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# Calling the GeographicLib C++ library from Python
The geodesic routines in GeographicLib have been implemented as a
[native Python library](http://pypi.python.org/pypi/geographiclib).
For documentation see
https://geographiclib.sourceforge.io/Python/doc/
## ctypes
This is the official way to use C++ libraries from Python. See
https://docs.python.org/3/extending/extending.html#writing-extensions-in-c
Borys Kaboakov https://github.com/banderlog provided a sample invocation
in the PyGetIntersect directory.
## pybind11
See
https://github.com/pybind/pybind11
This seems to be a good alternative to boost-python. Still need to
provide spccific instructions.
## boost-python
It is also possible to call the C++ version of GeographicLib directly
from Python and this directory contains a small example,
`PyGeographicLib.cpp`, which uses boost-python and the `Geoid` class to
convert heights above the geoid to heights above the ellipsoid. More
information on calling boost-python, see
https://www.boost.org/doc/libs/release/libs/python
To build and install this interface, do
```bash
mkdir BUILD
cd BUILD
cmake -D CMAKE_INSTALL_PREFIX=~/.local ..
make
make install
```
This assumes that you have installed GeographicLib somewhere that cmake
can find it. If you want just to use the version of GeographicLib that
you have built in the top-level BUILD directory, include, e.g.,
```bash
-D GeographicLib_DIR=../../BUILD
```
in the invocation of cmake (the directory is relative to the source
directory, wrapper/python).
`make install` installs PyGeographicLib in
```
~/.local/lib/python2.7/site-packages
```
which is in the default search path for python 2.7. To convert 20m
above the geoid at 42N 75W to a height above the ellipsoid, do
```python
$ python
>>> from PyGeographicLib import Geoid
>>> geoid = Geoid("egm2008-1")
>>> geoid.EllipsoidHeight(42, -75, 20)
-10.671887499999997
>>> help(Geoid.EllipsoidHeight)
```
Notes:
* The geoid data (`egm2008-1`) should be installed somewhere that
GeographicLib knows about.
* This prescription applies to Linux machines. Similar steps can be
used on Windows and MacOSX machines.
* You will need the packages boost-python, boost-devel, python, and
python-devel installed.
* `CMakeLists.txt` specifies the version of python to look for (version
2.7). This must match that used in boost-python. To check do, e.g.,
```bash
ldd /usr/lib64/libboost_python.so
```
* `CMakeLists.txt` looks for a shared-library version of GeographicLib.
This is the default with cmake build on non-Windows platforms. On
Windows, use the cmake option `-D BUILD_SHARED_LIBS=ON` to specify
building a shared library.
Acknowledgment:
Thanks to Jonathan Takahashi <jtakahashi@gmail.com> for the sample code
in PyGeographicLib.cpp and the commands needed to compile and link this.
## Cython
An alternative to boost-python is provided by [Cython](https::/cython.org).
Sergey Serebryakov offers the github project
https://github.com/megaserg/geographiclib-cython-bindings
which provides a fast python interface to the geodesic routines.
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