Realising a permanent height referencing on FINO3

Bundesanstalt für Gewässerkunde
Dr.-Ing. Astrid Sudau

The aim of the project is the georeferencing of water leval data recorded on the FINO3. These are of interest for various applications (hydrology, research on water level rises, calibration points for satellite altimeter measurements etc.), but the water level data have no height reference. This means that a direct comparison between the water level data and the data recorded ashore has not so far been possible or the water level data cannot be used for a large number of applications (including all those referred to above). With the help of the GNSS1 system it is possible to achieve a height reference on the FINO3 and to georeference the recorded water level data exactly and/or provide them with an exact height reference by means of a single connecting measurement between the GNSS antenna and the radar gauge zero point.

The German Federal Maritime and Hydrographic Agency will install a radar gauge on the FINO3. This is a device, which continuously records the wave height and the water level with reference to an internal zero point. Similarly, water level data is also recorded ashore relative to the respective gauge zero points. For analyses it is therefore urgently necessary to take account of the exact height of the respective gauge zero points. The levels on land are linked to the official height reference system of the land survey offices at regular intervals and/or the height of the gauge zero point is determined with reference to the official system. The currently official heights (e.g. German Levelling Base Network 1992, DHHN92) are based on large-scale measuring campaigns carried out in the eighties. As a result of anthropogenous and tectonic influences the true height has changed in the course of time. In practical terms this means that differences arise between official and actual heights. On top of this the “German heights” have no connection with the height systems of the bordering countries. Gauge zero point heights with regard to official heights have only limited suitability therefore for scientific analyses or cross-border investigations.

For the last few years it has been possible to monitor heights and height changes with GNSS systems and to show them in a globally compatible reference system. Unlike with classical techniques of height transmission (geometric and hydrostatic levelling), large areas can be covered using GNSS systems. The heights derived with the help of GNSS systems are originally realised in a globally defined reference system and can be carried over into local reference systems.

Assembly of the GNSS antenna on the FINO3 mast
Assembly of the GNSS antenna on the FINO3 mast

1 GNSS = Global Navigation Satellite System –

GNSS Gauging stations of the BfG [=German Federal Institute for Hydrography] and the BKG [=German Federal Agency for Cartography and Geodesy] in the area of the German Bight GNSS Reference stations in Europe Altimeter satellite traces in the area of the German Bight. .

Practically speaking this means that the height details of the gauge zero points can be used cross-border with the results of comparable systems. A requirement for this is information on the geometric relationships between the GNSS systems and the gauge zero points. These must be uniquely recorded or monitored at regular intervals, depending on location.

Currently the BfG runs eighteen permanently working GNSS systems along the German Bight. What is common to all systems is that they are directly installed on the gauges and/or fixed gauge/gauge zero points and GNSS systems are mounted on one and the same physical structure. One of these stations is the FINO3. Using the GNSS system installed there a height reference can be set on the FINO3 and a gauge zero point height determined. This also means that the water level data recorded with the help of the radar gauge can be directly compared with the gauge ashore equipped with GNSS and/or the the height details of the gauge zero points down to a few millimetres.

The data from the FINO3 are of special interest, as this is a guage, which is installed relatively far from the coast. This facilitates the direct measurement of water level data on open seas, which is of particular interest for modelling the surface of the sea and validating the results of satellite altimetry. Consequently, the combination of GNSS system / radar gauge on the FINO3 also fulfils an interface function between classical point-by-point gauge measurement and large-scale measurements by satellite altimetry.