With the coverage of gravity data we have computed, with very high precision, a reference surface called the "geoid" (P. Pâquet et al., 1997). This surface corresponds to an extension of mean sea level beneath the continents. It is relatively smooth, but its greatest deviation over Belgium exceeds 2 meters. This surface is a necessary step in going from another reference surface (terrestrial ellipsoid) to the surface topography. New techniques of precise positioning by satellites (GPS) give a precision of the order of one centimetre over distances of 100 km or more. These measurements are referred to the ellipsoid. In order to convert them to physical heights corresponding to those obtained by precise levelling, it is necessary to know the geoid with the same precision, that is, to within a centimetre.

References :

P. Pâquet, Z. Jiang, M. Everaerts, "A new Belgian Geoid determination",
in Springer editor : "Proceedings of Grageomar 96 symposium", 1997.