GeographicLib is a small set of C++ classes for performing conversions
between geographic, UTM, UPS, MGRS, geocentric, and local cartesian
coordinates, for gravity (e.g., EGM2008), geoid height, and geomagnetic
field (e.g., WMM2010) calculations, and for solving geodesic problems.

The library may be used from .NET applications using the NETGeographicLib
wrapper library.  It is a suitable replacement for the core functionality
provided by geotrans.

WWW: http://geographiclib.sourceforge.net/