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
We deal with height data every day and everybody knows up and down. The water of the rivers flows downhill and the heights are zero at sea level. Gravity does not only cause that we stand with both feet on the ground, it is also of importance with respect to the determination of our heights. The influence of the changing gravitational pull from the sun and the moon towards sea level becomes clearly visible as tides. Compared to this the earth’s gravity field has a considerably higher but temporally constant impact on the form of the sea level. Without knowledge of gravity no unambiguous heights above sea level can be computed. In addition, the sea surface does not have the same level at all places due to different water temperatures and salt content. The determination of uniform heights is therefore no easy task. In a global view, it is not yet solved satisfactorily until today.
The classical measuring technique for the realization of height reference systems is the geometrical leveling. The height reference system applied in Germany was determined several times with this technique during the last 150 years. The heights refer to the zero level determined by the Amsterdam tide gauge and are named heights above the base height level (NHN, Normalhöhennull – Normal Height Zero). There is a multitude of different height reference systems in the European countries that differ especially by the zero level with reference to different tide gauges. Due to this, differences in the decimeter range may occur at the borders of neighboring countries.
Over the last years Global Satellite Navigation Systems (GNSS) such as the Global Positioning System (GPS) and the European equivalent GALILEO have become indispensable to the realization of a manifold of surveying activities. However, GNSS heights can not be used directly since they are determined purely geometrically and do not refer to the sea level. Therefore, a model of the height reference surface is necessary for the determination of practice-relevant physical heights from GNSS observations. This model is called geoid or quasigeoid in geodesy.