Here is a list of all related documentation pages:

Introduction
Installing GeographicLib
Getting started
Utility programs
Code organization
GeographicLib in other languages
Geoid height
Gravity models
Normal gravity
Magnetic models
Geodesics on an ellipsoid of revolution
Finding nearest neighbors
Geodesics on a triaxial ellipsoid
Jacobi's conformal projection
Rhumb lines
Great Ellipses
Transverse Mercator projection
Geocentric coordinates
Auxiliary latitudes
Support for high precision arithmetic
Change log