GeoidEval (version 2.4) computes the height of the geoid above the WGS84 ellipsoid using interpolation in a grid of values for the earth gravity models, EGM84, or EGM96, EGM2008. The RMS error in the interpolated height is about 1 mm. Give the position in terms of latitude and longitude, for example (these all refer to the position of Timbuktu):
16.776 -3.009
16d47' -3d1'
W3°0'34" N16°46'33"
3:0:34W 16:46:33N
The coordinates can also be given in UTM, UPS, or MGRS coordinates (see
the documentation on the
GeoConvert utility).
The height of the geoid above the ellipsoid, N, is sometimes called the geoid undulation. It can be used to convert a height above the ellipsoid, h, to the corresponding height above the geoid (the orthometric height, roughly the height above mean sea level), H, using the relations
h = N + H; H = −N + h.
GeoidEval, which is a simple wrapper of the GeographicLib::Geoid class, is one of the utilities provided with GeographicLib. This web interface illustrates a subset of its capabilities. If you wish to use GeoidEval directly, download and compile GeographicLib. A description of the methods is given here.