Comparison of a local geoid model with a precise levelling network

This work presents the results of a GPS-LEV measurement campaign carried out on a number of leveling benchmarks part of the Italian national high-precision leveling network, for an extension of some tenths of kilometers around the GPS permanent station of Perugia (UNPG - part of the IGS, EUREF and ASI networks), in central Italy. A comparison has been made between the geoid undulations locally derived from GPS-LEV and the values given by some geoid models, both global (EGM 96) and national (ITALGEO 95, ITALGEO 99). The results show that the estimated geoidal heights derived from the correction of GPS-measured ellipsoidal heights with any geoid model need to be “fitted” in some way to the orthometric heights given by the leveling network covering the survey area, thus to the vertical datum as "realised" by the leveling network. The reason could be the “smoothness” of the geoid model and/or a slight difference in the datum definition between the GPS survey and the geoid model itself. Some different fitting techniques have been experimented, obtaining acceptable results for technical purposes.