A Preliminary Geoid Model for Africa

Precise geoid models are essential for the conversion of GPS-derived heights to heights above sea level. Such a model is under development for the continent of Africa, as part of the African Geoid Project. A uniform 5′ grid of gravity anomalies has been derived from terrestrial gravity data and has been combined with a 5′ grid for the marine areas derived from satellite altimetry. The combined data set has been used with the EGM96 geopotential model in a remove-restore process to compute the geoid using two-dimensional convolution. The final result is a 5′ grid of geoidal heights covering the land mass of Africa. There are significant gaps in the available terrestrial gravity data — these gaps, coupled with the effects of errors in the DEM used for calculating the G₁ term and in interpolating gravity anomalies, mean that the accuracy of this geoid model will be variable and generally less than desirable. Nevertheless, comparison with GPS/levelling data covering a small part of South Africa shows an RMS agreement of better than 10cm. Over a larger region (all of Egypt) the agreement is less satisfactory.