The egm08_25.gtx file has been generated with GeographicLib (from http://lists.maptools.org/pipermail/proj/2012-April/006273.html) GeographicLib now includes an example code, GeoidToGTX, that takes one of the earth gravity models (egm84, egm96, or egm2008), computes the geoid heights on a regular grid, and writes the results out as a gtx file. See http://geographiclib.sourceforge.net/html/gravity.html#gravityparallel Generation: GeoidToGTX egm2008 24 egm08_25.gtx