Impact of the New GRACE-Derived Global Geopotential Model and SRTM Data on the Geoid Heights in Algeria

In Algeria and since 2000, two geoid models have been published by the Geodetic Laboratory of the National Centre of Space Techniques using different data sets and techniques. Although these results were satisfactory and internally consistent they do not have the required accuracy to transform a GPS ellipsoidal height to an orthometric height. Recently, the quantity and quality of terrestrial gravity data slightly increased, and especially several new GGM from the recent satellite missions were released. At the same time, the new high resolution SRTM (Shuttle Radar Topography Mission) global DEM was constructed. Logically, these new data represent improvements that must be included in a new geoid for Algeria. The main purpose of this paper is to study the impact of the above new data on the geoid heights in Algeria. For this reason, a new gravimetric geoid determination has been carried out including these new data. The method used in the computation of the geoid has been the Stokes integral in convolution form. This solution was based on the land gravity data supplied by the BGI (Bureau Gravimetrique International), Digital Elevation Model derived from SRTM for topographic correction and the optimal Grace-derived GGM EIGEN-GL04C. This gravimetric geoid and previous geoids existing for this area, are compared to the geoid undulations corresponding to 62 GPS/levelling points located in northern part of Algeria. The study shows that the new gravimetric geoid model agrees considerably better with GPS/levelling than any other local geoid models. Its standard deviations are 27 and 25 cm before and after fitting using a four-parameters model as corrector surface. The new geoid model will be used in low accuracy scientific applications and in low-order levelling network densification with regard to the national levelling network coverage considered as good in the north and becomes poor in the south and West of the country. The availability and accuracy of the land gravity data remain insufficient to agree with GPS/levelling at the sub-centimeter level.