These files give the height of geoid above the ellipsoid on a
rectangular grid. It's necessary to download and install at least one
of these files if you want to use the Geoid class in GeographicLib.
The data is provided either as a compressed tar (extension .tar.bz2), a
zip file, or a windows installer (extension .exe). If you're unsure
which one to install, start with
For more information, see