Recent Investigations Toward Achieving a One Centimeter Geoid

Abstract

New research is discussed that relates to improving techniques of hybrid geoid generation and the use of Helmert condensation of surface free-air gravity anomalies. New approaches are examined for modeling signal remaining between a regional gravimetric geoid, G99SSS, and a database of GPS-derived ellipsoid heights at 6169 spirit-leveled benchmarks. One approach reduced the remaining unmodeled signal by one centimeter or about 20% over the approach used for GEOID99. Helmert condensation also offers a solution to improving regional geoids. This approach models the gravitational effects of the terrain masses using fast Fourier transforms for outer-zone (>4°) contributions and spherical, space-domain calculations for the inner-zone. The effects of the 3D masses are removed from the surface anomalies, which are then downward continued to the geoid where the effects of the 2D masses (condensed onto the geoid) are restored. This approach removes the approximation of Faye gravity anomalies and permits downward continuation in the smoother gravity field where the terrain has been removed. Preliminary investigations have begun on this approach that demonstrate the viability of modeling the 3D and 2D terrain masses as compared to other approaches.