B4: Investigations on the unification of height systems

Height systems are defined by long-term averages of tide gauge registrations. The mean sea level, fixed in this way, doesn't coincide, however, with a global height reference surface, the geoid. Moreover, mean sea level changes with time and - due to vertical tectonic movements - tide gauges may fake an apparent sea level change. The sea surface topography (the deviation between geoid and sea level) is the key information for the unification of height systems.

Large scale structure of the sea surface topography, estimated by an 8 year mean sea level from TOPEX/Poseidon minus the EGM96 geoid	Sketch concerning the comparison of altimetric sea surface heights and tide gauges linked by GPS to a geocentric system.

Satellite altimetry can map the sea level relative to a geocentric system and can be used to transfer sea level between different tide gauges. The use of altimetry in coastal areas is, however, difficult, because the radar echo is still (or already) contaminated by land surface and the atmospheric range corrections change considerably near the coast. In addition, a sufficient precise knowledge on the geoid is required in the neighborhood of the height reference points. Following topics are to be investigated:

• Discrepancies of different height systems comparing GPS and tide gauge registrations with altimetry and geoid;
• Extrapolation of altimetric sea surface heights and the absolute comparison with ellipsoidal heights of particular tide gauges;
• True changes of sea level by analysis of tide gauge registrations and an estimation of vertical tectonic motions of the tide gauge station.

The investigations are mainly performed in cooperation with partners from South America and shall contribute to the SIRGAS-Project, Phase III.

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