Evaluation of global geopotential models in determining the quasi-geoid for Southern Africa

Precise geoid models are needed to enable GPS-derived heights to be converted to the local height datum. A major component of these models is a global geopotential model. Several candidate models are assessed in the context of forming the basis for a regional quasi-geoid for Southern Africa. These candidates include the EGM96 and models based upon data from the GRACE satellite mission. EGM96 is particularly unreliable in areas where no terrestrial gravity data exist. The GRACE models perform best when only medium (up to degree 120) harmonic expansions are used, minimising the contribution of predicted gravity anomalies. Comparisons with 62 GPS/levelling data points in the western part of South Africa indicate a RMS fit of 13cm for the best combination. This fit can be improved to 4cm if the quasi-geoid model is allowed to tilt in two directions.