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A procedure for combining gravimetric geoid models and independent geoid data , with an example in the North Sea region

The main goal of the study is to obtain a consistent height and depth reference system in the form of a geoid for the Dutch mainland and marine area. For this purpose a procedure has been developed and tested to combine available gravity data and external data from satellite altimetry, GPS and levelling in an optimal manner. After all, local gravimetric geoid models from a combination of a global geopotential model and local gravity anomalies, usually contain errors of dm-level on wavelengths longer than 50 km. One of the main causes for this is the limited precision of the global models. External geoid information on discrete points, like GPS and levelling sites on land and altimeter tracks combined with permanent sea surface topography at sea is often available with cm-precision. These points can be used to correct for the medium and longer wavelength errors in the gravimetric geoid. The problem is to find an adequate functional representation of the correction surface. The authors have developed a method to investigate the form of this correction, and find empirical representations depending on area size. Once a class of functions have been selected the most suitable can be found from a statistical testing procedure. The initial purely gravimetric geoid is adjusted in longer wavelengths by means of the external geoid data. The preliminary North Sea geoid GEONZ97 has a precision of better than 4 cm at sea and along the Dutch coast. As soon as instantaneous GPS height components in off-shore applications reach a comparable accuracy, tides and meteorological response can be eliminated in an efficient and effective way simply by subtracting the geoid.