GPS datum (geographic model) to use for any GPS links: this
sets the geographic model to use for GPS links. The
following models are supported:
WGS84 is the World Geodetic System », 1984
EUR50 is the European 1950 system used for Spanish maps
EUR79 is the European 1979 system used on many other European
maps.
The chosen model is used to adjust all GPS links in the project.