Height Reference Systems
- The German height reference system
- The European height reference system
- Determination of the height reference surface of Germany
- GCG2011 - German Combined QuasiGeoid 2011
Determination of the height reference surface of Germany
Geoid and gravity field models
The geoid as natural reference surface is the equipotential surface of the Earth’s gravity field that approximates the mean sea level at the best. In a global view, the geoid differs by up to ±100 m from a geometrically defined reference surface, the ellipsoid. The differences depend on the density spread in the interior of the earth and hence on its gravity field. Satellite-based measurements of the gravity field and the sea surface and the global gravity field models based on this provide large-scale structures with a spatial extent of about 100 km. Smaller details cannot be resolved from space. Terrestrial measurements are indispensable for the determination of accurate models of the height reference surface.
Height reference surfaces in Germany
Owing to the growing employment of satellite-based measuring techniques in practical surveying the need arose to determine the height reference surface of Germany with an accuracy of a few centimeters. Since the mid-nineties, the BKG has done considerable work in this field together with the surveying administrations of the Länder. Furthermore, the BKG has developed a database for the determination of the height reference surface of Germany.
The basis for the determination of the quasigeoid model is formed by terrestrial and airborne gravity measurements, global gravity field models, digital terrain models, and a network of observation points in which ellipsoidal heights as well as levelling heights are available.
The height reference surface used in Germany is also called quasigeoid or NHN surface. It varies between 34 m in the area of the Baltic Sea and 50 m in the Alps. The horizontal gradients of the quasigeoid can amount to up to 10 m per km. Therefore, quasigeoid variations have to be considered also for local height determinations with GPS and GALILEO.
In 1998 a first model of the quasigeoid was established for the territory of the new Länder. Further solutions followed for the whole Federal Republic with the "Satellite-Geodetic Nivellitic Quasigeoid (SNG01)" from March 2003 and the "German Combined QuasiGeoid 2005 (GCG05)" from August 2005. The "German Combined QuasiGeoid 2011 (GCG2011)" from January 2012 is the actual version. These models were determined by combining two independent solutions of the Institut für Erdmessung of the Leibniz Universität Hannover and of the Federal Agency for Cartography and Geodesy.