Improving local gravimetric geoid models with external data

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 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 of investiging the form of this correction, and finding empirical representations depending on area size. Once a class of functions have been selected the most suitable can be found by a statistical testing procedure.