Detecting the Baltic Sea Level Surface with GPS-Measurements and Comparing it with the Local Geoid Model

Abstract

Gravimetric geoid NKG04 (Forsberg et al., NKG Geoid Meeting, Copenhagen) is derived by KMS using all available gravimetric data from the region. Some areas, also close to the Estonia are not completely covered by gravity measurements, example the eastern part of the Gulf of Finland. Baltic Sea is measured by airborn gravimetry with accuracy probably 2 mGal. Our idea was to compare the geoid on the sea areas against the independent method like GPS-levelling on the mainland. Main problem have been of course how to remove water tilt during the campaign

The existent GPS device on board a ship stores data every second and determines the heights with an accuracy of a few centimetres (using the kinematic method with post-processing, several base stations close to the ferry line). As a result, it is possible to observe the present relative water level profile in reference to the ellipsoid. If we take into account the tilt of the water level at the moment of measurement, we can observe the relative change of the geoid using independent methodology, which serves as a comparison to the gravimetric geoid solution

With this method we explored some areas on the Baltic Sea covered with regular ferry lines where the geoid profile changes faster. One such area lies about 30 km north of the island of Hiiumaa, where the geoid has a “lump”: the separation of the geoid from the ellipsoid changes by 1 m over a 70-km distance starting from Paldiski; further towards Sweden the original separation is restored. In addition, we analyzed all the Estonian and Swedish profiles, also using GPS data from Swedish base stations

We also performed the same kind of measurements on ferries running the regular line Tallinn- St. Petersburg-Helsinki-Tallinn. Those measurements were of particular interest as there was no gravimetric data available for the eastern part of the Gulf of Finland

As a third track, measurements were performed on liners running between Sillamäe and Kotka

The results show that the 150-km geoid NKG04 profile close to Hiiumaa did not differ any more than 15 cm from the GPS-measured level surface. The influence of the water tilt was more or less eliminated using the available tide gauge data. The profile between Tallinn and St. Petersburg manifested similar difference. Most of the measurements were repeated several times on the same profile