Comparisons of GPS-Derived Orthometric Heights Using Local Geometric Geoid Models1

The purpose of the research was to compare measuring orthometric heights using differential leveling, static global positioning observations, and real-time kinematic (RTK) Global Positioning System (GPS) observations in the vicinity of the University of Conn. campus. Height observations were recorded at temporary and permanent monuments using differential leveling, static GPS, and RTK. Heights at permanent monuments with published elevations were observed using each method and served as control and to check the quality of all measurements. Three-dimensional reference ellipsoid coordinates derived from GPS observations and published orthometric heights were used to create local geoid models, which were then used to convert ellipsoid heights observed with the static GPS and RTK to orthometric heights. The resulting values were used to compare between the measuring methods. In general, static GPS performed better than RTK. Several polynomial surface models of different orders were fitted to the geoid heights, from a constant up to a quadratic. No meaningful distinctions could be drawn between the models indicating that, for a small geographical area, shifting the national geoid model, GEOID03, by a constant vertical amount performs as well as the more complicated approaches. Experience showed that multiple, independent RTK observations on the same day are not sufficient to ensure freedom from systematic errors.