graticule-a.png "Fig. 1(a)"
Fig. 1(a):
The graticule for the spherical transverse Mercator projection.
The equator lies on \e y = 0.
Compare this with Lee, Fig. 1 (right), which shows the graticule for
half a sphere, but note that in his notation \e x and \e y have switched
meanings.  The graticule for the ellipsoid differs from that for a
sphere only in that the latitude lines have shifted slightly.  (The
conformal transformation from an ellipsoid to a sphere merely relabels
the lines of latitude.)  This projection places the point latitude =
0&deg;, longitude = 90&deg; at infinity.

\image html gauss-krueger-graticule-a.png "Fig. 1(b)"
Fig. 1(b):
The graticule for the Gauss-Kr&uuml;ger transverse Mercator projection.
The equator lies on \e y = 0 for longitude &lt; 81&deg;; beyond
this, it arcs up to meet \e y = 1.  Compare this with Lee, Fig. 45
(upper), which shows the graticule for the International Ellipsoid.  Lee,
Fig. 46, shows the graticule for the entire ellipsoid.  This projection
(like the Thompson projection) projects the ellipsoid to a finite area.

\image html thompson-tm-graticule-a.png "Fig. 1(c)"
Fig. 1(c):
The graticule for the Thompson transverse Mercator projection.  The
equator lies on \e y = 0 for longitude &lt; 81&deg;; at longitude =
81&deg;, it turns by 120&deg; and heads for \e y = 1.
Compare this with Lee, Fig. 43, which shows the graticule for the
International Ellipsoid.  Lee, Fig. 44, shows the graticule for the
entire ellipsoid.  This projection (like the Gauss-Kr&uuml;ger
projection) projects the ellipsoid to a finite area.

\image html gauss-krueger-error.png "Fig. 2"
Fig. 2:
The truncation error for the series for the Gauss-Kr&uuml;ger transverse
Mercator projection.  The blue curves show the truncation error for the
order of the series \e J = 2 (top) thru \e J = 12 (bottom).  The red
curves show the combined truncation and round-off errors for
 - float and \e J = 4 (top)
 - double and \e J = 6 (middle)
 - long double and \e J = 8 (bottom)

\image html gauss-krueger-graticule.png "Fig. 3(a)"
Fig. 3(a):
The graticule for the extended domain.  The blue lines show
latitude and longitude at multiples of 10&deg;.  The green lines
show 1&deg; intervals for longitude in [80&deg;, 90&deg;] and latitude in
[&minus;5&deg;, 10&deg;].

\image html gauss-krueger-convergence-scale.png "Fig. 3(b)"
Fig. 3(b):
The convergence and scale for the Gauss-Kr&uuml;ger
transverse Mercator projection in the extended domain.  The blue lines
emanating from the top left corner (the north pole) are lines of
constant convergence.  Convergence = 0&deg; is given by the
dog-legged line joining the points (0,1), (0,0), (1.71,0),
(1.71,&minus;&infin;).
Convergence = 90&deg; is given by the line y = 1.  The other
lines show multiples of 10&deg; between 0&deg; and
90&deg;.  The other blue, the green and the black lines show
scale = 1 thru 2 at intervals of 0.1, 2 thru 15 at intervals of 1, and
15 thru 35 at intervals of 5.  Multiples of 5 are shown in black,
multiples of 1 are shown in blue, and the rest are shown in green.
Scale = 1 is given by the line segment (0,0) to (0,1).  The red line
shows the equator between lon = 81&deg; and 90&deg;.  The
scale and convergence at the branch point are 1/\e e = 10 and
0&deg;, respectively.

\image html thompson-tm-graticule.png "Fig. 3(c)"
Fig. 3(c):
The graticule for the Thompson transverse Mercator
projection for the extended domain.  The range of the projection is the
rectangular region shown
 - 0 &le; \e u &le; K(<i>e</i><sup>2</sup>),
   0 &le; \e v &le; K(1 &minus; <i>e</i><sup>2</sup>)
 .
The coloring of the lines is the same as Fig. 3(a), except that latitude
lines extended down to &minus;10&deg; and a red line has been added
showing the line \e y = 0 for \e x &gt; 1.71 in the Gauss-Kr&uuml;ger
projection (Fig. 3(a)).  The extended Thompson projection figure has
reflection symmetry on all the four sides of Fig. 3(c).

<center>
Back to \ref greatellipse.  Forward to \ref geocentric.  Up to \ref contents.
</center>

**********************************************************************/
/**
\page geocentric Geocentric coordinates

<center>
Back to \ref transversemercator.  Forward to \ref auxlat.  Up to \ref contents.
</center>

The implementation of Geocentric::Reverse is adapted from
 - H. Vermeille,
   <a href="https://doi.org/10.1007/s00190-002-0273-6">
   Direct transformation from geocentric coordinates to geodetic
   coordinates</a>, J. Geodesy 76, 451--454 (2002).

This provides a closed-form solution but can't directly be applied close to
the center of the earth.  Several changes have been made to remove this
restriction and to improve the numerical accuracy.  Now the method is
accurate for all inputs (even if \e h is infinite).  The changes are
described in Appendix B of
 - C. F. F. Karney,
   <a href="https://arxiv.org/abs/1102.1215v1">Geodesics
   on an ellipsoid of revolution</a>,
   Feb. 2011; preprint
   <a href="https://arxiv.org/abs/1102.1215v1">arxiv:1102.1215v1</a>.
 .
Vermeille similarly updated his method in
 - H. Vermeille,
   <a href="https://doi.org/10.1007/s00190-010-0419-x">
   An analytical method to transform geocentric into
   geodetic coordinates</a>, J. Geodesy 85, 105--117 (2011).

The problems encountered near the center of the ellipsoid are:
 - There's a potential division by zero in the definition of \e s. The
   equations are easily reformulated to avoid this problem.
 - <i>t</i><sup>3</sup> may be negative.  This is OK; we just take the
   real root.
 - The solution for \e t may be complex.  However this leads to 3 real roots
   for <i>u</i>/\e r.  It's then just a matter of picking the one that computes
   the geodetic result which minimizes |<i>h</i>| and which avoids large
   round-off errors.
 - Some of the equations result in a large loss of accuracy due to
   subtracting nearly equal quantities.  E.g., \e k = sqrt(\e u + \e v +
   <i>w</i><sup>2</sup>) &minus; \e w is inaccurate if \e u + \e v is small;
   we can fix this by writing \e k = (\e u + \e v)/(sqrt(\e u + \e v +
   <i>w</i><sup>2</sup>) + \e w).

The error is computed as follows.  Write a version of
Geocentric::WGS84.Forward which uses long doubles (including using long
doubles for the WGS84 parameters).  Generate random (long double)
geodetic coordinates (\e lat0, \e lon0, \e h0) and use the "long double"
WGS84.Forward to obtain the corresponding (long double) geocentric
coordinates (\e x0, \e y0, \e z0).  [We restrict \e h0 so that \e h0
&ge; &minus; \e a (1 &minus; <i>e</i><sup>2</sup>) / sqrt(1 &minus;
<i>e</i><sup>2</sup> sin<sup>2</sup>\e lat0), which ensures that (\e
lat0, \e lon0, \e h0) is the principal geodetic inverse of (\e x0, \e
y0, \e z0).]  Because the forward calculation is numerically stable and
because long doubles (on Linux systems using g++) provide 11 bits
additional accuracy (about 3.3 decimal digits), we regard this set of
test data as exact.

Apply the double version of WGS84.Reverse to (\e x0, \e y0, \e z0) to
compute the approximate geodetic coordinates (\e lat1, \e lon1, \e h1).
Convert (\e lat1 &minus; \e lat0, \e lon1 &minus; \e lon0) to a
distance, \e ds, on the surface of the ellipsoid and define \e err =
hypot(\e ds, \e h1 &minus; \e h0).  For |<i>h0</i>| &lt; 5000&nbsp;km, we
have \e err &lt; 7&nbsp;nm (7 nanometers).

This methodology is not very useful very far from the globe, because the
absolute errors in the approximate geodetic height become large, or
within 50&nbsp;km of the center of the earth, because of errors in
computing the approximate geodetic latitude.  To illustrate the second
issue, the maximum value of \e err for \e h0 &lt; 0 is about 80&nbsp;mm.
The error is maximum close to the circle given by geocentric coordinates
satisfying hypot(\e x, \e y) = \e a <i>e</i><sup>2</sup> (=
42.7&nbsp;km), \e z = 0.  (This is the center of meridional curvature
for \e lat = 0.)  The geodetic latitude for these points is \e lat = 0.
However, if we move 1&nbsp;nm towards the center of the earth, the
geodetic latitude becomes 0.04", a distance of 1.4&nbsp;m from the
equator.  If, instead, we move 1&nbsp;nm up, the geodetic latitude
becomes 7.45", a distance of 229&nbsp;m from the equator.  In light of
this, Reverse does quite well in this vicinity.

To obtain a practical measure of the error for the general case we define
- <i>err</i><sub>h</sub> = |\e h1 &minus; <i>h0</i>| / max(1, \e h0 / \e a)
- for \e h0 &gt; 0, <i>err</i><sub>out</sub> = \e ds
- for \e h0 &lt; 0, apply the long double version of WGS84.Forward to (\e
  lat1, \e lon1, \e h1) to give (\e x1, \e y1, \e z1) and compute
  <i>err</i><sub>in</sub> = hypot(\e x1 &minus; \e x0, \e y1 &minus; \e
  y0, \e z1 &minus; \e z0).
.
We then find <i>err</i><sub>h</sub> &lt; 8&nbsp;nm,
<i>err</i><sub>out</sub> &lt; 4&nbsp;nm, and <i>err</i><sub>in</sub> <
7&nbsp;nm (1&nbsp;nm = 1 nanometer).

The testing has been confined to the WGS84 ellipsoid.  The method will
work for all ellipsoids used in terrestrial geodesy.  However, the
central region, which leads to multiple real roots for the cubic
equation in Reverse, pokes outside the ellipsoid (at the poles) for
ellipsoids with \e e &gt; 1/sqrt(2); Reverse has not been analyzed for this
case.  Similarly ellipsoids which are very nearly spherical may yield
inaccurate results due to underflow; in the other hand, the case of the
sphere, \e f = 0, is treated specially and gives accurate results.

Other comparable methods are K. M. Borkowski,
<a href="https://doi.org/10.1007/BF00643807"> Transformation
of geocentric to geodetic coordinates without approximations</a>,
Astrophys. Space Sci. 139, 1--4 (1987)
(<a href="https://doi.org/10.1007/BF00656995"> erratum</a>)
and T. Fukushima,
<a href="https://doi.org/10.1007/s001900050271"> Fast transform from
geocentric to geodetic coordinates</a>, J. Geodesy 73, 603--610 (1999).
However the choice of independent variables in these methods leads
to a loss of accuracy for points near the equatorial plane.

<center>
Back to \ref transversemercator.  Forward to \ref auxlat.  Up to \ref contents.
</center>

**********************************************************************/
/**
\page auxlat Auxiliary latitudes

<center>
Back to \ref geocentric.  Forward to \ref highprec.  Up to \ref contents.
</center>

Go to
 - \ref auxlatformula
 - \ref auxlattable
 - \ref auxlaterror

Six latitudes are used by GeographicLib:
- &phi;, the (geographic) latitude;
- &beta;, the parametric latitude;
- &theta;, the geocentric latitude;
- &mu;, the rectifying latitude;
- &chi;, the conformal latitude;
- &xi;, the authalic latitude.
.
The last five of these are called <i>auxiliary latitudes</i>.  These
quantities are all defined in the
<a href="https://en.wikipedia.org/wiki/Latitude#Auxiliary_latitudes">
Wikipedia article on latitudes</a>.  The Ellipsoid class contains
methods for converting all of these and the geographic latitude.

In addition there's the isometric latitude, &psi;, defined by &psi; =
gd<sup>&minus;1</sup>&chi; = sinh<sup>&minus;1</sup>&nbsp;tan&chi; and
&chi; = gd&psi; = tan<sup>&minus;1</sup>&nbsp;sinh&psi;.  This is not an
angle-like variable (for example, it diverges at the poles) and so we
don't treat it further here.  However conversions between &psi; and any
of the auxiliary latitudes is easily accomplished via an intermediate
conversion to &chi;.

The relations between &phi;, &beta;, and &theta; are all simple
elementary functions.  The latitudes &chi; and &xi; can be expressed as
elementary functions of &phi;; however, these functions can only be
inverted iteratively.  The rectifying latitude &mu; as a function of
&phi; (or &beta;) involves the incomplete elliptic integral of the
second kind (which is not an elementary function) and this needs to be
inverted iteratively.  The Ellipsoid class evaluates all the auxiliary
latitudes (and the corresponding inverse relations) in terms of their
basic definitions.

An alternative method of evaluating these auxiliary latitudes is in
terms of trigonometric series.  This offers some advantages:
- these series give a uniform way of expressing any latitude in terms of
  any other latitude;
- the evaluation may be faster, particularly if
  <a href="https://en.wikipedia.org/wiki/Clenshaw_algorithm#Meridian_arc_length_on_the_ellipsoid">
  Clenshaw summation</a> is used;
- provided that the flattening is sufficiently small, the result may be
  more accurate (because the round-off errors are smaller).

Here we give the complete matrix of relations between all six latitudes;
there are 30 (=&nbsp;6&nbsp;&times;&nbsp;5) such relations.  These
expansions complement the work of
- O. S. Adams,
  <a href="https://docs.lib.noaa.gov/rescue/cgs_specpubs/QB275U35no671921.pdf">
  Latitude developments connected with geodesy and cartography</a>,
  Spec. Pub. 67 (US Coast and Geodetic Survey, 1921).
- P. Osborne,
  <a href="https://dx.doi.org/10.5281/zenodo.35392">
  The Mercator Projections</a> (2013), Chap. 5.
- S. Orihuela,
  <a href="https://sites.google.com/site/geodesiafich/funciones_latitud.pdf">
  Funciones de Latitud</a> (2013).
.
Here, the expansions are in terms of the third flattening <i>n</i> =
(<i>a</i>&nbsp;&minus;&nbsp;<i>b</i>)/(<i>a</i>&nbsp;+&nbsp;<i>b</i>).
This choice of expansion parameter results in expansions in which half
the coefficients vanish for all relations between &phi;, &beta;,
&theta;, and &mu;.  In addition, the expansions converge for |<i>n</i>|
&lt; 1 or <i>b</i>/<i>a</i> &isin; (0,&nbsp;&infin;).  These expansions
were obtained with the the maxima code, <a
href="auxlat.mac">auxlat.mac</a>.

Adams (1921) uses the eccentricity squared <i>e</i><sup>2</sup> as the
expansion parameter, but the resulting series only converge for
|<i>e</i><sup>2</sup>| &lt; 1 or <i>b</i>/<i>a</i> &isin;
(0,&nbsp;&radic;2).  In addition, it is shown in \ref auxlaterror, that
the errors when the series are truncated are much worse than for the
corresponding series in \e n.

\b NOTE: The assertions about convergence above are too optimistic.
Some of the series do indeed converge for |<i>n</i>| &lt; 1.  However
others have a smaller radius of convergence.  More on this later&hellip;

\section auxlatformula Series approximations for conversions

Here are the relations between &phi;, &beta;, &theta;, and &mu; carried
out to 4th order in <i>n</i>:
\f[
\begin{align}
\beta-\phi&=\textstyle{}
-n\sin 2\phi
+\frac{1}{2}n^{2}\sin 4\phi
-\frac{1}{3}n^{3}\sin 6\phi
+\frac{1}{4}n^{4}\sin 8\phi
-\ldots\\
\phi-\beta&=\textstyle{}
+n\sin 2\beta
+\frac{1}{2}n^{2}\sin 4\beta
+\frac{1}{3}n^{3}\sin 6\beta
+\frac{1}{4}n^{4}\sin 8\beta
+\ldots\\
\theta-\phi&=\textstyle{}
-\bigl(2n-2n^{3}\bigr)\sin 2\phi
+\bigl(2n^{2}-4n^{4}\bigr)\sin 4\phi
-\frac{8}{3}n^{3}\sin 6\phi
+4n^{4}\sin 8\phi
-\ldots\\
\phi-\theta&=\textstyle{}
+\bigl(2n-2n^{3}\bigr)\sin 2\theta
+\bigl(2n^{2}-4n^{4}\bigr)\sin 4\theta
+\frac{8}{3}n^{3}\sin 6\theta
+4n^{4}\sin 8\theta
+\ldots\\
\theta-\beta&=\textstyle{}
-n\sin 2\beta
+\frac{1}{2}n^{2}\sin 4\beta
-\frac{1}{3}n^{3}\sin 6\beta
+\frac{1}{4}n^{4}\sin 8\beta
-\ldots\\
\beta-\theta&=\textstyle{}
+n\sin 2\theta
+\frac{1}{2}n^{2}\sin 4\theta
+\frac{1}{3}n^{3}\sin 6\theta
+\frac{1}{4}n^{4}\sin 8\theta
+\ldots\\
\mu-\phi&=\textstyle{}
-\bigl(\frac{3}{2}n-\frac{9}{16}n^{3}\bigr)\sin 2\phi
+\bigl(\frac{15}{16}n^{2}-\frac{15}{32}n^{4}\bigr)\sin 4\phi
-\frac{35}{48}n^{3}\sin 6\phi
+\frac{315}{512}n^{4}\sin 8\phi
-\ldots\\
\phi-\mu&=\textstyle{}
+\bigl(\frac{3}{2}n-\frac{27}{32}n^{3}\bigr)\sin 2\mu
+\bigl(\frac{21}{16}n^{2}-\frac{55}{32}n^{4}\bigr)\sin 4\mu
+\frac{151}{96}n^{3}\sin 6\mu
+\frac{1097}{512}n^{4}\sin 8\mu
+\ldots\\
\mu-\beta&=\textstyle{}
-\bigl(\frac{1}{2}n-\frac{3}{16}n^{3}\bigr)\sin 2\beta
-\bigl(\frac{1}{16}n^{2}-\frac{1}{32}n^{4}\bigr)\sin 4\beta
-\frac{1}{48}n^{3}\sin 6\beta
-\frac{5}{512}n^{4}\sin 8\beta
-\ldots\\
\beta-\mu&=\textstyle{}
+\bigl(\frac{1}{2}n-\frac{9}{32}n^{3}\bigr)\sin 2\mu
+\bigl(\frac{5}{16}n^{2}-\frac{37}{96}n^{4}\bigr)\sin 4\mu
+\frac{29}{96}n^{3}\sin 6\mu
+\frac{539}{1536}n^{4}\sin 8\mu
+\ldots\\
\mu-\theta&=\textstyle{}
+\bigl(\frac{1}{2}n+\frac{13}{16}n^{3}\bigr)\sin 2\theta
-\bigl(\frac{1}{16}n^{2}-\frac{33}{32}n^{4}\bigr)\sin 4\theta
-\frac{5}{16}n^{3}\sin 6\theta
-\frac{261}{512}n^{4}\sin 8\theta
-\ldots\\
\theta-\mu&=\textstyle{}
-\bigl(\frac{1}{2}n+\frac{23}{32}n^{3}\bigr)\sin 2\mu
+\bigl(\frac{5}{16}n^{2}-\frac{5}{96}n^{4}\bigr)\sin 4\mu
+\frac{1}{32}n^{3}\sin 6\mu
+\frac{283}{1536}n^{4}\sin 8\mu
+\ldots\\
\end{align}
\f]

Here are the remaining relations (including &chi; and &xi;) carried out
to 3rd order in <i>n</i>:
\f[
\begin{align}
\chi-\phi&=\textstyle{}
-\bigl(2n-\frac{2}{3}n^{2}-\frac{4}{3}n^{3}\bigr)\sin 2\phi
+\bigl(\frac{5}{3}n^{2}-\frac{16}{15}n^{3}\bigr)\sin 4\phi
-\frac{26}{15}n^{3}\sin 6\phi
+\ldots\\
\phi-\chi&=\textstyle{}
+\bigl(2n-\frac{2}{3}n^{2}-2n^{3}\bigr)\sin 2\chi
+\bigl(\frac{7}{3}n^{2}-\frac{8}{5}n^{3}\bigr)\sin 4\chi
+\frac{56}{15}n^{3}\sin 6\chi
+\ldots\\
\chi-\beta&=\textstyle{}
-\bigl(n-\frac{2}{3}n^{2}\bigr)\sin 2\beta
+\bigl(\frac{1}{6}n^{2}-\frac{2}{5}n^{3}\bigr)\sin 4\beta
-\frac{1}{15}n^{3}\sin 6\beta
+\ldots\\
\beta-\chi&=\textstyle{}
+\bigl(n-\frac{2}{3}n^{2}-\frac{1}{3}n^{3}\bigr)\sin 2\chi
+\bigl(\frac{5}{6}n^{2}-\frac{14}{15}n^{3}\bigr)\sin 4\chi
+\frac{16}{15}n^{3}\sin 6\chi
+\ldots\\
\chi-\theta&=\textstyle{}
+\bigl(\frac{2}{3}n^{2}+\frac{2}{3}n^{3}\bigr)\sin 2\theta
-\bigl(\frac{1}{3}n^{2}-\frac{4}{15}n^{3}\bigr)\sin 4\theta
-\frac{2}{5}n^{3}\sin 6\theta
-\ldots\\
\theta-\chi&=\textstyle{}
-\bigl(\frac{2}{3}n^{2}+\frac{2}{3}n^{3}\bigr)\sin 2\chi
+\bigl(\frac{1}{3}n^{2}-\frac{4}{15}n^{3}\bigr)\sin 4\chi
+\frac{2}{5}n^{3}\sin 6\chi
+\ldots\\
\chi-\mu&=\textstyle{}
-\bigl(\frac{1}{2}n-\frac{2}{3}n^{2}+\frac{37}{96}n^{3}\bigr)\sin 2\mu
-\bigl(\frac{1}{48}n^{2}+\frac{1}{15}n^{3}\bigr)\sin 4\mu
-\frac{17}{480}n^{3}\sin 6\mu
-\ldots\\
\mu-\chi&=\textstyle{}
+\bigl(\frac{1}{2}n-\frac{2}{3}n^{2}+\frac{5}{16}n^{3}\bigr)\sin 2\chi
+\bigl(\frac{13}{48}n^{2}-\frac{3}{5}n^{3}\bigr)\sin 4\chi
+\frac{61}{240}n^{3}\sin 6\chi
+\ldots\\
\xi-\phi&=\textstyle{}
-\bigl(\frac{4}{3}n+\frac{4}{45}n^{2}-\frac{88}{315}n^{3}\bigr)\sin 2\phi
+\bigl(\frac{34}{45}n^{2}+\frac{8}{105}n^{3}\bigr)\sin 4\phi
-\frac{1532}{2835}n^{3}\sin 6\phi
+\ldots\\
\phi-\xi&=\textstyle{}
+\bigl(\frac{4}{3}n+\frac{4}{45}n^{2}-\frac{16}{35}n^{3}\bigr)\sin 2\xi
+\bigl(\frac{46}{45}n^{2}+\frac{152}{945}n^{3}\bigr)\sin 4\xi
+\frac{3044}{2835}n^{3}\sin 6\xi
+\ldots\\
\xi-\beta&=\textstyle{}
-\bigl(\frac{1}{3}n+\frac{4}{45}n^{2}-\frac{32}{315}n^{3}\bigr)\sin 2\beta
-\bigl(\frac{7}{90}n^{2}+\frac{4}{315}n^{3}\bigr)\sin 4\beta
-\frac{83}{2835}n^{3}\sin 6\beta
-\ldots\\
\beta-\xi&=\textstyle{}
+\bigl(\frac{1}{3}n+\frac{4}{45}n^{2}-\frac{46}{315}n^{3}\bigr)\sin 2\xi
+\bigl(\frac{17}{90}n^{2}+\frac{68}{945}n^{3}\bigr)\sin 4\xi
+\frac{461}{2835}n^{3}\sin 6\xi
+\ldots\\
\xi-\theta&=\textstyle{}
+\bigl(\frac{2}{3}n-\frac{4}{45}n^{2}+\frac{62}{105}n^{3}\bigr)\sin 2\theta
+\bigl(\frac{4}{45}n^{2}-\frac{32}{315}n^{3}\bigr)\sin 4\theta
-\frac{524}{2835}n^{3}\sin 6\theta
-\ldots\\
\theta-\xi&=\textstyle{}
-\bigl(\frac{2}{3}n-\frac{4}{45}n^{2}+\frac{158}{315}n^{3}\bigr)\sin 2\xi
+\bigl(\frac{16}{45}n^{2}-\frac{16}{945}n^{3}\bigr)\sin 4\xi
-\frac{232}{2835}n^{3}\sin 6\xi
+\ldots\\
\xi-\mu&=\textstyle{}
+\bigl(\frac{1}{6}n-\frac{4}{45}n^{2}-\frac{817}{10080}n^{3}\bigr)\sin 2\mu
+\bigl(\frac{49}{720}n^{2}-\frac{2}{35}n^{3}\bigr)\sin 4\mu
+\frac{4463}{90720}n^{3}\sin 6\mu
+\ldots\\
\mu-\xi&=\textstyle{}
-\bigl(\frac{1}{6}n-\frac{4}{45}n^{2}-\frac{121}{1680}n^{3}\bigr)\sin 2\xi
-\bigl(\frac{29}{720}n^{2}-\frac{26}{945}n^{3}\bigr)\sin 4\xi
-\frac{1003}{45360}n^{3}\sin 6\xi
-\ldots\\
\xi-\chi&=\textstyle{}
+\bigl(\frac{2}{3}n-\frac{34}{45}n^{2}+\frac{46}{315}n^{3}\bigr)\sin 2\chi
+\bigl(\frac{19}{45}n^{2}-\frac{256}{315}n^{3}\bigr)\sin 4\chi
+\frac{248}{567}n^{3}\sin 6\chi
+\ldots\\
\chi-\xi&=\textstyle{}
-\bigl(\frac{2}{3}n-\frac{34}{45}n^{2}+\frac{88}{315}n^{3}\bigr)\sin 2\xi
+\bigl(\frac{1}{45}n^{2}-\frac{184}{945}n^{3}\bigr)\sin 4\xi
-\frac{106}{2835}n^{3}\sin 6\xi
-\ldots\\
\end{align}
\f]

\section auxlattable Series approximations in tabular form

Finally, this is a listing of all the coefficients for the expansions
carried out to 8th order in <i>n</i>.  Here's how to interpret this
data: the 5th line for &phi;&nbsp;&minus;&nbsp;&theta; is <tt>[32/5, 0,
-32, 0]</tt>; this means that the coefficient of sin(10&theta;) is
[(32/5)<i>n</i><sup>5</sup>&nbsp;&minus;
32<i>n</i><sup>7</sup>&nbsp;+&nbsp;<i>O</i>(<i>n</i><sup>9</sup>)].
<p>&beta;&nbsp;&minus;&nbsp;&phi;:<br><tt><small>
&nbsp;&nbsp;&nbsp;[-1, 0, 0, 0, 0, 0, 0, 0]<br>
&nbsp;&nbsp;&nbsp;[1/2, 0, 0, 0, 0, 0, 0]<br>
&nbsp;&nbsp;&nbsp;[-1/3, 0, 0, 0, 0, 0]<br>
&nbsp;&nbsp;&nbsp;[1/4, 0, 0, 0, 0]<br>
&nbsp;&nbsp;&nbsp;[-1/5, 0, 0, 0]<br>
&nbsp;&nbsp;&nbsp;[1/6, 0, 0]<br>
&nbsp;&nbsp;&nbsp;[-1/7, 0]<br>
&nbsp;&nbsp;&nbsp;[1/8]<br>
</small></tt>
<p>&phi;&nbsp;&minus;&nbsp;&beta;:<br><tt><small>
&nbsp;&nbsp;&nbsp;[1, 0, 0, 0, 0, 0, 0, 0]<br>
&nbsp;&nbsp;&nbsp;[1/2, 0, 0, 0, 0, 0, 0]<br>
&nbsp;&nbsp;&nbsp;[1/3, 0, 0, 0, 0, 0]<br>
&nbsp;&nbsp;&nbsp;[1/4, 0, 0, 0, 0]<br>
&nbsp;&nbsp;&nbsp;[1/5, 0, 0, 0]<br>
&nbsp;&nbsp;&nbsp;[1/6, 0, 0]<br>
&nbsp;&nbsp;&nbsp;[1/7, 0]<br>
&nbsp;&nbsp;&nbsp;[1/8]<br>
</small></tt>
<p>&theta;&nbsp;&minus;&nbsp;&phi;:<br><tt><small>
&nbsp;&nbsp;&nbsp;[-2, 0, 2, 0, -2, 0, 2, 0]<br>
&nbsp;&nbsp;&nbsp;[2, 0, -4, 0, 6, 0, -8]<br>
&nbsp;&nbsp;&nbsp;[-8/3, 0, 8, 0, -16, 0]<br>
&nbsp;&nbsp;&nbsp;[4, 0, -16, 0, 40]<br>
&nbsp;&nbsp;&nbsp;[-32/5, 0, 32, 0]<br>
&nbsp;&nbsp;&nbsp;[32/3, 0, -64]<br>
&nbsp;&nbsp;&nbsp;[-128/7, 0]<br>
&nbsp;&nbsp;&nbsp;[32]<br>
</small></tt>
<p>&phi;&nbsp;&minus;&nbsp;&theta;:<br><tt><small>
&nbsp;&nbsp;&nbsp;[2, 0, -2, 0, 2, 0, -2, 0]<br>
&nbsp;&nbsp;&nbsp;[2, 0, -4, 0, 6, 0, -8]<br>
&nbsp;&nbsp;&nbsp;[8/3, 0, -8, 0, 16, 0]<br>
&nbsp;&nbsp;&nbsp;[4, 0, -16, 0, 40]<br>
&nbsp;&nbsp;&nbsp;[32/5, 0, -32, 0]<br>
&nbsp;&nbsp;&nbsp;[32/3, 0, -64]<br>
&nbsp;&nbsp;&nbsp;[128/7, 0]<br>
&nbsp;&nbsp;&nbsp;[32]<br>
</small></tt>
<p>&theta;&nbsp;&minus;&nbsp;&beta;:<br><tt><small>
&nbsp;&nbsp;&nbsp;[-1, 0, 0, 0, 0, 0, 0, 0]<br>
&nbsp;&nbsp;&nbsp;[1/2, 0, 0, 0, 0, 0, 0]<br>
&nbsp;&nbsp;&nbsp;[-1/3, 0, 0, 0, 0, 0]<br>
&nbsp;&nbsp;&nbsp;[1/4, 0, 0, 0, 0]<br>
&nbsp;&nbsp;&nbsp;[-1/5, 0, 0, 0]<br>
&nbsp;&nbsp;&nbsp;[1/6, 0, 0]<br>
&nbsp;&nbsp;&nbsp;[-1/7, 0]<br>
&nbsp;&nbsp;&nbsp;[1/8]<br>
</small></tt>
<p>&beta;&nbsp;&minus;&nbsp;&theta;:<br><tt><small>
&nbsp;&nbsp;&nbsp;[1, 0, 0, 0, 0, 0, 0, 0]<br>
&nbsp;&nbsp;&nbsp;[1/2, 0, 0, 0, 0, 0, 0]<br>
&nbsp;&nbsp;&nbsp;[1/3, 0, 0, 0, 0, 0]<br>
&nbsp;&nbsp;&nbsp;[1/4, 0, 0, 0, 0]<br>
&nbsp;&nbsp;&nbsp;[1/5, 0, 0, 0]<br>
&nbsp;&nbsp;&nbsp;[1/6, 0, 0]<br>
&nbsp;&nbsp;&nbsp;[1/7, 0]<br>
&nbsp;&nbsp;&nbsp;[1/8]<br>
</small></tt>
<p>&mu;&nbsp;&minus;&nbsp;&phi;:<br><tt><small>
&nbsp;&nbsp;&nbsp;[-3/2, 0, 9/16, 0, -3/32, 0, 57/2048, 0]<br>
&nbsp;&nbsp;&nbsp;[15/16, 0, -15/32, 0, 135/2048, 0, -105/4096]<br>
&nbsp;&nbsp;&nbsp;[-35/48, 0, 105/256, 0, -105/2048, 0]<br>
&nbsp;&nbsp;&nbsp;[315/512, 0, -189/512, 0, 693/16384]<br>
&nbsp;&nbsp;&nbsp;[-693/1280, 0, 693/2048, 0]<br>
&nbsp;&nbsp;&nbsp;[1001/2048, 0, -1287/4096]<br>
&nbsp;&nbsp;&nbsp;[-6435/14336, 0]<br>
&nbsp;&nbsp;&nbsp;[109395/262144]<br>
</small></tt>
<p>&phi;&nbsp;&minus;&nbsp;&mu;:<br><tt><small>
&nbsp;&nbsp;&nbsp;[3/2, 0, -27/32, 0, 269/512, 0, -6607/24576, 0]<br>
&nbsp;&nbsp;&nbsp;[21/16, 0, -55/32, 0, 6759/4096, 0, -155113/122880]<br>
&nbsp;&nbsp;&nbsp;[151/96, 0, -417/128, 0, 87963/20480, 0]<br>
&nbsp;&nbsp;&nbsp;[1097/512, 0, -15543/2560, 0, 2514467/245760]<br>
&nbsp;&nbsp;&nbsp;[8011/2560, 0, -69119/6144, 0]<br>
&nbsp;&nbsp;&nbsp;[293393/61440, 0, -5962461/286720]<br>
&nbsp;&nbsp;&nbsp;[6459601/860160, 0]<br>
&nbsp;&nbsp;&nbsp;[332287993/27525120]<br>
</small></tt>
<p>&mu;&nbsp;&minus;&nbsp;&beta;:<br><tt><small>
&nbsp;&nbsp;&nbsp;[-1/2, 0, 3/16, 0, -1/32, 0, 19/2048, 0]<br>
&nbsp;&nbsp;&nbsp;[-1/16, 0, 1/32, 0, -9/2048, 0, 7/4096]<br>
&nbsp;&nbsp;&nbsp;[-1/48, 0, 3/256, 0, -3/2048, 0]<br>
&nbsp;&nbsp;&nbsp;[-5/512, 0, 3/512, 0, -11/16384]<br>
&nbsp;&nbsp;&nbsp;[-7/1280, 0, 7/2048, 0]<br>
&nbsp;&nbsp;&nbsp;[-7/2048, 0, 9/4096]<br>
&nbsp;&nbsp;&nbsp;[-33/14336, 0]<br>
&nbsp;&nbsp;&nbsp;[-429/262144]<br>
</small></tt>
<p>&beta;&nbsp;&minus;&nbsp;&mu;:<br><tt><small>
&nbsp;&nbsp;&nbsp;[1/2, 0, -9/32, 0, 205/1536, 0, -4879/73728, 0]<br>
&nbsp;&nbsp;&nbsp;[5/16, 0, -37/96, 0, 1335/4096, 0, -86171/368640]<br>
&nbsp;&nbsp;&nbsp;[29/96, 0, -75/128, 0, 2901/4096, 0]<br>
&nbsp;&nbsp;&nbsp;[539/1536, 0, -2391/2560, 0, 1082857/737280]<br>
&nbsp;&nbsp;&nbsp;[3467/7680, 0, -28223/18432, 0]<br>
&nbsp;&nbsp;&nbsp;[38081/61440, 0, -733437/286720]<br>
&nbsp;&nbsp;&nbsp;[459485/516096, 0]<br>
&nbsp;&nbsp;&nbsp;[109167851/82575360]<br>
</small></tt>
<p>&mu;&nbsp;&minus;&nbsp;&theta;:<br><tt><small>
&nbsp;&nbsp;&nbsp;[1/2, 0, 13/16, 0, -15/32, 0, 509/2048, 0]<br>
&nbsp;&nbsp;&nbsp;[-1/16, 0, 33/32, 0, -1673/2048, 0, 2599/4096]<br>
&nbsp;&nbsp;&nbsp;[-5/16, 0, 349/256, 0, -2989/2048, 0]<br>
&nbsp;&nbsp;&nbsp;[-261/512, 0, 963/512, 0, -43531/16384]<br>
&nbsp;&nbsp;&nbsp;[-921/1280, 0, 5545/2048, 0]<br>
&nbsp;&nbsp;&nbsp;[-6037/6144, 0, 16617/4096]<br>
&nbsp;&nbsp;&nbsp;[-19279/14336, 0]<br>
&nbsp;&nbsp;&nbsp;[-490925/262144]<br>
</small></tt>
<p>&theta;&nbsp;&minus;&nbsp;&mu;:<br><tt><small>
&nbsp;&nbsp;&nbsp;[-1/2, 0, -23/32, 0, 499/1536, 0, -14321/73728, 0]<br>
&nbsp;&nbsp;&nbsp;[5/16, 0, -5/96, 0, 6565/12288, 0, -201467/368640]<br>
&nbsp;&nbsp;&nbsp;[1/32, 0, -77/128, 0, 2939/4096, 0]<br>
&nbsp;&nbsp;&nbsp;[283/1536, 0, -4037/7680, 0, 1155049/737280]<br>
&nbsp;&nbsp;&nbsp;[1301/7680, 0, -19465/18432, 0]<br>
&nbsp;&nbsp;&nbsp;[17089/61440, 0, -442269/286720]<br>
&nbsp;&nbsp;&nbsp;[198115/516096, 0]<br>
&nbsp;&nbsp;&nbsp;[48689387/82575360]<br>
</small></tt>
<p>&chi;&nbsp;&minus;&nbsp;&phi;:<br><tt><small>
&nbsp;&nbsp;&nbsp;[-2, 2/3, 4/3, -82/45, 32/45, 4642/4725, -8384/4725, 1514/1323]<br>
&nbsp;&nbsp;&nbsp;[5/3, -16/15, -13/9, 904/315, -1522/945, -2288/1575, 142607/42525]<br>
&nbsp;&nbsp;&nbsp;[-26/15, 34/21, 8/5, -12686/2835, 44644/14175, 120202/51975]<br>
&nbsp;&nbsp;&nbsp;[1237/630, -12/5, -24832/14175, 1077964/155925, -1097407/187110]<br>
&nbsp;&nbsp;&nbsp;[-734/315, 109598/31185, 1040/567, -12870194/1216215]<br>
&nbsp;&nbsp;&nbsp;[444337/155925, -941912/184275, -126463/72765]<br>
&nbsp;&nbsp;&nbsp;[-2405834/675675, 3463678/467775]<br>
&nbsp;&nbsp;&nbsp;[256663081/56756700]<br>
</small></tt>
<p>&phi;&nbsp;&minus;&nbsp;&chi;:<br><tt><small>
&nbsp;&nbsp;&nbsp;[2, -2/3, -2, 116/45, 26/45, -2854/675, 16822/4725, 189416/99225]<br>
&nbsp;&nbsp;&nbsp;[7/3, -8/5, -227/45, 2704/315, 2323/945, -31256/1575, 141514/8505]<br>
&nbsp;&nbsp;&nbsp;[56/15, -136/35, -1262/105, 73814/2835, 98738/14175, -2363828/31185]<br>
&nbsp;&nbsp;&nbsp;[4279/630, -332/35, -399572/14175, 11763988/155925, 14416399/935550]<br>
&nbsp;&nbsp;&nbsp;[4174/315, -144838/6237, -2046082/31185, 258316372/1216215]<br>
&nbsp;&nbsp;&nbsp;[601676/22275, -115444544/2027025, -2155215124/14189175]<br>
&nbsp;&nbsp;&nbsp;[38341552/675675, -170079376/1216215]<br>
&nbsp;&nbsp;&nbsp;[1383243703/11351340]<br>
</small></tt>
<p>&chi;&nbsp;&minus;&nbsp;&beta;:<br><tt><small>
&nbsp;&nbsp;&nbsp;[-1, 2/3, 0, -16/45, 2/5, -998/4725, -34/4725, 1384/11025]<br>
&nbsp;&nbsp;&nbsp;[1/6, -2/5, 19/45, -22/105, -2/27, 1268/4725, -12616/42525]<br>
&nbsp;&nbsp;&nbsp;[-1/15, 16/105, -22/105, 116/567, -1858/14175, 1724/51975]<br>
&nbsp;&nbsp;&nbsp;[17/1260, -8/105, 2123/14175, -26836/155925, 115249/935550]<br>
&nbsp;&nbsp;&nbsp;[-1/105, 128/4455, -424/6237, 140836/1216215]<br>
&nbsp;&nbsp;&nbsp;[149/311850, -31232/2027025, 210152/4729725]<br>
&nbsp;&nbsp;&nbsp;[-499/225225, 30208/6081075]<br>
&nbsp;&nbsp;&nbsp;[-68251/113513400]<br>
</small></tt>
<p>&beta;&nbsp;&minus;&nbsp;&chi;:<br><tt><small>
&nbsp;&nbsp;&nbsp;[1, -2/3, -1/3, 38/45, -1/3, -3118/4725, 4769/4725, -25666/99225]<br>
&nbsp;&nbsp;&nbsp;[5/6, -14/15, -7/9, 50/21, -247/270, -14404/4725, 193931/42525]<br>
&nbsp;&nbsp;&nbsp;[16/15, -34/21, -5/3, 17564/2835, -36521/14175, -1709614/155925]<br>
&nbsp;&nbsp;&nbsp;[2069/1260, -28/9, -49877/14175, 2454416/155925, -637699/85050]<br>
&nbsp;&nbsp;&nbsp;[883/315, -28244/4455, -20989/2835, 48124558/1216215]<br>
&nbsp;&nbsp;&nbsp;[797222/155925, -2471888/184275, -16969807/1091475]<br>
&nbsp;&nbsp;&nbsp;[2199332/225225, -1238578/42525]<br>
&nbsp;&nbsp;&nbsp;[87600385/4540536]<br>
</small></tt>
<p>&chi;&nbsp;&minus;&nbsp;&theta;:<br><tt><small>
&nbsp;&nbsp;&nbsp;[0, 2/3, 2/3, -2/9, -14/45, 1042/4725, 18/175, -1738/11025]<br>
&nbsp;&nbsp;&nbsp;[-1/3, 4/15, 43/45, -4/45, -712/945, 332/945, 23159/42525]<br>
&nbsp;&nbsp;&nbsp;[-2/5, 2/105, 124/105, 274/2835, -1352/945, 13102/31185]<br>
&nbsp;&nbsp;&nbsp;[-55/126, -16/105, 21068/14175, 1528/4725, -2414843/935550]<br>
&nbsp;&nbsp;&nbsp;[-22/45, -9202/31185, 20704/10395, 60334/93555]<br>
&nbsp;&nbsp;&nbsp;[-90263/155925, -299444/675675, 40458083/14189175]<br>
&nbsp;&nbsp;&nbsp;[-8962/12285, -3818498/6081075]<br>
&nbsp;&nbsp;&nbsp;[-4259027/4365900]<br>
</small></tt>
<p>&theta;&nbsp;&minus;&nbsp;&chi;:<br><tt><small>
&nbsp;&nbsp;&nbsp;[0, -2/3, -2/3, 4/9, 2/9, -3658/4725, 76/225, 64424/99225]<br>
&nbsp;&nbsp;&nbsp;[1/3, -4/15, -23/45, 68/45, 61/135, -2728/945, 2146/1215]<br>
&nbsp;&nbsp;&nbsp;[2/5, -24/35, -46/35, 9446/2835, 428/945, -95948/10395]<br>
&nbsp;&nbsp;&nbsp;[83/126, -80/63, -34712/14175, 4472/525, 29741/85050]<br>
&nbsp;&nbsp;&nbsp;[52/45, -2362/891, -17432/3465, 280108/13365]<br>
&nbsp;&nbsp;&nbsp;[335882/155925, -548752/96525, -48965632/4729725]<br>
&nbsp;&nbsp;&nbsp;[51368/12285, -197456/15795]<br>
&nbsp;&nbsp;&nbsp;[1461335/174636]<br>
</small></tt>
<p>&chi;&nbsp;&minus;&nbsp;&mu;:<br><tt><small>
&nbsp;&nbsp;&nbsp;[-1/2, 2/3, -37/96, 1/360, 81/512, -96199/604800, 5406467/38707200, -7944359/67737600]<br>
&nbsp;&nbsp;&nbsp;[-1/48, -1/15, 437/1440, -46/105, 1118711/3870720, -51841/1209600, -24749483/348364800]<br>
&nbsp;&nbsp;&nbsp;[-17/480, 37/840, 209/4480, -5569/90720, -9261899/58060800, 6457463/17740800]<br>
&nbsp;&nbsp;&nbsp;[-4397/161280, 11/504, 830251/7257600, -466511/2494800, -324154477/7664025600]<br>
&nbsp;&nbsp;&nbsp;[-4583/161280, 108847/3991680, 8005831/63866880, -22894433/124540416]<br>
&nbsp;&nbsp;&nbsp;[-20648693/638668800, 16363163/518918400, 2204645983/12915302400]<br>
&nbsp;&nbsp;&nbsp;[-219941297/5535129600, 497323811/12454041600]<br>
&nbsp;&nbsp;&nbsp;[-191773887257/3719607091200]<br>
</small></tt>
<p>&mu;&nbsp;&minus;&nbsp;&chi;:<br><tt><small>
&nbsp;&nbsp;&nbsp;[1/2, -2/3, 5/16, 41/180, -127/288, 7891/37800, 72161/387072, -18975107/50803200]<br>
&nbsp;&nbsp;&nbsp;[13/48, -3/5, 557/1440, 281/630, -1983433/1935360, 13769/28800, 148003883/174182400]<br>
&nbsp;&nbsp;&nbsp;[61/240, -103/140, 15061/26880, 167603/181440, -67102379/29030400, 79682431/79833600]<br>
&nbsp;&nbsp;&nbsp;[49561/161280, -179/168, 6601661/7257600, 97445/49896, -40176129013/7664025600]<br>
&nbsp;&nbsp;&nbsp;[34729/80640, -3418889/1995840, 14644087/9123840, 2605413599/622702080]<br>
&nbsp;&nbsp;&nbsp;[212378941/319334400, -30705481/10378368, 175214326799/58118860800]<br>
&nbsp;&nbsp;&nbsp;[1522256789/1383782400, -16759934899/3113510400]<br>
&nbsp;&nbsp;&nbsp;[1424729850961/743921418240]<br>
</small></tt>
<p>&xi;&nbsp;&minus;&nbsp;&phi;:<br><tt><small>
&nbsp;&nbsp;&nbsp;[-4/3, -4/45, 88/315, 538/4725, 20824/467775, -44732/2837835, -86728/16372125, -88002076/13956067125]<br>
&nbsp;&nbsp;&nbsp;[34/45, 8/105, -2482/14175, -37192/467775, -12467764/212837625, -895712/147349125, -2641983469/488462349375]<br>
&nbsp;&nbsp;&nbsp;[-1532/2835, -898/14175, 54968/467775, 100320856/1915538625, 240616/4209975, 8457703444/488462349375]<br>
&nbsp;&nbsp;&nbsp;[6007/14175, 24496/467775, -5884124/70945875, -4832848/147349125, -4910552477/97692469875]<br>
&nbsp;&nbsp;&nbsp;[-23356/66825, -839792/19348875, 816824/13395375, 9393713176/488462349375]<br>
&nbsp;&nbsp;&nbsp;[570284222/1915538625, 1980656/54729675, -4532926649/97692469875]<br>
&nbsp;&nbsp;&nbsp;[-496894276/1915538625, -14848113968/488462349375]<br>
&nbsp;&nbsp;&nbsp;[224557742191/976924698750]<br>
</small></tt>
<p>&phi;&nbsp;&minus;&nbsp;&xi;:<br><tt><small>
&nbsp;&nbsp;&nbsp;[4/3, 4/45, -16/35, -2582/14175, 60136/467775, 28112932/212837625, 22947844/1915538625, -1683291094/37574026875]<br>
&nbsp;&nbsp;&nbsp;[46/45, 152/945, -11966/14175, -21016/51975, 251310128/638512875, 1228352/3007125, -14351220203/488462349375]<br>
&nbsp;&nbsp;&nbsp;[3044/2835, 3802/14175, -94388/66825, -8797648/10945935, 138128272/147349125, 505559334506/488462349375]<br>
&nbsp;&nbsp;&nbsp;[6059/4725, 41072/93555, -1472637812/638512875, -45079184/29469825, 973080708361/488462349375]<br>
&nbsp;&nbsp;&nbsp;[768272/467775, 455935736/638512875, -550000184/147349125, -1385645336626/488462349375]<br>
&nbsp;&nbsp;&nbsp;[4210684958/1915538625, 443810768/383107725, -2939205114427/488462349375]<br>
&nbsp;&nbsp;&nbsp;[387227992/127702575, 101885255158/54273594375]<br>
&nbsp;&nbsp;&nbsp;[1392441148867/325641566250]<br>
</small></tt>
<p>&xi;&nbsp;&minus;&nbsp;&beta;:<br><tt><small>
&nbsp;&nbsp;&nbsp;[-1/3, -4/45, 32/315, 34/675, 2476/467775, -70496/8513505, -18484/4343625, 29232878/97692469875]<br>
&nbsp;&nbsp;&nbsp;[-7/90, -4/315, 74/2025, 3992/467775, 53836/212837625, -4160804/1915538625, -324943819/488462349375]<br>
&nbsp;&nbsp;&nbsp;[-83/2835, 2/14175, 7052/467775, -661844/1915538625, 237052/383107725, -168643106/488462349375]<br>
&nbsp;&nbsp;&nbsp;[-797/56700, 934/467775, 1425778/212837625, -2915326/1915538625, 113042383/97692469875]<br>
&nbsp;&nbsp;&nbsp;[-3673/467775, 390088/212837625, 6064888/1915538625, -558526274/488462349375]<br>
&nbsp;&nbsp;&nbsp;[-18623681/3831077250, 41288/29469825, 155665021/97692469875]<br>
&nbsp;&nbsp;&nbsp;[-6205669/1915538625, 504234982/488462349375]<br>
&nbsp;&nbsp;&nbsp;[-8913001661/3907698795000]<br>
</small></tt>
<p>&beta;&nbsp;&minus;&nbsp;&xi;:<br><tt><small>
&nbsp;&nbsp;&nbsp;[1/3, 4/45, -46/315, -1082/14175, 11824/467775, 7947332/212837625, 9708931/1915538625, -5946082372/488462349375]<br>
&nbsp;&nbsp;&nbsp;[17/90, 68/945, -338/2025, -16672/155925, 39946703/638512875, 164328266/1915538625, 190673521/69780335625]<br>
&nbsp;&nbsp;&nbsp;[461/2835, 1102/14175, -101069/467775, -255454/1563705, 236067184/1915538625, 86402898356/488462349375]<br>
&nbsp;&nbsp;&nbsp;[3161/18900, 1786/18711, -189032762/638512875, -98401826/383107725, 110123070361/488462349375]<br>
&nbsp;&nbsp;&nbsp;[88868/467775, 80274086/638512875, -802887278/1915538625, -200020620676/488462349375]<br>
&nbsp;&nbsp;&nbsp;[880980241/3831077250, 66263486/383107725, -296107325077/488462349375]<br>
&nbsp;&nbsp;&nbsp;[37151038/127702575, 4433064236/18091198125]<br>
&nbsp;&nbsp;&nbsp;[495248998393/1302566265000]<br>
</small></tt>
<p>&xi;&nbsp;&minus;&nbsp;&theta;:<br><tt><small>
&nbsp;&nbsp;&nbsp;[2/3, -4/45, 62/105, 778/4725, -193082/467775, -4286228/42567525, 53702182/212837625, 182466964/8881133625]<br>
&nbsp;&nbsp;&nbsp;[4/45, -32/315, 12338/14175, 92696/467775, -61623938/70945875, -32500616/273648375, 367082779691/488462349375]<br>
&nbsp;&nbsp;&nbsp;[-524/2835, -1618/14175, 612536/467775, 427003576/1915538625, -663111728/383107725, -42668482796/488462349375]<br>
&nbsp;&nbsp;&nbsp;[-5933/14175, -8324/66825, 427770788/212837625, 421877252/1915538625, -327791986997/97692469875]<br>
&nbsp;&nbsp;&nbsp;[-320044/467775, -9153184/70945875, 6024982024/1915538625, 74612072536/488462349375]<br>
&nbsp;&nbsp;&nbsp;[-1978771378/1915538625, -46140784/383107725, 489898512247/97692469875]<br>
&nbsp;&nbsp;&nbsp;[-2926201612/1915538625, -42056042768/488462349375]<br>
&nbsp;&nbsp;&nbsp;[-2209250801969/976924698750]<br>
</small></tt>
<p>&theta;&nbsp;&minus;&nbsp;&xi;:<br><tt><small>
&nbsp;&nbsp;&nbsp;[-2/3, 4/45, -158/315, -2102/14175, 109042/467775, 216932/2627625, -189115382/1915538625, -230886326/6343666875]<br>
&nbsp;&nbsp;&nbsp;[16/45, -16/945, 934/14175, -7256/155925, 117952358/638512875, 288456008/1915538625, -11696145869/69780335625]<br>
&nbsp;&nbsp;&nbsp;[-232/2835, 922/14175, -25286/66825, -7391576/54729675, 478700902/1915538625, 91546732346/488462349375]<br>
&nbsp;&nbsp;&nbsp;[719/4725, 268/18711, -67048172/638512875, -67330724/383107725, 218929662961/488462349375]<br>
&nbsp;&nbsp;&nbsp;[14354/467775, 46774256/638512875, -117954842/273648375, -129039188386/488462349375]<br>
&nbsp;&nbsp;&nbsp;[253129538/1915538625, 2114368/34827975, -178084928947/488462349375]<br>
&nbsp;&nbsp;&nbsp;[13805944/127702575, 6489189398/54273594375]<br>
&nbsp;&nbsp;&nbsp;[59983985827/325641566250]<br>
</small></tt>
<p>&xi;&nbsp;&minus;&nbsp;&mu;:<br><tt><small>
&nbsp;&nbsp;&nbsp;[1/6, -4/45, -817/10080, 1297/18900, 7764059/239500800, -9292991/302702400, -25359310709/1743565824000, 39534358147/2858202547200]<br>
&nbsp;&nbsp;&nbsp;[49/720, -2/35, -29609/453600, 35474/467775, 36019108271/871782912000, -14814966289/245188944000, -13216941177599/571640509440000]<br>
&nbsp;&nbsp;&nbsp;[4463/90720, -2917/56700, -4306823/59875200, 3026004511/30648618000, 99871724539/1569209241600, -27782109847927/250092722880000]<br>
&nbsp;&nbsp;&nbsp;[331799/7257600, -102293/1871100, -368661577/4036032000, 2123926699/15324309000, 168979300892599/1600593426432000]<br>
&nbsp;&nbsp;&nbsp;[11744233/239500800, -875457073/13621608000, -493031379277/3923023104000, 1959350112697/9618950880000]<br>
&nbsp;&nbsp;&nbsp;[453002260127/7846046208000, -793693009/9807557760, -145659994071373/800296713216000]<br>
&nbsp;&nbsp;&nbsp;[103558761539/1426553856000, -53583096419057/500185445760000]<br>
&nbsp;&nbsp;&nbsp;[12272105438887727/128047474114560000]<br>
</small></tt>
<p>&mu;&nbsp;&minus;&nbsp;&xi;:<br><tt><small>
&nbsp;&nbsp;&nbsp;[-1/6, 4/45, 121/1680, -1609/28350, -384229/14968800, 12674323/851350500, 7183403063/560431872000, -375027460897/125046361440000]<br>
&nbsp;&nbsp;&nbsp;[-29/720, 26/945, 16463/453600, -431/17325, -31621753811/1307674368000, 1117820213/122594472000, 30410873385097/2000741783040000]<br>
&nbsp;&nbsp;&nbsp;[-1003/45360, 449/28350, 3746047/119750400, -32844781/1751349600, -116359346641/3923023104000, 151567502183/17863765920000]<br>
&nbsp;&nbsp;&nbsp;[-40457/2419200, 629/53460, 10650637121/326918592000, -13060303/766215450, -317251099510901/8002967132160000]<br>
&nbsp;&nbsp;&nbsp;[-1800439/119750400, 205072597/20432412000, 146875240637/3923023104000, -2105440822861/125046361440000]<br>
&nbsp;&nbsp;&nbsp;[-59109051671/3923023104000, 228253559/24518894400, 91496147778023/2000741783040000]<br>
&nbsp;&nbsp;&nbsp;[-4255034947/261534873600, 126430355893/13894040160000]<br>
&nbsp;&nbsp;&nbsp;[-791820407649841/42682491371520000]<br>
</small></tt>
<p>&xi;&nbsp;&minus;&nbsp;&chi;:<br><tt><small>
&nbsp;&nbsp;&nbsp;[2/3, -34/45, 46/315, 2458/4725, -55222/93555, 2706758/42567525, 16676974/30405375, -64724382148/97692469875]<br>
&nbsp;&nbsp;&nbsp;[19/45, -256/315, 3413/14175, 516944/467775, -340492279/212837625, 158999572/1915538625, 85904355287/37574026875]<br>
&nbsp;&nbsp;&nbsp;[248/567, -15958/14175, 206834/467775, 4430783356/1915538625, -7597644214/1915538625, 2986003168/37574026875]<br>
&nbsp;&nbsp;&nbsp;[16049/28350, -832976/467775, 62016436/70945875, 851209552/174139875, -375566203/39037950]<br>
&nbsp;&nbsp;&nbsp;[15602/18711, -651151712/212837625, 3475643362/1915538625, 5106181018156/488462349375]<br>
&nbsp;&nbsp;&nbsp;[2561772812/1915538625, -10656173804/1915538625, 34581190223/8881133625]<br>
&nbsp;&nbsp;&nbsp;[873037408/383107725, -5150169424688/488462349375]<br>
&nbsp;&nbsp;&nbsp;[7939103697617/1953849397500]<br>
</small></tt>
<p>&chi;&nbsp;&minus;&nbsp;&xi;:<br><tt><small>
&nbsp;&nbsp;&nbsp;[-2/3, 34/45, -88/315, -2312/14175, 27128/93555, -55271278/212837625, 308365186/1915538625, -17451293242/488462349375]<br>
&nbsp;&nbsp;&nbsp;[1/45, -184/945, 6079/14175, -65864/155925, 106691108/638512875, 149984636/1915538625, -101520127208/488462349375]<br>
&nbsp;&nbsp;&nbsp;[-106/2835, 772/14175, -14246/467775, 5921152/54729675, -99534832/383107725, 10010741462/37574026875]<br>
&nbsp;&nbsp;&nbsp;[-167/9450, -5312/467775, 75594328/638512875, -35573728/273648375, 1615002539/75148053750]<br>
&nbsp;&nbsp;&nbsp;[-248/13365, 2837636/638512875, 130601488/1915538625, -3358119706/488462349375]<br>
&nbsp;&nbsp;&nbsp;[-34761247/1915538625, -3196/3553875, 46771947158/488462349375]<br>
&nbsp;&nbsp;&nbsp;[-2530364/127702575, -18696014/18091198125]<br>
&nbsp;&nbsp;&nbsp;[-14744861191/651283132500]<br>
</small></tt>

\section auxlaterror Truncation errors

There are two sources of error when using these series.  The truncation
error arises from retaing terms up to a certain order in \e n; it is the
absolute difference between the value of the truncated series compared
with the exact latitude (evaluated with exact arithmetic).  In addition,
using standard double-precision arithmetic entails accumulating
round-off errors so that at the end of a complex calculation a few of
the trailing bits of the result are wrong.

Here's a table of the truncation errors.  The errors are given in "units
in the last place (ulp)" where 1 ulp = 2<sup>&minus;53</sup> radian =
6.4 &times; 10<sup>&minus;15</sup> degree = 2.3 &times;
10<sup>&minus;11</sup> arcsecond which is a measure of the round-off
error for double precision.  Here is some rough guidance on how to
interpret these errors:
- if the truncation error is less than 1 ulp, then round-off errors
  dominate;
- if the truncation error is greater than 8 ulp, then truncation errors
  dominate;
- otherwise, round-off and truncation errors are comparable.
.
The truncation errors are given accurate to 2 significant figures.

<center>
<table>
<caption>Auxiliary latitude truncation errors (ulp)</caption>
<tr>
<th rowspan="2"> expression
<th colspan="2"> [<i>f</i> = 1/150, order = 6]
<th colspan="2"> [<i>f</i> = 1/297, order = 5]
<tr>
<th> <i>n</i> series <th> <i>e</i><sup>2</sup> series
<th> <i>n</i> series <th> <i>e</i><sup>2</sup> series
<tr><td>  &beta; &minus; &phi;   <td><tt><center> 0.0060&nbsp; </center></tt><td><tt><center> 28&nbsp;&nbsp;&nbsp; </center></tt><td><tt><center> &nbsp;0.035&nbsp; </center></tt><td><tt><center> &nbsp;41&nbsp;&nbsp;&nbsp; </center></tt>
<tr><td>   &phi; &minus; &beta;  <td><tt><center> 0.0060&nbsp; </center></tt><td><tt><center> 28&nbsp;&nbsp;&nbsp; </center></tt><td><tt><center> &nbsp;0.035&nbsp; </center></tt><td><tt><center> &nbsp;41&nbsp;&nbsp;&nbsp; </center></tt>
<tr><td> &theta; &minus; &phi;   <td><tt><center> 2.9&nbsp;&nbsp;&nbsp;&nbsp; </center></tt><td><tt><center> 82&nbsp;&nbsp;&nbsp; </center></tt><td><tt><center> &nbsp;6.0&nbsp;&nbsp;&nbsp; </center></tt><td><tt><center> 120&nbsp;&nbsp;&nbsp; </center></tt>
<tr><td>   &phi; &minus; &theta; <td><tt><center> 2.9&nbsp;&nbsp;&nbsp;&nbsp; </center></tt><td><tt><center> 82&nbsp;&nbsp;&nbsp; </center></tt><td><tt><center> &nbsp;6.0&nbsp;&nbsp;&nbsp; </center></tt><td><tt><center> 120&nbsp;&nbsp;&nbsp; </center></tt>
<tr><td> &theta; &minus; &beta;  <td><tt><center> 0.0060&nbsp; </center></tt><td><tt><center> 28&nbsp;&nbsp;&nbsp; </center></tt><td><tt><center> &nbsp;0.035&nbsp; </center></tt><td><tt><center> &nbsp;41&nbsp;&nbsp;&nbsp; </center></tt>
<tr><td>  &beta; &minus; &theta; <td><tt><center> 0.0060&nbsp; </center></tt><td><tt><center> 28&nbsp;&nbsp;&nbsp; </center></tt><td><tt><center> &nbsp;0.035&nbsp; </center></tt><td><tt><center> &nbsp;41&nbsp;&nbsp;&nbsp; </center></tt>
<tr><td>    &mu; &minus; &phi;   <td><tt><center> 0.037&nbsp;&nbsp; </center></tt><td><tt><center> 41&nbsp;&nbsp;&nbsp; </center></tt><td><tt><center> &nbsp;0.18&nbsp;&nbsp; </center></tt><td><tt><center> &nbsp;60&nbsp;&nbsp;&nbsp; </center></tt>
<tr><td>   &phi; &minus; &mu;    <td><tt><center> 0.98&nbsp;&nbsp;&nbsp; </center></tt><td><tt><center> 59&nbsp;&nbsp;&nbsp; </center></tt><td><tt><center> &nbsp;2.3&nbsp;&nbsp;&nbsp; </center></tt><td><tt><center> &nbsp;84&nbsp;&nbsp;&nbsp; </center></tt>
<tr><td>    &mu; &minus; &beta;  <td><tt><center> 0.00069 </center></tt><td><tt><center> &nbsp;5.8&nbsp; </center></tt><td><tt><center> &nbsp;0.0024 </center></tt><td><tt><center> &nbsp;&nbsp;9.6&nbsp; </center></tt>
<tr><td>  &beta; &minus; &mu;    <td><tt><center> 0.13&nbsp;&nbsp;&nbsp; </center></tt><td><tt><center> 12&nbsp;&nbsp;&nbsp; </center></tt><td><tt><center> &nbsp;0.35&nbsp;&nbsp; </center></tt><td><tt><center> &nbsp;19&nbsp;&nbsp;&nbsp; </center></tt>
<tr><td>    &mu; &minus; &theta; <td><tt><center> 0.24&nbsp;&nbsp;&nbsp; </center></tt><td><tt><center> 30&nbsp;&nbsp;&nbsp; </center></tt><td><tt><center> &nbsp;0.67&nbsp;&nbsp; </center></tt><td><tt><center> &nbsp;40&nbsp;&nbsp;&nbsp; </center></tt>
<tr><td> &theta; &minus; &mu;    <td><tt><center> 0.099&nbsp;&nbsp; </center></tt><td><tt><center> 23&nbsp;&nbsp;&nbsp; </center></tt><td><tt><center> &nbsp;0.23&nbsp;&nbsp; </center></tt><td><tt><center> &nbsp;33&nbsp;&nbsp;&nbsp; </center></tt>
<tr><td>   &chi; &minus; &phi;   <td><tt><center> 0.78&nbsp;&nbsp;&nbsp; </center></tt><td><tt><center> 43&nbsp;&nbsp;&nbsp; </center></tt><td><tt><center> &nbsp;2.1&nbsp;&nbsp;&nbsp; </center></tt><td><tt><center> &nbsp;64&nbsp;&nbsp;&nbsp; </center></tt>
<tr><td>   &phi; &minus; &chi;   <td><tt><center> 9.0&nbsp;&nbsp;&nbsp;&nbsp; </center></tt><td><tt><center> 71&nbsp;&nbsp;&nbsp; </center></tt><td><tt><center> 17&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; </center></tt><td><tt><center> 100&nbsp;&nbsp;&nbsp; </center></tt>
<tr><td>   &chi; &minus; &beta;  <td><tt><center> 0.018&nbsp;&nbsp; </center></tt><td><tt><center> &nbsp;3.7&nbsp; </center></tt><td><tt><center> &nbsp;0.11&nbsp;&nbsp; </center></tt><td><tt><center> &nbsp;&nbsp;6.4&nbsp; </center></tt>
<tr><td>  &beta; &minus; &chi;   <td><tt><center> 1.7&nbsp;&nbsp;&nbsp;&nbsp; </center></tt><td><tt><center> 16&nbsp;&nbsp;&nbsp; </center></tt><td><tt><center> &nbsp;3.4&nbsp;&nbsp;&nbsp; </center></tt><td><tt><center> &nbsp;24&nbsp;&nbsp;&nbsp; </center></tt>
<tr><td>   &chi; &minus; &theta; <td><tt><center> 0.18&nbsp;&nbsp;&nbsp; </center></tt><td><tt><center> 31&nbsp;&nbsp;&nbsp; </center></tt><td><tt><center> &nbsp;0.56&nbsp;&nbsp; </center></tt><td><tt><center> &nbsp;43&nbsp;&nbsp;&nbsp; </center></tt>
<tr><td> &theta; &minus; &chi;   <td><tt><center> 0.87&nbsp;&nbsp;&nbsp; </center></tt><td><tt><center> 23&nbsp;&nbsp;&nbsp; </center></tt><td><tt><center> &nbsp;1.9&nbsp;&nbsp;&nbsp; </center></tt><td><tt><center> &nbsp;32&nbsp;&nbsp;&nbsp; </center></tt>
<tr><td>   &chi; &minus; &mu;    <td><tt><center> 0.022&nbsp;&nbsp; </center></tt><td><tt><center> &nbsp;0.56 </center></tt><td><tt><center> &nbsp;0.11&nbsp;&nbsp; </center></tt><td><tt><center> &nbsp;&nbsp;0.91 </center></tt>
<tr><td>    &mu; &minus; &chi;   <td><tt><center> 0.31&nbsp;&nbsp;&nbsp; </center></tt><td><tt><center> &nbsp;1.2&nbsp; </center></tt><td><tt><center> &nbsp;0.86&nbsp;&nbsp; </center></tt><td><tt><center> &nbsp;&nbsp;2.0&nbsp; </center></tt>
<tr><td>    &xi; &minus; &phi;   <td><tt><center> 0.015&nbsp;&nbsp; </center></tt><td><tt><center> 39&nbsp;&nbsp;&nbsp; </center></tt><td><tt><center> &nbsp;0.086&nbsp; </center></tt><td><tt><center> &nbsp;57&nbsp;&nbsp;&nbsp; </center></tt>
<tr><td>   &phi; &minus; &xi;    <td><tt><center> 0.34&nbsp;&nbsp;&nbsp; </center></tt><td><tt><center> 53&nbsp;&nbsp;&nbsp; </center></tt><td><tt><center> &nbsp;1.1&nbsp;&nbsp;&nbsp; </center></tt><td><tt><center> &nbsp;75&nbsp;&nbsp;&nbsp; </center></tt>
<tr><td>    &xi; &minus; &beta;  <td><tt><center> 0.00042 </center></tt><td><tt><center> &nbsp;6.3&nbsp; </center></tt><td><tt><center> &nbsp;0.0039 </center></tt><td><tt><center> &nbsp;10&nbsp;&nbsp;&nbsp; </center></tt>
<tr><td>  &beta; &minus; &xi;    <td><tt><center> 0.040&nbsp;&nbsp; </center></tt><td><tt><center> 10&nbsp;&nbsp;&nbsp; </center></tt><td><tt><center> &nbsp;0.15&nbsp;&nbsp; </center></tt><td><tt><center> &nbsp;15&nbsp;&nbsp;&nbsp; </center></tt>
<tr><td>    &xi; &minus; &theta; <td><tt><center> 0.28&nbsp;&nbsp;&nbsp; </center></tt><td><tt><center> 28&nbsp;&nbsp;&nbsp; </center></tt><td><tt><center> &nbsp;0.75&nbsp;&nbsp; </center></tt><td><tt><center> &nbsp;38&nbsp;&nbsp;&nbsp; </center></tt>
<tr><td> &theta; &minus; &xi;    <td><tt><center> 0.040&nbsp;&nbsp; </center></tt><td><tt><center> 23&nbsp;&nbsp;&nbsp; </center></tt><td><tt><center> &nbsp;0.11&nbsp;&nbsp; </center></tt><td><tt><center> &nbsp;33&nbsp;&nbsp;&nbsp; </center></tt>
<tr><td>    &xi; &minus; &mu;    <td><tt><center> 0.015&nbsp;&nbsp; </center></tt><td><tt><center> &nbsp;0.79 </center></tt><td><tt><center> &nbsp;0.058&nbsp; </center></tt><td><tt><center> &nbsp;&nbsp;1.5&nbsp; </center></tt>
<tr><td>    &mu; &minus; &xi;    <td><tt><center> 0.0043&nbsp; </center></tt><td><tt><center> &nbsp;0.54 </center></tt><td><tt><center> &nbsp;0.018&nbsp; </center></tt><td><tt><center> &nbsp;&nbsp;1.1&nbsp; </center></tt>
<tr><td>    &xi; &minus; &chi;   <td><tt><center> 0.60&nbsp;&nbsp;&nbsp; </center></tt><td><tt><center> &nbsp;1.9&nbsp; </center></tt><td><tt><center> &nbsp;1.5&nbsp;&nbsp;&nbsp; </center></tt><td><tt><center> &nbsp;&nbsp;3.6&nbsp; </center></tt>
<tr><td>   &chi; &minus; &xi;    <td><tt><center> 0.023&nbsp;&nbsp; </center></tt><td><tt><center> &nbsp;0.53 </center></tt><td><tt><center> &nbsp;0.079&nbsp; </center></tt><td><tt><center> &nbsp;&nbsp;0.92 </center></tt>
</table>
</center>

\if SKIP
 0  beta phi  ,0.0060!,28!!!,!0.035!,!41!!!
 1   phi beta ,0.0060!,28!!!,!0.035!,!41!!!
 2 theta phi  ,2.9!!!!,82!!!,!6.0!!!,120!!!
 3   phi theta,2.9!!!!,82!!!,!6.0!!!,120!!!
 4 theta beta ,0.0060!,28!!!,!0.035!,!41!!!
 5  beta theta,0.0060!,28!!!,!0.035!,!41!!!
 6    mu phi  ,0.037!!,41!!!,!0.18!!,!60!!!
 7   phi mu   ,0.98!!!,59!!!,!2.3!!!,!84!!!
 8    mu beta ,0.00069,!5.8!,!0.0024,!!9.6!
 9  beta mu   ,0.13!!!,12!!!,!0.35!!,!19!!!
10    mu theta,0.24!!!,30!!!,!0.67!!,!40!!!
11 theta mu   ,0.099!!,23!!!,!0.23!!,!33!!!
12   chi phi  ,0.78!!!,43!!!,!2.1!!!,!64!!!
13   phi chi  ,9.0!!!!,71!!!,17!!!!!,100!!!
14   chi beta ,0.018!!,!3.7!,!0.11!!,!!6.4!
15  beta chi  ,1.7!!!!,16!!!,!3.4!!!,!24!!!
16   chi theta,0.18!!!,31!!!,!0.56!!,!43!!!
17 theta chi  ,0.87!!!,23!!!,!1.9!!!,!32!!!
18   chi mu   ,0.022!!,!0.56,!0.11!!,!!0.91
19    mu chi  ,0.31!!!,!1.2!,!0.86!!,!!2.0!
20    xi phi  ,0.015!!,39!!!,!0.086!,!57!!!
21   phi xi   ,0.34!!!,53!!!,!1.1!!!,!75!!!
22    xi beta ,0.00042,!6.3!,!0.0039,!10!!!
23  beta xi   ,0.040!!,10!!!,!0.15!!,!15!!!
24    xi theta,0.28!!!,28!!!,!0.75!!,!38!!!
25 theta xi   ,0.040!!,23!!!,!0.11!!,!33!!!
26    xi mu   ,0.015!!,!0.79,!0.058!,!!1.5!
27    mu xi   ,0.0043!,!0.54,!0.018!,!!1.1!
28    xi chi  ,0.60!!!,!1.9!,!1.5!!!,!!3.6!
29   chi xi   ,0.023!!,!0.53,!0.079!,!!0.92
\endif

The 2nd and 3rd columns show the results for the SRMmax ellipsoid, \e f
= 1/150, retaining 6th order terms in the series expansion.  The 4th and
5th columns show the results for the International ellipsoid, \e f =
1/297, retaining 5th order terms in the series expansion.  The 2nd and
4th columns give the errors for the series expansions in terms of \e n
given in this section (appropriately truncated).  The 3rd and 5th
columns give the errors when the series are reexpanded in terms of
<i>e</i><sup>2</sup> = 4\e n/(1 + \e n)<sup>2</sup> and truncated
retaining the <i>e</i><sup>12</sup> and <i>e</i><sup>10</sup> terms
respectively.

Some observations:
- For production use, the 6th order series in \e n are recommended.  For
  \e f = 1/150, the resulting errors are close to the round-off limit.
  The errors in the 6th order series scale as <i>f</i><sup>7</sup>; so
  the errors with \e f = 1/297 are about 120 times smaller.
- It's inadvisable to use the 5th order series in \e n; this order is
  barely acceptable for \e f = 1/297 and the errors grow as
  <i>f</i><sup>6</sup> as \e f is increased.
- In all cases, the expansions in terms of <i>e</i><sup>2</sup> are
  considerably less accurate than the corresponding series in \e n.
- For every series converting between &phi; and any of &theta;, &mu;,
  &chi;, or &xi;, the series where &beta; is substituted for &phi; is
  more accurate.  Considering that the transformation between &phi; and
  &beta; is so simple, tan&beta; = (1 - \e f) tan&phi;, it sometimes
  makes sense to use &beta; internally as the basic measure of latitude.
  (This is the case with geodesic calculations.)

<center>
Back to \ref geocentric.  Forward to \ref highprec.  Up to \ref contents.
</center>

**********************************************************************/
/**
\page highprec Support for high precision arithmetic

<center>
Back to \ref auxlat.  Forward to \ref changes.  Up to \ref contents.
</center>

One of the goals with the algorithms in GeographicLib is to deliver
accuracy close to the limits for double precision.  In order to develop
such algorithms it is very useful to be have accurate test data.  For
this purpose, I used Maxima's bfloat capability, which support arbitrary
precision floating point arithmetic.  As of version 1.37, such
high-precision test data can be generated directly by GeographicLib by
compiling it with <code>GEOGRAPHICLIB_PRECISION</code> equal to 4 or 5.

Here's what you should know:
 - This is mainly for use for algorithm developers.  It's not
   recommended for installation for all users on a system.
 - Configuring with <code>-D GEOGRAPHICLIB_PRECISION=4</code> gives quad
   precision (113-bit precision) via boost::multiprecision::float128;
   this requires:
   - <a href="https://www.boost.org"> Boost</a>, version 1.64 or later,
   - the quadmath library (the package names are libquadmath
     and libquadmath-devel),
   - the use of g++.
 - Configuring with <code>-D GEOGRAPHICLIB_PRECISION=5</code> gives
   arbitrary precision via mpfr::mpreal; this requires:
   - <a href="https://www.mpfr.org"> MPFR</a>, version 3.0 or later,
   - <a href="http://www.holoborodko.com/pavel/mpfr"> MPFR C++</a>
     (version 3.6.9, dated 2022-01-18, or later; version 3.6.9 also
     requires the fixes given in pull requests #15),
   - a compiler which supports the explicit cast operator (e.g., g++ 4.5
     or later, Visual Studio 12 2013 or later).
 - MPFR, MPFR C++, and Boost all come with their own licenses.  Be sure
   to respect these.
 - The indicated precision is used for <b>all</b> floating point
   arithmetic.  Thus, you can't compare the results of different
   precisions within a single invocation of a program.  Instead, you can
   create a file of accurate test data at a high precision and use this
   to test the algorithms at double precision.
 - With MPFR, the precision should be set (using Utility::set_digits)
   just once before any other GeographicLib routines are called.
   Calling this function after other GeographicLib routines will lead to
   inconsistent results (because the precision of some constants like
   Math::pi() is set when the functions are first called).
 - All the \ref utilities call Utility::set_digits() (with no
   arguments).  This causes the precision (in bits) to be determined by
   the <code>GEOGRAPHICLIB_DIGITS</code> environment variable.  If this
   is not defined the precision is set to 256 bits (about 77 decimal
   digits).
 - The accuracy of most calculations should increase as the precision
   increases (and typically only a few bits of accuracy should be lost).
   We can distinguish 4 sources of error:
   - Round-off errors; these are reliably reduced when the precision is
     increased.  For the most part, the algorithms used by GeographicLib
     are carefully tuned to minimize round-off errors, so that only a
     few bits of accuracy are lost.
   - Convergence errors due to not iterating certain algorithms to
     convergence.  However, all iterative methods used by GeographicLib
     converge quadratically (the number of correct digits doubles on
     each iteration) so that full convergence is obtained for
     "reasonable" precisions (no more than, say, 100 decimal digits or
     about 340 bits).  An exception is thrown if the convergence
     criterion is not met when using high precision arithmetic.
   - Truncation errors.  Some classes (namely, Geodesic and
     TransverseMercator) use series expansion to approximate the true
     solution.  Additional terms in the series are used for high
     precision, however there's always a finite truncation error which
     varies as some power of the flattening.  On the other hand,
     GeodesicExact and TransverseMercatorExact are somewhat slower
     classes offering the same functionality implemented with
     EllipticFunction.  These classes provide arbitrary accuracy.
     (However, a caveat is that the evaluation of the area in
     GeodesicExact still uses a series (albeit of considerably higher
     order).  So the area calculations are always have a finite
     truncation error.)
   - Quantization errors.  Geoid, GravityModel, and MagneticModel all
     depend on external data files.  The coefficient files for
     GravityModel and MagneticModel store the coefficients as IEEE
     doubles (and perhaps these coefficients can be regarded as exact).
     However, with Geoid, the data files for the geoid heights are
     quantized at 3mm leading to an irreducible &plusmn;1.5mm
     quantization error.  On the other hand, all the physical constants
     used by GeographicLib, e.g., the flattening of the WGS84 ellipsoid,
     are evaluated as exact decimal numbers.
 - Where might high accuracy be important?
   - checking the truncation error of series approximations;
   - checking for excessive round-off errors (typically due to subtraction);
   - checking the round-off error in computing areas of many-sided polygons;
   - checking the summation of high order spherical harmonic expansions
     (where underflow and overflow may also be a problem).
 - Because only a tiny number of people will be interested in using this
   facility:
   - the cmake support for the required libraries is rudimentary;
   - however geographiclib-config.cmake does export
     <code>GEOGRAPHICLIB_PRECISION</code> and
     <code>GEOGRAPHICLIB_HIGHPREC_LIBRARIES</code>, the libraries
     providing the support for high-precision arithmetic;
   - support for the C++11 mathematical functions and the explicit cast
     operator is required;
   - quad precision is only available on Linux;
   - mpfr has been mostly tested on Linux (but it works on Windows with
     Visual Studio 12 and MacOS too).

The following steps needed to be taken

 - Phase 1, make sure you can switch easily between double, float, and
   long double.
   - use <code>\#include &lt;cmath&gt;</code> instead of <code>\#include
     &lt;math.h&gt;</code>;
   - use, e.g., <code>std::sqrt</code> instead of <code>sqrt</code> in
     header files (similarly for <code>sin</code>, <code>cos</code>,
     <code>atan2</code>, etc.);
   - use <code>using namespace std;</code> and plain <code>sqrt</code>,
     etc., in code files;
   - express all convergence criteria in terms of\code
     numeric_limits<double>::epsilon() \endcode
     etc., instead of using "magic constants", such as 1.0e-15;
   - use <code>typedef double real;</code> and replace all occurrences of
     <code>double</code> by <code>real</code>;
   - write all literals by, e.g., <code>real(0.5)</code>.  Some
     constants might need the L suffix, e.g., <code>real f =
     1/real(298.257223563L)</code> (but see below);
   - Change the typedef of <code>real</code> to <code>float</code> or
     <code>long double</code>, compile, and test.  In this way, the
     library can be run with any of the three basic floating point
     types.
   - If you want to run the library with multiple floating point types
     within a single executable, then make all your classes take a
     template parameter specifying the floating-point type and
     instantiate your classes with the floating-point types that you
     plan to use.  I did not take this approach with GeographicLib
     because double precision is suitable for the vast majority of
     applications and turning all the classes into templates classes
     would end up needlessly complicating (and bloating) the library.

 - Phase 2, changes to support arbitrary, but fixed, precision
   - Use, e.g., \code
     typedef boost::multiprecision::float128 real; \endcode
   - Change <code>std::sqrt(...)</code>, etc. to \code
     using std::sqrt;
     sqrt(...) \endcode
     (but note that <code>std::max</code> can stay).  It's only necessary
     to do this in header files (code files already have <code>using
     namespace std;</code>).
   - In the case of boost's multiprecision numbers, the C++11
     mathematical functions need special treatment, see Math.hpp.
   - If necessary, use series with additional terms to improve the
     accuracy.
   - Replace slowly converging root finding methods with rapidly
     converging methods.  In particular, the simple iterative method to
     determine the flattening from the dynamical form factor in
     NormalGravity converged too slowly; this was replaced by Newton's
     method.
   - If necessary, increase the maximum allowed iteration count in root
     finding loops.  Also throw an exception of the maximum iteration
     count is exceeded.
   - Write literal constants in a way that works for any precision, e.g.,
     \code
     real f = 1/( real(298257223563LL) / 1000000000 ); \endcode
     [Note that \code
     real f = 1/( 298 + real(257223563) / 1000000000 ); \endcode
     and <code>1/real(298.257223563L)</code> are susceptible to
     double rounding errors.  We normally want to avoid such errors when
     real is a double.]
   - For arbitrary constants, you might have to resort to macros \code
     #if   GEOGRAPHICLIB_PRECISION == 1
     #define REAL(x) x##F
     #elif GEOGRAPHICLIB_PRECISION == 2
     #define REAL(x) x
     #elif GEOGRAPHICLIB_PRECISION == 3
     #define REAL(x) x##L
     #elif GEOGRAPHICLIB_PRECISION == 4
     #define REAL(x) x##Q
     #else
     #define REAL(x) real(#x)
     #endif \endcode
     and then use \code
     real f = 1/REAL(298.257223563); \endcode
   - Perhaps use local static declarations to avoid the overhead of
     reevaluating constants, e.g., \code
     static inline real pi() {
       using std::atan2;
       // pi is computed just once
       static const real pi = atan2(real(0), real(-1));
       return pi;
     } \endcode
     This is <b>not</b> necessary for built-in floating point types,
     since the atan2 function will be evaluated at compile time.
   - In Utility::readarray and Utility::writearray, arrays of reals were
     treated as plain old data.  This assumption now no longer holds and
     these functions needed special treatment.
   - volatile declarations don't apply.

 - Phase 3, changes to support arbitrary precision which can be set at
   runtime.
   - The big change now is that the precision is not known at compile
     time.  All static initializations which involve floating point
     numbers need to be eliminated.
     - Some static variables (e.g., tolerances which are expressed in
       terms of <code>numeric_limits&lt;double&gt;\::epsilon()</code>)
       were made member variables and so initialized when the
       constructor was called.
     - Some simple static real arrays (e.g., the interpolating stencils
       for Geoid) were changed into integer arrays.
     - Some static variables where converted to static functions similar
       to the definition of pi() above.
     - All the static instances of classes where converted as follows
       \code
       // declaration
       static const Geodesic WGS84;
       // definition
       const Geodesic Geodesic::WGS84(Constants::WGS84_a(),
                                      Constants::WGS84_f());
       // use
       const Geodesic& geod = Geodesic::WGS84; \endcode
       becomes \code
       // declaration
       static const Geodesic& WGS84();
       // definition
       const Geodesic& Geodesic::WGS84() {
         static const Geodesic wgs84(Constants::WGS84_a(),
                                     Constants::WGS84_f());
         return wgs84;
         static const Geodesic& WGS84();
       }
       // use
       const Geodesic& geod = Geodesic::WGS84(); \endcode
       This is the so-called
       <a href="https://isocpp.org/wiki/faq/ctors#static-init-order-on-first-use">
       "construct on first use idiom"</a>.  This is the most disruptive
       of the changes since it requires a different calling convention
       in user code.  However the old static initializations were
       invoked every time a code linking to GeographicLib was started,
       even if the objects were not subsequently used.  The new method
       only initializes the static objects if they are used.
     .
   - <code>numeric_limits&lt;double&gt;\::digits</code> is no longer a
     compile-time constant.  It becomes
     <code>numeric_limits&lt;double&gt;\::digits()</code>.
   - Depending on the precision cos(&pi;/2) might be negative.
     Similarly atan(tan(&pi;/2)) may evaluate to &minus;&pi;/2.
     GeographicLib already handled this, because this happens with long
     doubles (64 bits in the fraction).
   - The precision needs to be set in each thread in a multi-processing
     applications (for an example, see
     <code>examples/GeoidToGTX.cpp</code>).
   - Passing numbers to functions by value incurs a substantially higher
     overhead than with doubles.  This could be avoided by passing such
     arguments by reference.  This was <b>not</b> done here because it
     would junk up the code to benefit a narrow application.
   - The constants in GeodesicExact, e.g., 831281402884796906843926125,
     can't be specified as long doubles nor long longs (since they have
     more than 64 significant bits):
     - first tried macro which expanded to a string (see the macro REAL
       above);
     - now use inline function to combine two long long ints each with at
       most 52 significant bits;
     - also needed to simplify one routine in GeodesicExact which took
       inordinately long (15 minutes) to compile using g++.

<center>
Back to \ref auxlat.  Forward to \ref changes.  Up to \ref contents.
</center>

**********************************************************************/
/**
\page changes Change log

<center>
Back to \ref highprec.  Up to \ref contents.
</center>

List of versions in reverse chronological order together with a brief
list of changes.  (Note: Old versions of the library use a year-month
style of numbering.  Now, the library uses a major and minor version
number.)  Recent versions of GeographicLib are available at
<a href="https://sourceforge.net/projects/geographiclib/files/distrib-C++/">
https://sourceforge.net/projects/geographiclib/files/distrib-C++/</a>.
Older versions are in
<a href="https://sourceforge.net/projects/geographiclib/files/distrib/">
https://sourceforge.net/projects/geographiclib/files/distrib/</a>.

The corresponding documentation for these versions is obtained by
clicking on the &ldquo;Version <i>m.nn</i>&rdquo; links below.  Some of
the links in the documentation of older versions may be out of date (in
particular the links for the source code will not work if the code has
been migrated to the archive subdirectory).  All the releases are
available as tags &ldquo;r<i>m.nn</i>&rdquo; in the the "release" branch
of the
<a href="https://github.com/geographiclib/geographiclib/tree/release">
git repository for GeographicLib</a>.

\if SKIP
 - TODO: Planned updates
   - Change templating in PolygonArea so that the AddPoint and AddEdge
     methods are templated instead of the class itself.  This would
     allow a polygon to contain a mixture of geodesic and rhumb-line
     edges.
   - Generalize Geoid, UTMUPS, MGRS, GeoCoords to handle non-WGS84
     ellipsoids.
   - Add GreatEllipse class.
   - Implement Geodesic + TM + PolarStereographic on Jets.
   - Template versions of Gnomonic and AzimuthalEquidistant so that they
     can use either Geodesic and GeodesicExact.  Use typedefs so that
     Geodesic::Line points to GeodesicLine.
   - Add information on radius of convergence for aux latitude series.
   - Check Geocentric::Reverse for accuracy for extreme ellipsoids, \e e
     &gt; 1/sqrt(2).
   - Check Visual Studo "code analysis" results (from Analize VS
     menu).  Probably an awful lot of false positives.
   - Add Math routine for Clenshaw summation; may AuxiliaryLatitude
     class for conversions via the series?
   - Allow addition of points to NearestNeighbor objects (maybe by
     allowing larger buckets on construction or by just do additional
     splits as necessary).
   - Check whether boost/quadmath workarounds can be removed with boost 1.79.
\endif

 - <a href="https://geographiclib.sourceforge.io/C++/2.1.1">Version 2.1.1</a>
   (released 2022-07-25)
   - The <a href="Planimeter.1.html">Planimeter</a> utility used to
     accept polygon vertices as UTM/UPS or MGRS coordinates.  However,
     since these are converted to latitude and longitude using the WGS84
     ellipoid, this convention is incompatible with the -e option to
     specify the ellipsoid.  So now, you have to provide the
     \--geoconvert-input option to allow vertices to be specified in
     this way.
   - Fix sign error in DST::refine.
   - Relax overly strict convergence test for inverse problem in
     Geodesic and GeodesicExact.
   - Minor:
     - Use lookup table for selecting number of points for DST in
       GeodesicExact.
     - Relax one of the test thresholds in geodtest.cpp.

 - <a href="https://geographiclib.sourceforge.io/C++/2.1">Version 2.1</a>
   (released 2022-06-09)
   - Improve the accuracy of the area calculations in GeodesicExact and
     the <a href="Planimeter.1.html">Planimeter</a> utility with the -E
     option.  This now gives full double precision accuracy for
     <i>b</i>/\e a &isin; [0.01, 100] or \e f &isin; [-99, 0.99].
     (Previously the range for full accuracy was <i>b</i>/\e a &isin;
     [0.5, 2].) The method involves a direct computation of the Fourier
     series for the integrand of area integral which then allows the
     integral to be evaluated.
   - Add a new class DST for doing discrete sine transforms.  This is
     mainly for "internal" use, to compute the area in GeodesicExact.
     Internally, this uses the
     <a href="https://github.com/mborgerding/kissfft">kissfft</a>
     package by Mark Borgerding.
   - Fix various warnings from Visual Studio about arithmetic involving
     enums.
   - Minor improvement in the logic for the geodesic inverse problem to
     address a slow down that can occur with `GEOGRAPHICLIB_PRECISION =
     5` due to the time it takes to calculate, for example,
     `sin(10^(10^8))` accurately.
   .
 - <a href="https://geographiclib.sourceforge.io/C++/2.0">Version 2.0</a>
   (released 2022-05-06)
   - Remove non C++ implementations from this package.  These are now
     managed as separate packages.  See
     [https://geographiclib.sourceforge.io/doc/library.html#languages](
     ../../doc/library.html#languages).
   - This allowed the cmake interface to be rationalized, see \ref cmake
     - Replace `GEOGRAPHICLIB_LIB_TYPE` by `BUILD_SHARED_LIBS` (a
       standard cmake variable) and `BUILD_BOTH_LIBS`.
     - Remove `COMMON_INSTALL_PATH` and replace with variables to
       specify where various components are to be installed.
     - `GEOGRAPHICLIB_DOCUMENTATION` is now `BUILD_DOCUMENTATION`.
     - Remove `BUILD_NETGEOGRAPHICLIB`.
     - Remove legacy cmake config support (non-namespace).
     - `make exampleprograms` does a separate cmake configuration to
       mimic how a user's program might find GeographicLib.
     - `find_package (GeographicLib)` now sets and checks the value of
       `GEOGRAPHICLIB_PRECISION`.
     - Improve the separation of end-user and maintain cmake code.
   - More careful treatment of &plusmn;0&deg; and &plusmn;180&deg;.
     - These behave consistently with taking the limits
       - &plusmn;0 means &plusmn;&epsilon; as &epsilon; &rarr; 0+
       - &plusmn;180 means &plusmn;(180 &minus; &epsilon;) as &epsilon;
         &rarr; 0+
     - As a consequence, azimuths of +0&deg; and +180&deg; are reckoned
       to be east-going, as far as tracking the longitude with
       Geodesic::LONG_UNROLL and the area goes, while azimuths
       &minus;0&deg; and &minus;180&deg; are reckoned to be west-going.
     - When computing longitude differences, if &lambda;<sub>2</sub>
       &minus; &lambda;<sub>1</sub> = &plusmn;180&deg; (mod 360&deg;),
       then the sign is picked depending on the sign of the difference.
     - The normal range for returned longitudes and azimuths is
       [&minus;180&deg;, 180&deg;].
   - Programs in directory `tests` now allow testing to extend beyond
     invocations of the utility programs.
   - BUG FIXES:
     - Fixed bug where in the solution of the inverse geodesic problem where
       with lat1 = 0 and lat2 = nan was treated as equatorial.
     - Fixed roundoff corner case in geodesic area computation (only
       triggered with long double).
   - Minor changes in code:
     - Use fmin, fmax, fabs instead of min, max, abs, where appropriate.
     - Parameterize the conversion units for degrees, minutes, seconds as
       Math::qd, Math::dm, Math::ms.
     - Ellipsoid::MinorRadius() has be renamed Ellipsoid::PolarRadius().
     - Remove deprecated functions: XXX::MajorRadius, Utility::val, and
       placeholders (Math::cbrt, etc.) for C++11 math functions.
     - Can specify the comment character in Utility::ParseLine.
     - Rename various internal identifiers to avoid reserved names.
     - Remove unused internal variable in RhumbLine.
     - The documentation for EllipticFunction specifies restrictions on
       the arguments.
     - Put GeodesicExactC4.cpp back into GeodesicExact.cpp.
   - Library file now called `libGeographicLib.so`, etc., instead of
     `libGeographic.so`.
   - The .NET version of GeographicLib has been removed.
   .
 - <a href="https://geographiclib.sourceforge.io/1.52">Version 1.52</a>
   (released 2021-06-22)
   - Add MagneticModel::FieldGeocentric and
     MagneticCircle::FieldGeocentric to return the field in geocentric
     coordinates (thanks to Marcelo Banik de Padua).
   - Document realistic errors for PolygonAreaT and
     <a href="Planimeter.1.html">Planimeter</a>.
   - Geodesic routines: be more aggressive in preventing negative \e s12
     and \e m12 for short lines (all languages).
   - Fix bug in AlbersEqualArea for extreme prolate ellipsoids (plus
     general cleanup in the code).
   - Thanks to Thomas Warner, a sample of wrapping the C++ library, so
     it's accessible in Excel, is given in <code>wrapper/Excel</code>.
   - Minor changes
     - Work around inaccuracies in hypot routines in Visual Studio
       (win32), Python, and JavaScript.
     - Initialize reference argument to remquo (C++ and C).
     - Get ready to retire unused _exact in RhumbLine.
     - Declare RhumbLine copy constructor "= default".
     - Use C++11 "= delete" idiom to delete copy assignment and copy
       constructors in RhumbLine, Geoid, GravityModel, MagneticModel.
     - Fix MGRS::Forward to work around aggressive optimization leading to
       incorrect rounding.
     - Fix plain makefiles, Makefile.mk, so that PREFIX is handled
       properly.
     - Make cmake's GeographicLib_LIBRARIES point to namespace versions.
   - <b>NOTE:</b> In the next version (tentatively 2.0), I plan to split
     up the git repository and the packaging of GeographicLib into
     separate entities for each language.  This will simplify
     development and deployment of the library.
   - <b>WARNING:</b> The .NET version of GeographicLib will not be
     supported in the next version.
   .
 - <a href="https://geographiclib.sourceforge.io/1.51">Version 1.51</a>
   (released 2020-11-22)
   - C++11 compiler required for C++ library.  As a consequence:
     - The workaround implementations for C++11 routines (Math::hypot,
       Math::expm1, Math::log1p, Math::asinh, Math::atanh,
       Math::copysign, Math::cbrt, Math::remainder, Math::remquo,
       Math::round, Math::lround, Math::fma, Math::isfinite, and
       Math::isnan) are now deprecated.  Just use the versions in the
       std:: namespace instead.
     - SphericalEngine class, fix the namespace for using streamoff.
     - Some templated functions, e.g., Math::degree(), now have default
       template parameters, T = Math::real.
   - C99 compiler required for C library.
   - Reduce memory footprint in Java implementation.
   - New form of Utility::ParseLine to allow the syntax "KEY = VAL".
   - Add International Geomagnetic Reference Field (13th generation),
     igrf13, which approximates the main magnetic field of the earth for
     the period 1900--2025.
   - More symbols allowed with DMS decoding in C++, JS, and cgi-bin
     packages; see DMS::Decode.
   - Fix bug in cgi-bin argument processing which causes "+" to be
     misinterpreted.
   - Required minium version of CMake is now 3.7.0 (released
     2016-11-11).  This is to work around a bug in find_package for
     cmake 3.4 and 3.5.
   .
 - <a href="https://geographiclib.sourceforge.io/1.50.1">Version 1.50.1</a>
   (released 2019-12-13)
   - Add the World Magnetic Model 2020, wmm2020, covering the period
     2020--2025.  This is now the model returned by
     MagneticModel::DefaultMagneticName and is default magnetic model
     for <a href="MagneticField.1.html">MagneticField</a> (replacing
     wmm2015v2 which is only valid thru the end of 2019).
   - Include float instantiations of those templated Math functions
     which migrated to Math.cpp in version 1.50.
   - <b>WARNING:</b> The <i>next</i> version of GeographicLib will
     require a C++11 compliant compiler.  This means that the minimum
     version of Visual Studio will be Visual Studio 14 2015.  (This
     repeats the warning given with version 1.50.  It didn't apply to
     this version because this is a minor update.)
   .
 - <a href="https://geographiclib.sourceforge.io/1.50">Version 1.50</a>
   (released 2019-09-24)
   - BUG fixes:
     - Java + JavaScript implementations of PolygonArea::TestEdge counted
       the pole encirclings wrong.
     - Fix typo in JavaScript implementation which affected unsigned
       areas.
     - Adding edges to a polygon counted pole encirclings inconsistent
       with the way the adding point counted them.  This might have
       caused an incorrect result if a polygon vertex had longitude = 0.
       This affected all implementations except Fortran and MATLAB).
     - GARS::Forward: fix BUG in handling of longitude = &plusmn;180&deg;.
     - Fix bug in Rhumb class and
       <a href="RhumbSolve.1.html">RhumbSolve(1)</a> utiity which caused
       incorrect area to be reported if an endpoint is at a pole.
       Thanks to Natalia Sabourova for reporting this.
     - Fix bug in MATLAB routine mgrs_inv which resulted in incorrect
       results for UPS zones with prec = &minus;1.
     - In geodreckon.m geoddistance.m, suppress (innocuous) "warning:
       division by zero" messages from Octave.
     - In python implementation, work around problems caused by sin(inf)
       and fmod(inf) raising exceptions.
     - Geoid class, fix the use of streamoff.
   - The PolygonArea class, the
     <a href="Planimeter.1.html">Planimeter</a> utility, and their
     equivalents in C, Fortran, MATLAB, Java, JavaScript, Python, and
     Maxima can now handle arbitrarily complex polygons.  In the case of
     self-intersecting polygons the area is accumulated "algebraically",
     e.g., the areas of the 2 loops in a figure-8 polygon will partially
     cancel.
   - Changes in gravity and magnetic model handling
     - SphericalEngine::coeff::readcoeffs takes new optional argument
       \e truncate.
     - The constructors for GravityModel and MagneticModel allow the
       maximum degree and order to be specified.  The array of
       coefficients will then be truncated if necessary.
     - GravityModel::Degree(), GravityModel::Order(),
       MagneticModel::Degree(), MagneticModel::Order() return the
       maximum degree and order of all the components of a GravityModel
       or MagneticModel.
     - <a href="Gravity.1.html">Gravity</a> and
       <a href="MagneticField.1.html">MagneticField</a> utilities
       accept -N and -M options to to allow the maximum degree and order
       to be specified.
   - The <a href="GeodSolve.1.html">GeodSolve</a> allows fractional
     distances to be entered as fractions (with the <code>-F</code>
     flag).
   - MajorRadius() methods are now called EquatorialRadius() for the
     C++, Java, and .NET libraries.  "Equatorial" is more descriptive in
     the case of prolate ellipsoids.  MajorRadius() is retained for
     backward compatibility for C++ and Java but is deprecated.
   - Minimum version updates:
     - CMake = 3.1.0, released 2014-12-15.
     - Minimum g++ version = 4.8.0, released 2013-03-22.
     - Visual Studio 10 2010 (haven't been able to check Visual Studio
       2008 for a long time).
     - <b>WARNING:</b> The <i>next</i> version of GeographicLib will
       require a C++11 compliant compiler.  This means that the minimum
       version of Visual Studio will be Visual Studio 14 2015.
     - Minimum boost version = 1.64 needed for GEOGRAPHICLIB_PRECISION =
       4.
     - Java = 1.6; this allows the removal of epsilon, min, hypot,
       log1p, copysign, cbrt from GeoMath.
   - CMake updates:
     - Fine tune Visual Studio compatibility check in
       find_package(GeographicLib); this allows GeographicLib compiled
       with Visual Studio 14 2015 to be used with a project compiled
       with Visual Studio 15 2017 and 16 2019.
     - Suppress warnings with dotnet build.
     - Change CMake target names and add an interface library (thanks to
       Matthew Woehlke).
     - Remove pre-3.1.0 cruft and update the documentation to remove the
       need to call include_dirctories.
     - Add _d suffix to example and test programs.
     - Changer installation path for binary installer to the Windows
       default.
     - Add support for Intel compiler (for C++, C, Fortran).  This
       entails supplying the <code>-fp-model precise</code> flag to
       prevent the compiler from incorrectly simplying <code>(a + b) +
       c</code> and <code>0.0 + x</code>.
   - Add version 2 of the World Magnetic Model 2015, wmm2015v2.  This
     is now the default magnetic model for
     <a href="MagneticField.1.html">MagneticField</a> (replacing wmm2015
     which is now deprecated).  Coming in 2019-12: the wmm2020 model.
   - The -f flag in the scripts geographiclib-get-geoids,
     geographiclib-get-gravity, and geographiclib-get-magnetic, allows
     you to load new models (not yet in the set defined by "all").  This
     is in addition to its original role of allowing you to overwrite
     existing models.
   - Changes in math function support:
     - Move some of the functionality from Math.hpp to Math.cpp to make
       compilation of package which depend on GeographicLib less
       sensitive to the current compiler environment.
     - Add Math::remainder, Math::remquo, Math::round, and Math::lround.
       Also add implementations of remainder, remquo to C
       implementation.
     - Math::cbrt, Math::atanh, and Math::asinh now preserve the sign of
       &minus;0.  (Also: C, Java, JavaScript, Python, MATLAB.  Not
       necessary: Fortran because sign is a built-in function.)
     - JavaScript: fall back to Math.hypot, Math.cbrt, Math.log1p,
       Math.atanh if they are available.
   - When parsing DMS strings ignore various non-breaking spaces (C++
     and JavaScript).
   - Improve code coverage in the tests of geodesic algorithms (C++, C,
     Java, JavaScript, Python, MATLAB, Fortran).
   - Old deprecated NormalGravity::NormalGravity constructor removed.
   - Additions to the documentation:
     - add documentation links to ge{distance,reckon}.m;
     - clarify which solution is returned for Geocentric::Reverse.
   .
 - <a href="https://geographiclib.sourceforge.io/1.49">Version 1.49</a>
   (released 2017-10-05)
   - Add the Enhanced Magnetic Model 2017, emm2017.  This is valid for
     2000 thru the end of 2021.
   - Avoid potential problems with the order of initializations in DMS,
     GARS, Geohash, Georef, MGRS, OSGB, SphericalEngine; this only would
     have been an issue if GeographicLib objects were instantiated
     globally.  Now no GeographicLib initialization code should be run
     prior to the entry of main().
   - To support the previous fix, add an overload,
     Utility::lookup(const char* s, char c).
   - NearestNeighbor::Search throws an error if \e pts is the wrong size
     (instead of merely returning no results).
   - Use complex arithmetic for Clenshaw sums in TransverseMercator and
     tranmerc_{fwd,inv}.m.
   - Changes in cmake support:
     - fix compiler flags for GEOGRAPHICLIB_PRECISION = 4;
     - add CONVERT_WARNINGS_TO_ERRORS option (default OFF), if ON then
       compiler warnings are treated as errors.
   - Fix warnings about implicit conversions of doubles to bools in C++,
     C, and JavaScript packages.
   - Binary installers for Windows now use Visual Studio 14 2015.
   .
 - <a href="https://geographiclib.sourceforge.io/1.48">Version 1.48</a>
   (released 2017-04-09)
   - The "official" URL for GeographicLib is now
     https://geographiclib.sourceforge.io (instead of
     http://geographiclib.sourceforge.net).
   - The default range for longitude and azimuth is now
     (&minus;180&deg;, 180&deg;], instead of [&minus;180&deg;,
     180&deg;).  This was already the case for the C++ library; now the
     change has been made to the other implementations (C, Fortran,
     Java, JavaScript, Python, MATLAB, and Maxima).
   - Changes to NearestNeighbor:
     - fix BUG in reading a NearestNeighbor object from a stream which
       sometimes incorrectly caused a "Bad index" exception to be
       thrown;
     - add NearestNeighbor::operator<<, NearestNeighbor::operator>>,
       NearestNeighbor::swap, std::swap(GeographicLib::NearestNeighbor&,
       GeographicLib::NearestNeighbor&);
   - Additions to the documentation:
     - add documentation on \ref nearest;
     - \ref normalgravity documentation is now on its own page and now
       has an illustrative figure;
     - document the \ref auxlaterror in the series for auxiliary
       latitudes.
   - Fix BUGS in MATLAB function geodreckon with mixed scalar and array
     arguments.
   - Workaround bug in math.fmod for Python 2.7 on 32-bit Windows
     machines.
   - Changes in cmake support:
     - add USE_BOOST_FOR_EXAMPLES option (default OFF), if ON search for
       Boost libraries for building examples;
     - add APPLE_MULTIPLE_ARCHITECTURES option (default OFF), if ON
       build for both i386 and x86_64 on Mac OS X systems;
     - don't add flag for C++11 for g++ 6.0 (since it's not needed).
   - Fix compiler warnings with Visual Studio 2017 and for the C
     library.
   .
 - <a href="https://geographiclib.sourceforge.io/1.47">Version 1.47</a>
   (released 2017-02-15)
   - Add NearestNeighbor class.
   - Improve accuracy of area calculation (fixing a flaw introduced in
     version 1.46); fix applied in Geodesic, GeodesicExact, and the
     implementations in C, Fortran, Java, JavaScript, Python, MATLAB,
     and Maxima.
   - Generalize NormalGravity to allow oblate and prolate ellipsoids.
     As a consequence a new form of constructor,
     NormalGravity::NormalGravity, has been introduced and the old
     form is now <b>deprecated</b> (and because the signatures of the two
     constructors are similar, the compiler will warn about the use of
     the old one).
   - Changes in Math class:
     - Math::sincosd, Math::sind, Math::cosd only return &minus;0 for
       the case sin(&minus;0);
     - Math::atan2d and Math::AngNormalize return results in
       (&minus;180&deg;, 180&deg;]; this may affect the longitudes and
       azimuth returned by several other functions.
   - Add Utility::trim() and Utility::val<T>(); Utility::num<T>() is now
     <b>deprecated</b>.
   - Changes in cmake support:
     - remove support of PACKAGE_PATH and INSTALL_PATH in cmake
       configuration;
     - fix to FindGeographicLib.cmake to make it work on Debian systems;
     - use $<TARGET_PDB_FILE:tgt> (cmake version &ge; 3.1);
     - use NAMESPACE for exported targets;
     - geographiclib-config.cmake exports GEOGRAPHICLIB_DATA,
       GEOGRAPHICLIB_PRECISION, and GeographicLib_HIGHPREC_LIBRARIES.
   - Add pkg-config support for cmake and autoconf builds.
   - Minor fixes:
     - fix the order of declarations in C library, incorporating the
       patches in version 1.46.1;
     - fix the packaging of the Python library, incorporating the
       patches in version 1.46.3;
     - restrict junit dependency in the Java package to testing scope
       (thanks to Mick Killianey);
     - various behind-the-scenes fixes to EllipticFunction;
     - fix documentation and default install location for Windows binary
       installers;
     - fix clang compiler warnings in GeodesicExactC4 and
       TransverseMercator.
   .
 - <a href="https://geographiclib.sourceforge.io/1.46">Version 1.46</a>
   (released 2016-02-15)
   - The following BUGS have been fixed:
     - the -w flag to <a href="Planimeter.1.html">Planimeter(1)</a> was
       being ignored;
     - in the Java package, the wrong longitude was being returned with
       direct geodesic calculation with a negative distance when
       starting point was at a pole (this bug was introduced in version
       1.44);
     - in the JavaScript package, PolygonArea.TestEdge contained a
       misspelling of a variable name and other typos (problem found by
       threepointone).
   - INCOMPATIBLE CHANGES:
     - make the -w flag (to swap the default order of latitude and
       longitude) a toggle for all \ref utilities;
     - the -a option to <a href="GeodSolve.1.html">GeodSolve(1)</a> now
       toggles (instead of sets) arc mode;
     - swap order \e coslon and \e sinlon arguments in CircularEngine
       class.
   - Remove deprecated functionality:
     - remove gradient calculation from the Geoid class and
       <a href="GeoidEval.1.html">GeoidEval(1)</a> (this was inaccurate
       and of dubious utility);
     - remove reciprocal flattening functions, InverseFlattening in many
       classes and Constants::WGS84_r(); stop treating flattening &gt; 1
       as the reciprocal flattening in constructors;
     - remove DMS::Decode(string), DMS::DecodeFraction,
       EllipticFunction:m, EllipticFunction:m1, Math::extradigits,
       Math::AngNormalize2, PolygonArea::TestCompute;
     - stop treating LONG_NOWRAP as an alias for LONG_UNROLL in
       Geodesic (and related classes) and Rhumb;
     - stop treating full/schmidt as aliases for FULL/SCHMIDT in
       SphericalEngine (and related classes);
     - remove qmake project file src/GeographicLib.pro because QtCreator
       can handle cmake projects now;
     - remove deprecated Visual Studio 2005 project and solution files.
   - Changes to GeodesicLine and GeodesicLineExact classes; these
     changes (1) simplify the process of computing waypoints on a
     geodesic given two endpoints and (2) allow a GeodesicLine to be
     defined which is consistent with the solution of the inverse
     problem (in particular Geodesic::InverseLine the specification of
     south-going lines which pass the poles in a westerly direction by
     setting sin &alpha;<sub>1</sub> = &minus;0):
     - the class stores the distance \e s13 and arc length \e a13 to a
       reference point 3; by default these quantities are NaNs;
     - GeodesicLine::SetDistance (and GeodesicLine::SetArc) specify the
       distance (and arc length) to point 3;
     - GeodesicLine::Distance (and GeodesicLine::Arc) return the
       distance (and arc length) to point 3;
     - new methods Geodesic::InverseLine and Geodesic::DirectLine return
       a GeodesicLine with the reference point 3 defined as point 2 of
       the corresponding geodesic calculation;
     - these changes are also included in the C, Java, JavaScript, and
       Python packages.
   - Other changes to the geodesic routines:
     - more accurate solution of the inverse problem when longitude
       difference is close to 180&deg; (also in C, Fortran, Java,
       JavaScript, Python, MATLAB, and Maxima packages);
     - more accurate calculation of lon2 in the inverse calculation with
       LONG_UNROLL (also in Java, JavaScript, Python packages).
   - Changes to <a href="GeodSolve.1.html">GeodSolve(1)</a> utility:
     - the -I and -D options now specify geodesic line calculation via
       the standard inverse or direct geodesic problems;
     - rename -l flag to -L to parallel the new -I and -D flags (-l is
       is retained for backward compatibility but is <b>deprecated</b>),
       and similarly for <a href="GeodSolve.1.html">RhumbSolve(1)</a>;
     - the -F flag (in conjunction with the -I or -D flags) specifies that
       distances read on standard input are fractions of \e s13 or \e
       a13;
     - the -a option now toggles arc mode (noted above);
     - the -w option now toggles longitude first mode (noted above).
   - Changes to Math class:
     - Math::copysign added;
     - add overloaded version of Math::AngDiff which returns the error
       in the difference.  This allows a more accurate treatment of
       inverse geodesic problem when \e lon12 is close to 180&deg;;
     - Math::AngRound now converts tiny negative numbers to &minus;0
       (instead of +0), however &minus;0 is still converted to +0.
   - Add -S and -T options to
     <a href="GeoConvert.1.html">GeoConvert(1)</a>.
   - Add <a href="https://www.sphinx-doc.org/en/master/">Sphinx</a>
     documentation for Python package.
   - Samples of wrapping the C++ library, so it's accessible in other
     languages, are given in <code>wrapper/C</code>,
     <code>wrapper/python</code>, and <code>wrapper/matlab</code>.
   - Binary installers for Windows now use Visual Studio 12 2013.
   - Remove top-level pom.xml from release (it was specific to SRI).
   - A reminder: because of the JavaScript changes introduced in version
     1.45, you should remove the following installation directories from
     your system:
     - Windows: ${CMAKE_INSTALL_PREFIX}/doc/scripts
     - Others: ${CMAKE_INSTALL_PREFIX}/share/doc/GeographicLib/scripts
   .
 - <a href="https://geographiclib.sourceforge.io/1.45">Version 1.45</a>
   (released 2015-09-30)
   - Fix BUG in solution of inverse geodesic caused by misbehavior of
     some versions of Visual Studio on Windows (fmod(&minus;0.0, 360.0)
     returns +0.0 instead of &minus;0.0) and Octave (sind(&minus;0.0)
     returns +0.0 instead of &minus;0.0).  These bugs were exposed
     because max(&minus;0.0, +0.0) returns &minus;0.0 for some
     languages.
   - Geodesic::Inverse now correctly returns NaNs if one of the
     latitudes is a NaN.
   - Changes to JavaScript package:
     - thanks to help from Yurij Mikhalevich, it is a now a
       <a href="https://nodejs.org">node</a> package that can be
       installed with <a href="https://www.npmjs.com">npm</a>;
     - make install now installs the node package in
       <code>lib/node_modules/geographiclib</code>;
     - add unit tests using mocha;
     - add documentation via <a href="http://usejsdoc.org">JSDoc</a>;
     - fix bug Geodesic.GenInverse (this bug, introduced in version
       1.44, resulted in the wrong azimuth being reported for points at
       the pole).
   - Changes to Java package:
     - add implementation of ellipsoidal Gnomonic projection (courtesy
       of Sebastian Mattheis);
     - add unit tests using JUnit;
     - Math.toRadians and Math.toDegrees are used instead of
       GeoMath.degree (which is now removed), as a result&hellip;
     - Java version 1.2 (released 1998-12) or later is now required.
   - Changes to Python package:
     - add unit tests using the unittest framework;
     - fixed bug in normalization of the area.
   - Changes to MATLAB package:
     - fix array size mismatch in geoddistance by avoiding calls to
       subfunctions with zero-length arrays;
     - fix tranmerc_{fwd,inv} so that they work with arrays and
       mixed array/scalar arguments;
     - work around Octave problem which causes mgrs_fwd to return
       garbage with prec = 10 or 11;
     - add geographiclib_test.m to run a test suite.
   - Behavior of substituting 1/\e f for \e f if \e f &gt; 1 is now
     <b>deprecated</b>.  This behavior has been removed from the
     JavaScript, C, and Python implementations (it was never
     documented).  Maxima, MATLAB, and Fortran implementations never
     included this behavior.
   - Other changes:
     - fix bug, introduced in version 1.42, in the C++ implementation to
       the computation of area which causes NaNs to be returned in the
       case of a sphere;
     - fixed bug, introduced in version 1.44, in the detection of C++11
       math functions in configure.ac;
     - throw error on non-convergence in Gnomonic::Reverse if
       GEOGRAPHICLIB_PRECISION &gt; 3;
     - add geod_polygon_clear to C library;
     - turn illegal latitudes into NaNs for Fortran library;
     - add test suites for the C and Fortran libraries.
   .
 - <a href="https://geographiclib.sourceforge.io/1.44">Version 1.44</a>
   (released 2015-08-14)
   - Various changes to improve accuracy, e.g., by minimizing round-off
     errors:
     - Add Math::sincosd, Math::sind, Math::cosd which take their
       arguments in degrees.  These functions do exact range reduction
       and thus they obey exactly the elementary properties of the
       trigonometric functions, e.g., sin 9&deg; = cos 81&deg; = &minus;
       sin 123456789&deg;.
     - Math::AngNormalize now works for any angles, instead of angles in
       the range [&minus;540&deg;, 540&deg;); the function
       Math::AngNormalize2 is now <b>deprecated</b>.
     - This means that there is now no restriction on longitudes and
       azimuths; any values can be used.
     - Improve the accuracy of Math::atan2d.
     - DMS::Decode avoids unnecessary round-off errors; thus 7:33:36 and
       7.56 result in identical values.  DMS::Encode rounds ties to
       even.  These changes have also been made to DMS.js.
     - More accurate rounding in MGRS::Reverse and mgrs_inv.m; this
       change only makes a difference at sub-meter precisions.
     - With MGRS::Forward and mgrs_fwd.m, ensure that digits in lower
       precision results match those at higher precision; as a result,
       strings of trailing 9s are less likely to be generated.  This
       change only makes a difference at sub-meter precisions.
     - Replace the series for <i>A</i><sub>2</sub> in the Geodesic class
       with one with smaller truncation errors.
     - Geodesic::Inverse sets \e s12 to zero for coincident points at
       pole (instead of returning a tiny quantity).
     - Math::LatFix returns its argument if it is in [&minus;90&deg;,
       90&deg;]; if not, it returns NaN.
     - Using Math::LatFix, routines which don't check their arguments
       now interpret a latitude outside the legal range of
       [&minus;90&deg;, 90&deg;] as a NaN; such routines will return
       NaNs instead of finite but incorrect results; <b>caution</b>:
       code that (dangerously) relied on the "reasonable" results being
       returned for values of the latitude outside the allowed range
       will now malfunction.
   - All the \ref utilities accept the -w option to swap the
     latitude-longitude order on input and output (and where appropriate
     on the command-line arguments).  CartConvert now accepts the -p
     option to set the precision; now all of the utilities except
     GeoidEval accept -p.
   - Add classes for GARS, the Global Area Reference System, and for
     Georef, the World Geographic Reference System.
   - Changes to DMS::Decode and DMS.js:
     - tighten up the rules:
       - 30:70.0 and 30:60 are illegal (minutes and second must be
         strictly less than 60), however
       - 30:60.0 and 30:60. are legal (floating point 60 is OK, since it
         might have been generated by rounding 59.99&hellip;);
     - generalize a+b concept, introduced in version 1.42, to any number
       of pieces; thus 8+0:40-0:0:10 is interpreted as 8:39:50.
   - Documentation fixes:
     - update man pages to refer to
       <a href="GeoConvert.1.html">GeoConvert(1)</a> on handling of
       geographic coordinates;
     - document limitations of the series used for TransverseMercator;
     - hide the documentation of the computation of the gradient of the
       geoid height (now <b>deprecated</b>) in the Geoid class;
     - warn about the possible misinterpretation of 7.0E+1 by
       DMS::Decode;
     - \e swaplatlong optional argument of DMS::DecodeLatLon and
       various functions in the GeoCoords class is now called \e
       longfirst;
     - require Doxygen 1.8.7 or later.
   - More systematic treatment of version numbers:
     - Python: \__init\__.py defines \__version\__ and \__version_info\__;
     - JavaScript:
       - Math.js defines Constants.version and Constants.version_string;
       - version number included as comment in packed script
         geographiclib.js;
       - <a href="../scripts/geod-calc.html">geod-calc.html</a> and
         <a href="../scripts/geod-google.html">geod-google.html</a>
         report the version number;
       - https://geographiclib.sourceforge.io/scripts/ gives access to
         earlier versions of geographiclib.js as
         geographiclib-<i>m.nn</i>.js;
     - Fortran: add geover subroutine to return version numbers;
     - Maxima: geodesic.mac defines geod_version;
     - CGI scripts: these report the version numbers of the utilities.
   - BUG FIXES:
     - NormalGravity now works properly for a sphere (\e omega = \e f =
       \e J2 = 0), instead of returning NaNs (problem found by htallon);
     - CassiniSoldner::Forward and cassini_fwd.m now returns the correct
       azimuth for points at the pole.
   - MATLAB-specific fixes:
     - mgrs_fwd now treats treats prec &gt; 11 as prec = 11;
     - illegal letter combinations are now correctly detected by
       mgrs_inv;
     - fixed bug where mgrs_inv returned the wrong results for prec = 0
       strings and center = 0;
     - mgrs_inv now decodes prec = 11 strings properly;
     - routines now return array results with the right shape;
     - routines now properly handle mixed scalar and array arguments.
   - Add Accumulator<T>::operator*=(T y).
   - Geohash uses "invalid" instead of "nan" when the latitude or
     longitude is a nan.
   .
 - <a href="https://geographiclib.sourceforge.io/1.43">Version 1.43</a>
   (released 2015-05-23)
   - Add the Enhanced Magnetic Model 2015, emm2015.  This is valid for
     2000 thru the end of 2019.  This required some changes in the
     MagneticModel and MagneticCircle classes; so this model cannot be
     used with versions of GeographicLib prior to 1.43.
   - Fix BLUNDER in PolarStereographic constructor introduced in version
     1.42.  This affected UTMUPS conversions for UPS which could be
     incorrect by up to 0.5 km.
   - Changes in the LONG_NOWRAP option (added in version 1.39) in the
     Geodesic and GeodesicLine classes:
     - The option is now called LONG_UNROLL (a less negative sounding
       term); the original name, LONG_NOWRAP, is retained for backwards
       compatibility.
     - There were two bad BUGS in the implementation of this capability:
       (a)&nbsp;it gave <i>incorrect</i> results for west-going
       geodesics; (b)&nbsp;the option was <i>ignored</i> if used
       directly via the GeodesicLine class.  The first bug affected the
       implementations in all languages.  The second affected the
       implementation in C++ (GeodesicLine and GeodesicLineExact),
       JavaScript, Java, C, Python.  These bugs have now been FIXED.
     - The <a href="GeodSolve.1.html">GeodSolve</a> utility now accepts
       a -u option, which turns on the LONG_UNROLL treatment.  With this
       option <code>lon1</code> is reported as entered and
       <code>lon2</code> is given such that <code>lon2</code> &minus;
       <code>lon1</code> indicates how often and in what sense the
       geodesic has encircled the earth.  (This option also affects the
       value of longitude reported when an inverse calculation is run
       with the -f option.)
     - The inverse calculation with the JavaScript and Python libraries
       similarly sets <code>lon1</code> and <code>lon2</code> in output
       dictionary respecting the LONG_UNROLL flag.
     - The online version of
       <a href="https://geographiclib.sourceforge.io/cgi-bin/GeodSolve">
       GeodSolve</a> now offers an option to unroll the longitude.
     - To support these changes DMS::DecodeLatLon no longer reduces the
       longitude to the range [&minus;180&deg;, 180&deg;) and
       Math::AngRound now coverts &minus;0 to +0.
   - Add Math::polyval (also to C, Java, JavaScript, Fortran, Python
     versions of the library; this is a built-in function for
     MATLAB/Octave).  This evaluates a polynomial using Horner's method.
     The Maxima-generated code fragments for the evaluation of series in
     the Geodesic, TransverseMercator, and Rhumb classes and MATLAB
     routines for great ellipses have been replaced by Maxima-generated
     arrays of polynomial coefficients which are used as input to
     Math::polyval.
   - Add MGRS::Check() to verify that \e a, \e f,
     <i>k</i><sub>UTM</sub>, and <i>k</i><sub>UPS</sub> are consistent
     with the assumptions in the UTMUPS and MGRS classes.  This is
     invoked with <code>GeoConvert \--version</code>.  (This function
     was added to document and check the assumptions used in the UTMUPS
     and MGRS classes in case they are extended to deal with ellipsoids
     other than WS84.)
   - MATLAB function mgrs_inv now takes an optional \e center argument
     and strips white space from both beginning and end of the string.
   - Minor internal changes:
     - GeodSolve sets the geodesic mask so that unnecessary calculations
       are avoided;
     - some routines have migrated into a math class for the Python,
       Java, and JavaScript libraries.
   - A reminder: because of changes in the installation directories for
     non-Windows systems introduced in version 1.42, you should remove
     the following directories from your system:
     - ${CMAKE_INSTALL_PREFIX}/share/cmake/GeographicLib*
     - ${CMAKE_INSTALL_PREFIX}/libexec/GeographicLib/matlab
   .
 - <a href="https://geographiclib.sourceforge.io/1.42">Version 1.42</a>
   (released 2015-04-28)
   - DMS::Decode allows a single addition or subtraction operation,
     e.g., 70W+0:0:15.  This affects the GeoCoords class and the
     utilities (which use the DMS class for reading coordinates).
   - Add Math::norm, Math::AngRound, Math::tand, Math::atan2d,
     Math::eatanhe, Math::taupf, Math::tauf, Math::fma and remove
     duplicated (but private) functionality from other classes.
   - On non-Windows systems, the cmake config-style find_package files
     are now installed under ${CMAKE_INSTALL_PREFIX}/lib${LIB_SUFFIX}
     instead of ${CMAKE_INSTALL_PREFIX}/share, because the files are
     architecture-specific.  This change will let 32-bit and 64-bit
     versions coexist on the same machine (in lib and lib64).  You
     should remove the versions in the old "share" location.
   - MATLAB changes:
     - provide native MATLAB implementations for compiled interface
       functions, see `wrapper/octave`;
     - the compiled MATLAB interface is now <b>deprecated</b> and so the
       MATLAB_COMPILER option in the cmake build has been removed;
     - reorganize directories, so that
       - matlab/geographiclib contains the native matlab code;
       - matlab/geographiclib-legacy contains wrapper functions to mimic
         the previous compiled functionality;
     - the installed MATLAB code mirrors this layout, but the parent
       installation directory on non-Windows systems is
       ${CMAKE_INSTALL_PREFIX}/share (instead of
       ${CMAKE_INSTALL_PREFIX}/libexec), because the files are now
       architecture independent;
     - matlab/geographiclib is now packaged and distributed as
       MATLAB File Exchange package
       <a href="https://www.mathworks.com/matlabcentral/fileexchange/50605">
       50605</a> (this supersedes three earlier MATLAB packages);
     - point fix for geodarea.m to correct bug in area of polygons which
       encircle a pole multiple times (released as version 1.41.1 of
       MATLAB File Exchange package 39108, 2014-04-22).
   - artifactId for Java package changed from GeographicLib to
     GeographicLib-Java and the package is now deployed to
     Maven Central (thanks to Chris Bennight for help on this).
   - Fix autoconf mismatch of version numbers (which were inconsistent
     in versions 1.40 and 1.41).
   - Mark the computation of the gradient of the geoid height in the
     Geoid class and the <a href="GeoidEval.1.html">GeoidEval</a>
     utility as <b>deprecated</b>.
   - Work around the boost-quadmath bug with setprecision(0).
   - Deprecate use of Visual Studio 2005 "-vc8" project files in the
     windows directory.
   .
 - <a href="https://geographiclib.sourceforge.io/1.41">Version 1.41</a>
   (released 2015-03-09)
   - Fix bug in Rhumb::Inverse (with \e exact = true) and related
     functions which causes the wrong distance to be reported if one of
     the end points is at a pole.  Thanks to Thomas Murray for reporting
     this.
   - Add International Geomagnetic Reference Field (12th generation),
     igrf12, which approximates the main magnetic field of the earth for
     the period 1900--2020.
   - Split information about \ref jacobi to a separate section and
     include more material.
   .
 - <a href="https://geographiclib.sourceforge.io/1.40">Version 1.40</a>
   (released 2014-12-18)
   - Add the World Magnetic Model 2015, wmm2015.  This is now the
     default magnetic model for
     <a href="MagneticField.1.html">MagneticField</a> (replacing wmm2010
     which is valid thru the end of 2014).
   - Geodesic::Inverse didn't return NaN if one of the longitudes was a
     NaN (bug introduced in version 1.25).  Fixed in the C++, Java,
     JavaScript, C, Fortran, and Python implementations of the geodesic
     routines.  This bug was not present in the MATLAB version.
   - Fix bug in Utility::readarray and Utility::writearray which caused
     an exception in debug mode with zero-sized arrays.
   - Fix BLUNDER in OSGB::GridReference (found by kalderami) where the
     wrong result was returned if the easting or northing was negative.
   - OSGB::GridReference now returns "INVALID" if either coordinate is
     NaN.  Similarly a grid reference starting with "IN" results in NaNs
     for the coordinates.
   - Default constructor for GeoCoords corresponds to an undefined
     position (latitude and longitude = NaN), instead of the north pole.
   - Add an online version of <a href="RhumbSolve.1.html">RhumbSolve</a>
     at https://geographiclib.sourceforge.io/cgi-bin/RhumbSolve.
   - Additions to the documentation:
     - documentation on \ref triaxial-conformal;
     - a page on \ref auxlat (actually, this was added in version 1.39);
     - document the use of two single quotes to stand for a double quote
       in DMS (this feature was introduced in version 1.13).
   - The MATLAB function, geographiclibinterface, which compiles the
     wrapper routines for MATLAB now works with MATLAB on a Mac.
   .
 - <a href="https://geographiclib.sourceforge.io/1.39">Version 1.39</a>
   (released 2014-11-11)
   - GeographicLib usually normalizes longitudes to the range
     [&minus;180&deg;, 180&deg;).  However, when solving the direct
     geodesic and rhumb line problems, it is sometimes necessary to know
     how many lines the line encircled the earth by returning the
     longitude "unwrapped".  So the following changes have been made:
     - add a LONG_NOWRAP flag to \e mask enums for the \e outmask
       arguments for Geodesic, GeodesicLine, Rhumb, and RhumbLine;
     - similar changes have been made to the Python, JavaScript, and
       Java implementations of the geodesic routines;
     - for the C, Fortran, and MATLAB implementations the \e arcmode
       argument to the routines was generalized to allow a combination
       of ARCMODE and LONG_NOWRAP bits;
     - the Maxima version now returns the longitude unwrapped.
     .
     These changes were necessary to fix the PolygonAreaT::AddEdge (see
     the next item).
   - Changes in area calculations:
     - fix BUG in PolygonAreaT::AddEdge (also in C, Java, JavaScript,
       and Python implementations) which sometimes causes the wrong area
       to be returned if the edge spanned more than 180&deg;;
     - add area calculation to the Rhumb and RhumbLine classes and the
       <a href="RhumbSolve.1.html">RhumbSolve</a> utility (see \ref
       rhumbarea);
     - add PolygonAreaRhumb typedef for PolygonAreaT<Rhumb>;
     - add -R option to <a href="Planimeter.1.html">Planimeter</a> to use
       PolygonAreaRhumb (and -G option for the default geodesic polygon);
     - fix BLUNDER in area calculation in MATLAB routine geodreckon;
     - add area calculation to MATLAB/Octave routines for great ellipses
       (see \ref gearea).
   - Fix bad BUG in Geohash::Reverse; this was introduced in version
     1.37 and affected all platforms where unsigned longs are 32-bits.
     Thanks to Christian Csar for reporting and diagnosing this.
   - Binary installers for Windows are now built with Visual Studio 11
     2012 (instead of Visual Studio 10 2010).  Compiled MATLAB support
     still with version 2013a (64-bit).
   - Update GeographicLib.pro for builds with qmake to include all the
     source files.
   - Cmake updates:
     - include cross-compiling checks in cmake config file;
     - improve the way unsuitable versions are reported;
     - include_directories (${GeographicLib_INCLUDE_DIRS}) is no longer
       necessary with cmake 2.8.11 or later.
   - legacy/Fortran now includes drop-in replacements for the geodesic
     utilities from the NGS.
   - geographiclib-get-{geoids,gravity,magnetic} with no arguments now
     print the usage instead of loading the minimal sets.
   - Utility::date(const std::string&, int&, int&, int&) and hence the
     <a href="MagneticField.1.html">MagneticField</a> utility accepts
     the string "now" as a legal time (meaning today).
   .
 - <a href="https://geographiclib.sourceforge.io/1.38">Version 1.38</a>
   (released 2014-10-02)
   - On MacOSX, the installed package is relocatable (for cmake version
     2.8.12 and later).
   - On Mac OSX, GeographicLib can be installed using homebrew.
   - In cmake builds under Windows, set the output directories so that
     binaries and shared libraries are together.
   - Accept the minus sign as a synonym for - in DMS.{cpp,js}.
   - The cmake configuration file geographiclib-depends.cmake has been
     renamed to geographiclib-targets.cmake.
   - MATLAB/Octave routines for great ellipses added; see \ref
     greatellipse.
   - Provide man pages for geographiclib-get-{geoids,gravity,magnetic}.
   .
 - <a href="https://geographiclib.sourceforge.io/1.37">Version 1.37</a>
   (released 2014-08-08)
   - Add \ref highprec.
   - <b>INCOMPATIBLE CHANGE</b>: the static instantiations of various
     classes for the WGS84 ellipsoid have been changed to a
     <a href="https://isocpp.org/wiki/faq/ctors#static-init-order-on-first-use">
     "construct on first use idiom"</a>.  This avoids a lot of wasteful
     initialization before the user's code starts.  Unfortunately it
     means that existing source code that relies on any of the following
     static variables will need to be changed to a function call:
     - AlbersEqualArea::AzimuthalEqualAreaNorth
     - AlbersEqualArea::AzimuthalEqualAreaSouth
     - AlbersEqualArea::CylindricalEqualArea
     - Ellipsoid::WGS84
     - Geocentric::WGS84
     - Geodesic::WGS84
     - GeodesicExact::WGS84
     - LambertConformalConic::Mercator
     - NormalGravity::GRS80
     - NormalGravity::WGS84
     - PolarStereographic::UPS
     - TransverseMercator::UTM
     - TransverseMercatorExact::UTM
     .
     Thus, occurrences of, for example, \code
     const Geodesic& geod = Geodesic::WGS84; // version 1.36 and earlier
     \endcode
     need to be changed to \code
     const Geodesic& geod = Geodesic::WGS84(); // version 1.37 and later
     \endcode
     (note the parentheses!); alternatively use \code
     // works with all versions
     const Geodesic geod(Constants::WGS84_a(), Constants::WGS84_a()); \endcode
   - Incompatible change: the environment variables
     <code>{GEOID,GRAVITY,MAGNETIC}_{NAME,PATH}</code> are now prefixed
     with <code>GEOGRAPHICLIB_</code>.
   - Incompatible change for Windows XP: retire the Windows XP common
     data path.  If you're still using Windows XP, then you might have
     to move the folder <code>C:\\Documents and Settings\\All
     Users\\Application Data\\GeographicLib</code> to
     <code>C:\\ProgramData\\GeographicLib</code>.
   - All macro names affecting the compilation now start with
     <code>GEOGRAPHICLIB_</code>; this applies to
     <code>GEOID_DEFAULT_NAME</code>, <code>GRAVITY_DEFAULT_NAME</code>,
     <code>MAGNETIC_DEFAULT_NAME</code>, <code>PGM_PIXEL_WIDTH</code>,
     <code>HAVE_LONG_DOUBLE</code>, <code>STATIC_ASSERT</code>,
     <code>WORDS_BIGENDIAN</code>.
   - Changes to PolygonArea:
     - introduce PolygonAreaT which takes a geodesic class as a parameter;
     - PolygonArea and PolygonAreaExact are typedef'ed to
       PolygonAreaT<Geodesic> and PolygonAreaT<GeodesicExact>;
     - add -E option to <a href="Planimeter.1.html">Planimeter</a> to use
       PolygonAreaExact;
     - add -Q option to <a href="Planimeter.1.html">Planimeter</a> to
       calculate the area on the authalic sphere.
   - Add -p option to <a href="Planimeter.1.html">Planimeter</a>,
     <a href="ConicProj.1.html">ConicProj</a>,
     <a href="GeodesicProj.1.html">GeodesicProj</a>,
     <a href="TransverseMercatorProj.1.html">TransverseMercatorProj</a>.
   - Add Rhumb and RhumbLine classes and the
     <a href="RhumbSolve.1.html">RhumbSolve</a> utility; see \ref rhumb
     for more information.
   - Minor changes to NormalGravity:
     - add NormalGravity::J2ToFlattening and NormalGravity::FlatteningToJ2;
     - use Newton's method to determine \e f from \e J2;
     - in constructor, allow \e omega = 0 (i.e., treat the spherical case).
   - Add grs80 GravityModel, see \ref gravity.
   - Make geographiclib-get-{geoids,gravity,magnetic} scripts work on MacOS.
   - Minor changes:
     - simplify cross-platform support for C++11 mathematical functions;
     - change way area coefficients are given in GeodesicExact to improve
       compile times;
     - enable searching the online documentation;
     - add macros <code>GEOGRAPHICLIB_VERSION</code> and
       <code>GEOGRAPHICLIB_VERSION_NUM</code>;
     - add solution and project files for Visual Studio Express 2010.
   .
 - <a href="https://geographiclib.sourceforge.io/1.36">Version 1.36</a>
   (released 2014-05-13)
   - Changes to comply with NGA's prohibition of the use of the
     upper-case letters N/S to designate the hemisphere when displaying
     UTM/UPS coordinates:
     - UTMUPS::DecodeZone allows north/south as hemisphere
       designators (in addition to n/s);
     - UTMUPS::EncodeZone now encodes the hemisphere in
       <i>lower case</i> (to distinguish this use from a grid zone
       designator);
     - UTMUPS::EncodeZone takes an optional parameter \e
       abbrev to indicate whether to use n/s or north/south as the
       hemisphere designator;
     - GeoCoords::UTMUPSRepresentation and
       GeoCoords::AltUTMUPSRepresentation similarly accept
       the \e abbrev parameter;
     - <a href="GeoConvert.1.html">GeoConvert</a> uses the
       flags -a and -l to govern whether UTM/UPS output uses n/s
       (the -a flag) or north/south (the -l flag) to denote the
       hemisphere;
     - Fixed a bug what allowed +3N to be accepted as an alternative UTM
       zone designation (instead of 3N).
     .
     <b>WARNING:</b> The use of lower case n/s for the hemisphere might
     cause compatibility problems.  However DecodeZone has always
     accepted either case; so the issue will only arise with other
     software reading the zone information.  To avoid possible
     misinterpretation of the zone designator, consider calling
     EncodeZone with \e abbrev = false and GeoConvert with -l, so that
     north/south are used to denote the hemisphere.
   - MGRS::Forward with \e prec = &minus;1 will produce a grid
     zone designation.  Similarly MGRS::Reverse will
     decode a grid zone designation (and return \e prec = &minus;1).
   - Stop using the throw() declaration specification which is
     deprecated in C++11.
   - Add missing std:: qualifications to copy in LocalCartesion and
     Geocentric headers (bug found by Clemens).
   .
 - <a href="https://geographiclib.sourceforge.io/1.35">Version 1.35</a>
   (released 2014-03-13)
   - Fix blunder in UTMUPS::EncodeEPSG (found by Ben
     Adler).
   - MATLAB wrapper routines geodesic{direct,inverse,line} switch to
     "exact" routes if |<i>f</i>| &gt; 0.02.
   - GeodSolve.cgi allows ellipsoid to be set (and uses the -E option
     for <a href="GeodSolve.1.html">GeodSolve</a>).
   - Set title in HTML versions of man pages for the \ref utilities.
   - Changes in cmake support:
     - add _d to names of executables in debug mode of Visual Studio;
     - add support for Android (cmake-only), thanks to Pullan Yu;
     - check CPACK version numbers supplied on command line;
     - configured version of project-config.cmake.in is
       project-config.cmake (instead of geographiclib-config.cmake), to
       prevent find_package incorrectly using this file;
     - fix tests with multi-line output;
     - this release includes a file, pom.xml, which is used by an
       experimental build system (based on maven) at SRI.
   .
 - <a href="https://geographiclib.sourceforge.io/1.34">Version 1.34</a>
   (released 2013-12-11)
   - Many changes in cmake support:
     - minimum version of cmake needed increased to 2.8.4 (which was
       released in 2011-02);
     - allow building both shared and static libraries with <code>-D
       GEOGRAPHICLIB_LIB_TYPE=BOTH</code>;
     - both shared and static libraries (Release plus Debug) included in
       binary installer;
     - find_package uses COMPONENTS and GeographicLib_USE_STATIC_LIBS to
       select the library to use;
     - find_package version checking allows nmake and Visual Studio
       generators to interoperate on Windows;
     - find_package (GeographicLib &hellip;) requires that GeographicLib be
       capitalized correctly;
     - on Unix/Linux, don't include the version number in directory for
       the cmake configuration files;
     - defaults for GEOGRAPHICLIB_DOCUMENTATION and
       BUILD_NETGEOGRAPHICLIB are now OFF;
     - the GEOGRAPHICLIB_EXAMPLES configuration parameter is no longer
       used; cmake always configures to build the examples, but they are
       not built by default (instead build targets: exampleprograms and
       netexamples);
     - matlab-all target renamed to matlabinterface;
     - the configuration parameters PACKAGE_PATH and INSTALL_PATH are
       now deprecated (use CMAKE_INSTALL_PREFIX instead);
     - on Linux, the installed package is relocatable;
     - on MacOSX, the installed utilities can find the shared library.
   - Use a more precise value for OSGB::CentralScale().
   - Add Arc routines to Python interface.
   - The Geod utility has been removed; the same functionality lives on
     with <a href="GeodSolve.1.html">GeodSolve</a> (introduced in
     version 1.30).
   .
 - <a href="https://geographiclib.sourceforge.io/1.33">Version 1.33</a>
   (released 2013-10-08)
   - Add <a href="NET/index.html">NETGeographic .NET wrapper library</a>
     (courtesy of Scott Heiman).
   - Make inspector functions in Ellipsoid const.
   - Add Accumulator.cpp to instantiate Accumulator.
   - Defer some of the initialization of OSGB to when it
     is first called.
   - Fix bug in autoconf builds under MacOS.
   .
 - <a href="https://geographiclib.sourceforge.io/1.32">Version 1.32</a>
   (released 2013-07-12)
   - Generalize C interface for polygon areas to allow vertices to be
     specified incrementally.
   - Fix way flags for C++11 support are determined.
   .
 - <a href="https://geographiclib.sourceforge.io/1.31">Version 1.31</a>
   (released 2013-07-01)
   - Changes breaking binary compatibility (source compatibility is
     maintained):
     - overloaded versions of DMS::Encode,
       EllipticFunction::EllipticFunction, and
       GeoCoords::DMSRepresentation, have been eliminated
       by the use of optional arguments;
     - correct the declaration of first arg to
       UTMUPS::DecodeEPSG.
   - FIX BUG in GravityCircle constructor (found by
     Mathieu Peyr&eacute;ga) which caused bogus results for the gravity
     disturbance and gravity anomaly vectors.  (This only affected
     calculations using GravityCircle.  GravityModel calculations did
     not suffer from this bug.)
   - Improvements to the build:
     - add macros GEOGRAPHICLIB_VERSION_{MAJOR,MINOR,PATCH} to Config.h;
     - fix documentation for new version of perlpod;
     - improving setting of runtime path for Unix-like systems with cmake;
     - install PDB files when compiling with Visual Studio to aid
       debugging;
     - Windows binary release now uses MATLAB R2013a (64-bit) and
       uses the -largeArrayDims option.
     - fixes to the way the MATLAB interface routines are built (thanks
       to Phil Miller and Chris F.).
   - Changes to the geodesic routines:
     - add Java implementation of the geodesic routines (thanks to Skip
       Breidbach for the maven support);
     - FIX BUG: avoid altering input args in Fortran implementation;
     - more systematic treatment of very short geodesic;
     - fixes to Python port so that they work with version 3.x, in
       addition to 2.x (courtesy of Amato);
     - accumulate the perimeter and area of polygons via a double-wide
       accumulator in Fortran, C, and MATLAB implementations (this is
       already included in the other implementations);
     - port PolygonArea::AddEdge and
       PolygonArea::TestEdge to JavaScript and Python
       interfaces;
     - include documentation on \ref geodshort.
   - Unix scripts for downloading datasets,
     geographiclib-get-{geoids,gravity,magnetic}, skip already download
     models by default, unless the -f flag is given.
   - FIX BUGS: meridian convergence and scale returned by
     TransverseMercatorExact was wrong at a pole.
   - Improve efficiency of MGRS::Forward by avoiding the
     calculation of the latitude if possible (adapting an idea of Craig
     Rollins).
   .
 - <a href="https://geographiclib.sourceforge.io/1.30">Version 1.30</a>
   (released 2013-02-27)
   - Changes to geodesic routines:
     - FIX BUG in fail-safe mechanisms in Geodesic::Inverse;
     - the command line utility Geod is now called
       <a href="GeodSolve.1.html">GeodSolve</a>;
     - allow addition of polygon edges in PolygonArea;
     - add full Maxima implementation of geodesic algorithms.
   .
 - <a href="https://geographiclib.sourceforge.io/1.29">Version 1.29</a>
   (released 2013-01-16)
   - Changes to allow compilation with libc++ (courtesy of Kal Conley).
   - Add description of \ref triaxial to documentation.
   - Update journal reference for "Algorithms for geodesics".
   .
 - <a href="https://geographiclib.sourceforge.io/1.28">Version 1.28</a>
   (released 2012-12-11)
   - Changes to geodesic routines:
     - compute longitude difference exactly;
     - hence FIX BUG in area calculations for polygons with vertices very
       close to the prime meridian;
     - FIX BUG is geoddistance.m where the value of m12 was wrong for
       meridional geodesics;
     - add MATLAB implementations of the geodesic projections;
     - remove unneeded special code for geodesics which start at a pole;
     - include polygon area routine in C and Fortran implementations;
     - add doxygen documentation for C and Fortran libraries.
   .
 - <a href="https://geographiclib.sourceforge.io/1.27">Version 1.27</a>
   (released 2012-11-29)
   - Changes to geodesic routines:
     - add native MATLAB implementations: geoddistance.m, geodreckon.m,
       geodarea.m;
     - add C and Fortran implementations;
     - improve the solution of the direct problem so that the series
       solution is accurate to round off for |<i>f</i>| &lt; 1/50;
     - tighten up the convergence criteria for solution of the inverse
       problem;
     - no longer signal failures of convergence with NaNs (a slightly
       less accurate answer is returned instead).
   - Fix DMS::Decode double rounding BUG.
   - On MacOSX platforms with the cmake configuration, universal
     binaries are built.
   .
 - <a href="https://geographiclib.sourceforge.io/1.26">Version 1.26</a>
   (released 2012-10-22)
   - Replace the series used for geodesic areas by one with better
     convergence (this only makes an appreciable difference if
     |<i>f</i>| &gt; 1/150).
   .
 - <a href="https://geographiclib.sourceforge.io/1.25">Version 1.25</a>
   (released 2012-10-16)
   - Changes to geodesic calculations:
     - restart Newton's method in Geodesic::Inverse when it goes awry;
     - back up Newton's method with the bisection method;
     - Geodesic::Inverse now converges for any value of \e f;
     - add GeodesicExact and GeodesicLineExact which are formulated in
       terms of elliptic integrals and thus yield accurate results even
       for very eccentric ellipsoids;
     - the -E option to <a href="GeodSolve.1.html">Geod</a> invokes these
       exact classes.
   - Add functionality to EllipticFunction:
     - add all the traditional elliptic integrals;
     - remove restrictions on argument range for incomplete elliptic integrals;
     - allow imaginary modulus for elliptic integrals and elliptic functions;
     - make interface to the symmetric elliptic integrals public.
   - Allow Ellipsoid to be copied.
   - Changes to the build tools:
     - cmake uses folders in Visual Studio to reduce clutter;
     - allow precision of reals to be set in cmake;
     - fail gracefully in the absence of pod documentation tools;
     - remove support for maintainer tasks in Makefile.mk;
     - upgrade to automake 1.11.6 to fix the "make distcheck" security
       vulnerability; see
       https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2012-3386
   .
 - <a href="https://geographiclib.sourceforge.io/1.24">Version 1.24</a>
   (released 2012-09-22)
   - Allow the specification of the hemisphere in UTM coordinates in
     order to provide continuity across the equator:
     - add UTMUPS::Transfer;
     - add GeoCoords::UTMUPSRepresentation(bool, int) and
       GeoCoords::AltUTMUPSRepresentation(bool, int);
     - use the hemisphere letter in, e.g.,
       <a href="GeoConvert.1.html">GeoConvert</a> -u -z 31n.
   - Add UTMUPS::DecodeEPSG and
     UTMUPS::EncodeEPSG.
   - cmake changes:
     - restore support for cmake 2.4.x;
     - explicitly check version of doxygen.
   - Fix building under cygwin.
   - Document restrictions on \e f in \ref intro.
   - Fix Python interface to work with version 2.6.x.
   .
 - <a href="https://geographiclib.sourceforge.io/1.23">Version 1.23</a>
   (released 2012-07-17)
   - Documentation changes:
     - remove html documentation from distribution and use web links if
       doxygen is not available;
     - use doxygen tags to document exceptions;
     - begin migrating the documentation to using Greek letters where
       appropriate (requires doxygen 1.8.1.2 or later).
   - Add Math::AngNormalize and
     Math::AngNormalize2; the allowed range for longitudes
     and azimuths widened to [&minus;540&deg;, 540&deg;).
   - DMS::Decode understands more unicode symbols.
   - Geohash uses geohash code "nan" to stand for not a
     number.
   - Add Ellipsoid::NormalCurvatureRadius.
   - Various fixes in LambertConformalConic,
     TransverseMercator,
     PolarStereographic, and Ellipsoid to
     handle reverse projections of points near infinity.
   - Fix programming blunder in LambertConformalConic::Forward
     (incorrect results were returned if the tangent latitude was
     negative).
   .
 - <a href="https://geographiclib.sourceforge.io/1.22">Version 1.22</a>
   (released 2012-05-27)
   - Add Geohash and Ellipsoid classes.
   - FIX BUG in AlbersEqualArea for very prolate
     ellipsoids (<i>b</i><sup>2</sup> &gt; 2 <i>a</i><sup>2</sup>).
   - cmake changes:
     - optionally use PACKAGE_PATH and INSTALL_PATH to determine
       CMAKE_INSTALL_PREFIX;
     - use COMMON_INSTALL_PATH to determine layout of installation
       directories;
     - as a consequence, the installation paths for the documentation,
       and Python and MATLAB interfaces are shortened for Windows;
     - zip source distribution now uses DOS line endings;
     - the tests work in debug mode for Windows;
     - default setting of GEOGRAPHICLIB_DATA does not depend on
       CMAKE_INSTALL_PREFIX;
     - add a cmake configuration for build tree.
   .
 - <a href="https://geographiclib.sourceforge.io/1.21">Version 1.21</a>
   (released 2012-04-25)
   - Support colon-separated DMS output:
     - DMS::Encode and
       GeoCoords::DMSRepresentation generalized;
     - <a href="GeoConvert.1.html">GeoConvert</a> and
       <a href="GeodSolve.1.html">Geod</a> now accept a -: option.
   - <a href="GeoidEval.1.html">GeoidEval</a> does not print the gradient
     of the geoid height by default (because it's subject to large
     errors); give the -g option to get the gradient printed.
   - Work around optimization BUG in Geodesic::Inverse
     with tdm mingw g++ version 4.6.1.
   - autoconf fixed to ensure that that out-of-sources builds work;
     document this as the preferred method of using autoconf.
   - cmake tweaks:
     - simplify the configuration of doxygen;
     - allow the MATLAB compiler to be specified with the
       MATLAB_COMPILER option.
   .
 - <a href="https://geographiclib.sourceforge.io/1.20">Version 1.20</a>
   (released 2012-03-23)
   - cmake tweaks:
     - improve find_package's matching of compiler versions;
     - CMAKE_INSTALL_PREFIX set from CMAKE_PREFIX_PATH if available;
     - add "x64" to the package name for the 64-bit binary installer;
     - fix cmake warning with Visual Studio Express.
   - Fix SphericalEngine to deal with aggressive iterator
     checking by Visual Studio.
   - Fix transcription BUG is Geodesic.js.
   .
 - <a href="https://geographiclib.sourceforge.io/1.19">Version 1.19</a>
   (released 2012-03-13)
   - Slight improvement in Geodesic::Inverse for very
     short lines.
   - Fix argument checking tests in MGRS::Forward.
   - Add \--comment-delimiter and \--line-separator options to the \ref
     utilities.
   - Add installer for 64-bit Windows; the compiled MATLAB interface is
     supplied with the Windows 64-bit installer only.
   .
 - <a href="https://geographiclib.sourceforge.io/1.18">Version 1.18</a>
   (released 2012-02-18)
   - Improve documentation on configuration with cmake.
   - cmake's find_package ensures that the compiler versions match on Windows.
   - Improve documentation on compiling MATLAB interface.
   - Binary installer for Windows installs under C:/pkg-vc10 by default.
   .
 - <a href="https://geographiclib.sourceforge.io/1.17">Version 1.17</a>
   (released 2012-01-21)
   - Work around optimization BUG in Geodesic::Inverse
     with g++ version 4.4.0 (mingw).
   - Fix BUG in argument checking with OSGB::GridReference.
   - Fix missing include file in SphericalHarmonic2.
   - Add simple examples of usage for each class.
   - Add internal documentation to the cmake configuration files.
   .
 - <a href="https://geographiclib.sourceforge.io/1.16">Version 1.16</a>
   (released 2011-12-07)
   - Add calculation of the earth's gravitational field:
     - add NormalGravity GravityModel and
       GravityCircle classes;
     - add command line utility
       <a href="Gravity.1.html">Gravity</a>;
     - add \ref gravity;
     - add Constants::WGS84_GM(), Constants::WGS84_omega(), and
       similarly for GRS80.
   - Build uses GEOGRAPHICLIB_DATA to specify a common parent directory
      for geoid, gravity, and magnetic data (instead of
      GEOGRAPHICLIB_GEOID_PATH, etc.); similarly,
      <a href="GeoidEval.1.html">GeoidEval</a>,
      <a href="Gravity.1.html">Gravity</a>, and
      <a href="MagneticField.1.html">MagneticField</a>, look at the
      environment variable GEOGRAPHICLIB_DATA to locate the data.
   - Spherical harmonic software changes:
     - capitalize enums SphericalHarmonic::FULL and
       SphericalHarmonic::SCHMIDT (the lower case names
       are retained but deprecated);
     - optimize the sum by using a static table of square roots which is
       updated by SphericalEngine::RootTable;
     - avoid overflow for high degree models.
   - Magnetic software fixes:
     - fix documentation BUG in MagneticModel::Circle;
     - make MagneticModel constructor explicit;
     - provide default MagneticCircle constructor;
     - add additional inspector functions to
       MagneticCircle;
     - add -c option to <a href="MagneticField.1.html">MagneticField</a>;
     - default height to zero in
       <a href="MagneticField.1.html">MagneticField</a>.
   .
 - <a href="https://geographiclib.sourceforge.io/1.15">Version 1.15</a>
   (released 2011-11-08)
   - Add calculation of the earth's magnetic field:
     - add MagneticModel and MagneticCircle
       classes;
     - add command line utility
       <a href="MagneticField.1.html">MagneticField</a>;
     - add \ref magnetic;
     - add \ref magneticinst;
     - add \ref magneticformat;
     - add classes SphericalEngine,
       CircularEngine, SphericalHarmonic,
       SphericalHarmonic1, and
       SphericalHarmonic2. which sum spherical harmonic
       series.
   - Add Utility class to support I/O and date
     manipulation.
   - Cmake configuration includes a _d suffix on the library built in
     debug mode.
   - For the Python package, include manifest and readme files; don't
     install setup.py for non-Windows systems.
   - Include Doxygen tag file in distribution as doc/html/Geographic.tag.
   .
 - <a href="https://geographiclib.sourceforge.io/1.14">Version 1.14</a>
   (released 2011-09-30)
   - Ensure that geographiclib-config.cmake is relocatable.
   - Allow more unicode symbols to be used in DMS::Decode.
   - Modify <a href="GeoidEval.1.html">GeoidEval</a> so that it can be
     used to convert the height datum for LIDAR data.
   - Modest speed-up of Geodesic::Inverse.
   - Changes in Python interface:
     - FIX BUG in transcription of Geodesic::Inverse;
     - include setup.py for easy installation;
     - Python only distribution is available at
       https://pypi.python.org/pypi/geographiclib
   - Supply a minimal Qt qmake project file for library
     src/Geographic.pro.
   .
 - <a href="https://geographiclib.sourceforge.io/1.13">Version 1.13</a>
   (released 2011-08-13)
   - Changes to I/O:
     - allow : (colon) to be used as a DMS separator in
       DMS::Decode(const std::string&, flag&);
     - also accept Unicode symbols for degrees, minutes, and seconds
       (coded as UTF-8);
     - provide optional \e swaplatlong argument to various
       DMS and GeoCoords functions to make
       longitude precede latitude;
     - <a href="GeoConvert.1.html">GeoConvert</a> now has a -w option to
       make longitude precede latitude on input and output;
     - include a JavaScript version of DMS.
   - Slight improvement in starting guess for solution of geographic
     latitude in terms of conformal latitude in TransverseMercator,
     TransverseMercatorExact, and LambertConformalConic.
   - For most classes, get rid of const member variables so that the
     default copy assignment works.
   - Put Math and Accumulator in their own
     header files.
   - Remove unused "fast" Accumulator method.
   - Reorganize `wrapper/python`.
   - Withdraw some deprecated routines.
   - cmake changes:
     - include FindGeographic.cmake in distribution;
     - building with cmake creates and installs
       geographiclib-config.cmake;
     - better support for building a shared library under Windows.
   .
 - <a href="https://geographiclib.sourceforge.io/1.12">Version 1.12</a>
   (released 2011-07-21)
   - Change license to MIT/X11.
   - Add PolygonArea class and equivalent MATLAB function.
   - Provide JavaScript and Python implementations of geodesic routines.
   - Fix Windows installer to include runtime dlls for MATLAB.
   - Fix (innocuous) unassigned variable in Geodesic::GenInverse.
   - Geodesic routines in MATLAB return a12 as first column of aux return
     value (incompatible change).
   - A couple of code changes to enable compilation with Visual
     Studio 2003.
   .
 - <a href="https://geographiclib.sourceforge.io/1.11">Version 1.11</a>
   (released 2011-06-27)
   - Changes to <a href="Planimeter.1.html">Planimeter</a>:
     - add -l flag to <a href="Planimeter.1.html">Planimeter</a> for polyline
       calculations;
     - trim precision of area to 3 decimal places;
     - FIX BUG with pole crossing edges (due to compiler optimization).
   - <a href="GeodSolve.1.html">Geod</a> no longer reports the reduced
     length by default; however the -f flag still reports this and in
     addition gives the geodesic scales and the geodesic area.
   - FIX BUGS (compiler-specific) in inverse geodesic calculations.
   - FIX BUG: accommodate tellg() returning &minus;1 at end of string.
   - Change way flattening of the ellipsoid is specified:
     - constructors take \e f argument which is taken to be the
       flattening if \e f &lt; 1 and the inverse flattening otherwise
       (this is a compatible change for spheres and oblate ellipsoids, but it
       is an INCOMPATIBLE change for prolate ellipsoids);
     - the -e arguments to the \ref utilities are handled similarly; in
       addition, simple fractions, e.g., 1/297, can be used for the
       flattening;
     - introduce Constants::WGS84_f() for the WGS84
       flattening (and deprecate Constants::WGS84_r() for the inverse
       flattening);
     - most classes have a Flattening() member function;
     - InverseFlattening() has been deprecated (and now returns inf for a
       sphere, instead of 0).
   .
 - <a href="https://geographiclib.sourceforge.io/1.10">Version 1.10</a>
   (released 2011-06-11)
   - Improvements to MATLAB/Octave interface:
     - add {geocentric,localcartesian}{forward,reverse};
     - make geographiclibinterface more general;
     - install the source for the interface;
     - cmake compiles the interface if ENABLE_MATLAB=ON;
     - include compiled interface with Windows binary installer.
   - Fix various configuration issues
     - autoconf did not install Config.h;
     - cmake installed in man/man1 instead of share/man/man1;
     - cmake did not set the rpath on the tools.
   .
 - <a href="https://geographiclib.sourceforge.io/1.9">Version 1.9</a>
   (released 2011-05-28)
   - FIX BUG in area returned by
     <a href="Planimeter.1.html">Planimeter</a> for pole encircling polygons.
   - FIX BUG in error message reported when DMS::Decode reads the string
     "5d.".
   - FIX BUG in AlbersEqualArea::Reverse (lon0 not being used).
   - Ensure that all exceptions thrown in the \ref utilities are caught.
   - Avoid using catch within DMS.
   - Move Accumulator class from Planimeter.cpp to
     Constants.hpp.
   - Add Math::sq<T>.
   - Simplify \ref geoidinst
     - add geographiclib-get-geoids for Unix-like systems;
     - add installers for Windows.
   - Provide cmake support:
     - build binary installer for Windows;
     - include regression tests;
     - add \--input-string, \--input-file, \--output-file options to the
       \ref utilities to support tests.
   - Rename utility EquidistantTest as <a href="GeodesicProj.1.html">
     GeodesicProj</a> and TransverseMercatorTest as
     <a href="TransverseMercatorProj.1.html"> TransverseMercatorProj</a>.
   - Add <a href="ConicProj.1.html"> ConicProj</a>.
   - Reverse the initial sense of the -s option for
     <a href="Planimeter.1.html"> Planimeter</a>.
   - Migrate source from subversion to git.
   .
 - <a href="https://geographiclib.sourceforge.io/1.8">Version 1.8</a>
   (released 2011-02-22)
   - Optionally return rotation matrix from Geocentric and
     LocalCartesian.
   - For the \ref utilities, supply man pages, -h prints the synopsis,
     \--help prints the man page, \--version prints the version.
   - Use accurate summation in <a href="Planimeter.1.html">Planimeter</a>.
   - Add 64-bit targets for Visual Studio 2010.
   - Use templates for defining math functions and some constants.
   - Geoid updates
     - Add Geoid::DefaultGeoidPath and
       Geoid::DefaultGeoidName;
     - <a href="GeoidEval.1.html">GeoidEval</a> uses environment variable
       GEOGRAPHICLIB_GEOID_NAME as the default geoid;
     - Add \--msltohae and \--haetomsl as
       <a href="GeoidEval.1.html">GeoidEval</a> options (and don't
       document the single hyphen versions).
   - Remove documentation that duplicates papers on transverse Mercator
     and geodesics.
   .
 - <a href="https://geographiclib.sourceforge.io/1.7">Version 1.7</a>
   (released 2010-12-21)
   - FIX BUG in scale returned by LambertConformalConic::Reverse.
   - Add AlbersEqualArea projection.
   - Library created by Visual Studio is Geographic.lib instead of
     GeographicLib.lib (compatible with makefiles).
   - Make classes NaN aware.
   - Use cell arrays for MGRS strings in MATLAB.
   - Add solution/project files for Visual Studio 2010 (32-bit only).
   - Use C++11 static_assert and math functions, if available.
   .
 - <a href="https://geographiclib.sourceforge.io/1.6">Version 1.6</a>
   (released 2010-11-23)
   - FIX BUG introduced in Geoid in version 1.5 (found by
     Dave Edwards).
   .
 - <a href="https://geographiclib.sourceforge.io/1.5">Version 1.5</a>
   (released 2010-11-19)
   - Improve area calculations for small polygons.
   - Add -s and -r flags to <a href="Planimeter.1.html">Planimeter</a>.
   - Improve the accuracy of LambertConformalConic using
     divided differences.
   - FIX BUG in meridian convergence returned by
     LambertConformalConic::Forward.
   - Add optional threadsafe parameter to Geoid
     constructor.  WARNING: This changes may break binary compatibility
     with previous versions of GeographicLib.  However, the library is
     source compatible.
   - Add OSGB.
   - MATLAB and Octave interfaces to UTMUPS,
     MGRS, Geoid, Geodesic
     provided.
   - Minor changes
     - explicitly turn on optimization in Visual Studio 2008 projects;
     - add missing dependencies in some Makefiles;
     - move pi() and degree() from Constants to
       Math;
     - introduce Math::extended type to aid testing;
     - add Math::epi() and Math::edegree().
     - fixes to compile under cygwin;
     - tweak expression used to find latitude from conformal latitude.
   .
 - <a href="https://geographiclib.sourceforge.io/1.4">Version 1.4</a>
   (released 2010-09-12)
   - Changes to Geodesic and GeodesicLine:
     - FIX BUG in Geodesic::Inverse with prolate ellipsoids;
     - add area computations to Geodesic::Direct and Geodesic::Inverse;
     - add geodesic areas to geodesic test set;
     - make GeodesicLine constructor public;
     - change longitude series in Geodesic into Helmert-like form;
     - ensure that equatorial geodesics have cos(alpha0) = 0 identically;
     - generalize interface for Geodesic and GeodesicLine;
     - split GeodesicLine and Geodesic into different files;
     - signal convergence failure in Geodesic::Inverse with NaNs;
     - deprecate one function in Geodesic and two functions in
       GeodesicLine;
     - deprecate -n option for <a href="GeodSolve.1.html">Geod</a>.
     .
     <b>WARNING</b>: These changes may break binary compatibility with
     previous versions of GeographicLib.  However, the library is
     source compatible (with the proviso that
     GeographicLib/GeodesicLine.hpp may now need to be included).
   - Add the <a href="Planimeter.1.html">Planimeter</a> utility for
     computing the areas of geodesic polygons.
   - Improve iterative solution of Gnomonic::Reverse.
   - Add Geoid::ConvertHeight.
   - Add -msltohae, -haetomsl, and -z options to
     <a href="GeoidEval.1.html">GeoidEval</a>.
   - Constructors check that minor radius is positive.
   - Add overloaded Forward and Reverse functions to the projection
     classes which don't return the convergence (or azimuth) and scale.
   - Document function parameters and return values consistently.
   .
 - <a href="https://geographiclib.sourceforge.io/1.3">Version 1.3</a>
   (released 2010-07-21)
   - Add Gnomonic, the ellipsoid generalization of the
     gnomonic projection.
   - Add -g and -e options to
     <a href="GeodesicProj.1.html">EquidistantTest</a>.
   - Use fixed-point notation for output from
     <a href="CartConvert.1.html">CartConvert</a>,
     <a href="GeodesicProj.1.html">EquidistantTest</a>,
     <a href="TransverseMercatorProj.1.html">TransverseMercatorTest</a>.
   - PolarStereographic:
     - Improved conversion to conformal coordinates;
     - Fix bug with scale at opposite pole;
     - Complain if latitude out of range in SetScale.
   - Add Math::NaN().
   - Add long double version of hypot for Windows.
   - Add EllipticFunction::E(real).
   - Update references to Geotrans in MGRS documentation.
   - Speed up tmseries.mac.
   .
 - <a href="https://geographiclib.sourceforge.io/1.2">Version 1.2</a>
   (released 2010-05-21)
   - FIX BUGS in Geodesic,
     - wrong azimuth returned by Direct if point 2 is on a pole;
     - Inverse sometimes fails with very close points.
   - Improve calculation of scale in CassiniSoldner,
     - add GeodesicLine::Scale,
       GeodesicLine::EquatorialAzimuth, and
       GeodesicLine::EquatorialArc;
     - break friend connection between CassiniSoldner and Geodesic.
   - Add DMS::DecodeAngle and
     DMS::DecodeAzimuth.  Extend
     DMS::Decode and DMS::Encode to deal
     with distances.
   - Code and documentation changes in Geodesic and
     Geocentric for consistency with
     the forthcoming paper on geodesics.
   - Increase order of series using in Geodesic to 6 (full
     accuracy maintained for ellipsoid flattening &lt; 0.01).
   - Macro __NO_LONG_DOUBLE_MATH to disable use of long double.
   - Correct declaration of Math::isfinite to return a bool.
   - Changes in the \ref utilities,
     - improve error reporting when parsing command line arguments;
     - accept latitudes and longitudes in decimal degrees or degrees,
       minutes, and seconds, with optional hemisphere designators;
     - <a href="GeoConvert.1.html">GeoConvert</a> -z accepts zone or
       zone+hemisphere;
     - <a href="GeoidEval.1.html">GeoidEval</a> accepts any of the input
       formats used by <a href="GeoConvert.1.html">GeoConvert</a>;
     - <a href="CartConvert.1.html">CartConvert</a> allows the ellipsoid
       to be specified with -e.
   .
 - <a href="https://geographiclib.sourceforge.io/1.1">Version 1.1</a>
   (released 2010-02-09)
   - FIX BUG (introduced in 2009-03) in EllipticFunction::E(sn,cn,dn).
   - Increase accuracy of scale calculation in TransverseMercator and
     TransverseMercatorExact.
   - Code and documentation changes for consistency with
     <a  href="https://arxiv.org/abs/1002.1417">arXiv:1002.1417</a>
   .
 - <a href="https://geographiclib.sourceforge.io/1.0">Version 1.0</a>
   (released 2010-01-07)
   - Add autoconf configuration files.
   - BUG FIX: Improve initial guess for Newton's method in
     PolarStereographic::Reverse.  (Previously this failed to converge
     when the co-latitude exceeded about 130 deg.)
   - Constructors for TransverseMercator, TransverseMercatorExact,
     PolarStereographic, Geocentric, and Geodesic now check for obvious
     problems with their arguments and throw an exception if necessary.
   - Most classes now include inspector functions such as MajorRadius()
     so that you can determine how instances were constructed.
   - Add LambertConformalConic class.
   - Add PolarStereographic::SetScale to allow the
     latitude of true scale to be specified.
   - Add solution and project files for Visual Studio 2008.
   - Add GeographicErr for exceptions.
   - Geoid changes:
     - BUG FIX: fix typo in Geoid::Cache which could cause
       a segmentation fault in some cases when the cached area spanned
       the prime meridian.
     - Include sufficient edge data to allow heights to be returned for
       cached area without disk reads;
     - Add inspector functions to query the extent of the cache.
   .
 - <a href="https://geographiclib.sourceforge.io/2009-11">Version 2009-11</a>
   (released 2009-11-03)
   - Allow specification of "closest UTM zone" in UTMUPS
     and <a href="GeoConvert.1.html">GeoConvert</a> (via -t option).
   - Utilities now complain is there are too many tokens on input lines.
   - Include real-to-real versions of DMS::Decode and
     DMS::Encode.
   - More house-cleaning changes:
     - Ensure that functions which return results through reference
       arguments do not alter the arguments when an exception is thrown.
     - Improve accuracy of MGRS::Forward.
     - Include more information in some error messages.
     - Improve accuracy of inverse hyperbolic functions.
     - Fix the way Math functions handle different precisions.
   .
 - <a href="https://geographiclib.sourceforge.io/2009-10">Version 2009-10</a>
   (released 2009-10-18)
  - Change web site to https://geographiclib.sourceforge.io
  - Several house-cleaning changes:
    - Change from the a flat directory structure to a more easily
      maintained one.
    - Introduce Math class for common mathematical functions (in
      Constants.hpp).
    - Use Math::real as the type for all real quantities.  By default this
      is typedef'ed to double; and the library should be installed this
      way.
    - Eliminate const reference members of AzimuthalEquidistant,
      CassiniSoldner and LocalCartesian so that they may be copied.
    - Make several constructors explicit.  Disallow some constructors.
      Disallow copy constructor/assignment for Geoid.
    - Document least squares formulas in Geoid.cpp.
    - Use unsigned long long for files positions of geoid files in Geoid.
    - Introduce optional mgrslimits argument in UTMUPS::Forward and
      UTMUPS::Reverse to enforce stricter MGRS limits on eastings and
      northings.
    - Add 64-bit targets in Visual Studio project files.
   .
 - <a href="https://geographiclib.sourceforge.io/2009-09">Version 2009-09</a>
   (released 2009-09-01)
   - Add Geoid and
     <a href="GeoidEval.1.html">GeoidEval</a> utility.
   .
 - <a href="https://geographiclib.sourceforge.io/2009-08">Version 2009-08</a>
   (released 2009-08-14)
   - Add CassiniSoldner class and
     <a href="GeodesicProj.1.html">EquidistantTest</a> utility.
   - Fix bug in Geodesic::Inverse where NaNs were
     sometimes returned.
   - INCOMPATIBLE CHANGE: AzimuthalEquidistant now returns the reciprocal
     of the azimuthal scale instead of the reduced length.
   - Add -n option to <a href="GeoConvert.1.html">GeoConvert</a>.
   .
 - <a href="https://geographiclib.sourceforge.io/2009-07">Version 2009-07</a>
   (released 2009-07-16)
   - Speed up the series inversion code in tmseries.mac and geod.mac.
   - Reference Borkowski in section on \ref geocentric.
   .
 - <a href="https://geographiclib.sourceforge.io/2009-06">Version 2009-06</a>
   (released 2009-06-01)
   - Add routines to decode and encode zone+hemisphere to UTMUPS.
   - Clean up code in Geodesic.
   .
 - <a href="https://geographiclib.sourceforge.io/2009-05">Version 2009-05</a>
   (released 2009-05-01)
   - Improvements to Geodesic:
     - more economical series expansions,
     - return reduced length (as does the
       <a href="GeodSolve.1.html">Geod</a> utility),
     - improved calculation of starting point for inverse method,
     - use reduced length to give derivative for Newton's method.
   - Add AzimuthalEquidistant class.
   - Make Geocentric, TransverseMercator,
     and PolarStereographic classes work with prolate
     ellipsoids.
   - <a href="CartConvert.1.html">CartConvert</a> checks its inputs more
     carefully.
   - Remove reference to defunct Constants.cpp from GeographicLib.vcproj.
   .
 - <a href="https://geographiclib.sourceforge.io/2009-04">Version 2009-04</a>
   (released 2009-04-01)
   - Use compile-time constants to select the order of series in
     TransverseMercator.
   - 2x unroll of Clenshaw summation to avoid data shuffling.
   - Simplification of EllipticFunction::E.
   - Use STATIC_ASSERT for compile-time checking of constants.
   - Improvements to Geodesic:
     - compile-time option to change order of series used,
     - post Maxima code for generating the series,
     - tune the order of series for double,
     - improvements in the selection of starting points for Newton's
       method,
     - accept and return spherical arc lengths,
     - works with both oblate and prolate ellipsoids,
     - add -a, -e, -b options to the <a href="GeodSolve.1.html">Geod</a>
       utility.
   .
 - <a href="https://geographiclib.sourceforge.io/2009-03">Version 2009-03</a>
   (released 2009-03-01)
   - Add Geodesic and the
     <a href="GeodSolve.1.html">Geod</a> utility.
   - Declare when no exceptions are thrown by functions.
   - Minor changes to DMS class.
   - Use invf = 0 to mean a sphere in constructors to some classes.
   - The makefile creates a library and includes an install target.
   - Rename ECEF to Geocentric, ECEFConvert
     to <a href="CartConvert.1.html">CartConvert</a>.
   - Use inline functions to define constant doubles in Constants.hpp.
   .
 - <a href="https://geographiclib.sourceforge.io/2009-02">Version 2009-02</a>
   (released 2009-01-30)
   - Fix documentation of constructors (flattening &rarr; inverse
     flattening).
   - Use std versions of math functions.
   - Add ECEF and LocalCartesian classes
     and the ECEFConvert utility.
   - Gather the documentation on the \ref utilities onto one page.
   .
 - <a href="https://geographiclib.sourceforge.io/2009-01">Version 2009-01</a>
   (released 2009-01-12)
   - First proper release of library.
   - More robust TransverseMercatorExact:
     - Introduce \e extendp version of constructor,
     - Test against extended test data,
     - Optimize starting positions for Newton's method,
     - Fix behavior near all singularities,
     - Fix order dependence in C++ start-up code,
     - Improved method of computing scale and convergence.
   - Documentation on transverse Mercator projection.
   - Add MGRS, UTMUPS, etc.
   .
 - Version 2008-09
   - Ad hoc posting of information on the transverse Mercator projection.

<center>
Back to \ref highprec.  Up to \ref contents.
</center>
**********************************************************************/
}
