New investigation on the choice of the tailored geopotential model for Algeria

Abstract

Abstract The choice of the best geopotential model to reduce geodetic data is one of the critical steps in computing the geoid. Several studies have shown that the geopotential models tailored to regional or local gravity data are best suited for high precision geoid computations. Since 2000 a number of geoid models for Algeria have been produced by Geodetic Laboratory of the National Centre of Space Techniques. In particular 5′ × 5′ geoid models were generated in 2000, [ Benahmed Daho, S. A., 2000. The new gravimetric geoid in Algeria. IGeS Bulletin No. 10 of the International Geoid Service (IGeS). ISSN 1128-3955. pp. 78–84. ] and in 2004 [ Benahmed Daho, S.A., Fairhead, J.D., 2004. A new quasigeoid computation from gravity and GPS data in Algeria. Newton's Bulletin No. 2. A Joint Bulletin of the Bureau Gravimetrique International and of the International Geoid Service. ISSN 1810-8547. pp. 52–59.] using different data sets and techniques. Although these results were satisfactory and internally consistent they do no have the required accuracy to be able to transform a GPS ellipsoidal height to an orthometric height. During the same time and with the recent satellite missions CHAMP and GRACE several new global gravity models were released. These lead to substantial improvements of our knowledge of the long-wavelength part of the Earth's gravity field, and thereby of the long-wavelengths of the geoid. For the computation of a new gravimetric geoid model for Algeria we need a new investigation on the choice of the best and optimal geopotential model for the combined solution with local gravimetric and topographic data using the remove-restore technique. In this paper, an analysis was carried out to define the geopotential model, which fits best the local gravity field in Algeria. Six global geopotential models are used in this study: The new GRACE satellite-only and combined models EIGEN-GRACE02S and GGM02C, combined CHAMP and GRACE model EIGEN-CG01C, combined CHAMP and LAGEOS model EIGEN-GL04C, OSU91A and EGM96. The test of the fitting of these high order geopotential models to the gravity field in Algeria is based on the gravity data supplied by the B.G.I. and GETECH, and some of the precise GPS data collected from the international TYRGEONET ( TYRhenian GEOdynamical NETwork ), ALGEONET ( ALGerian GEOdynamical NETwork ) projects with baseline length ranging from about 1 to 1000 km. The comparisons were made at all gravity and GPS levelled data by computation of the residual data (i.e. observed data minus model). The geopotential model that provides the closest statistical fit to these data can be assumed to be the most suitable model to adopt for the determination of the new Algerian gravimetric geoid. The study shows that the newly released combined model (EIGEN-GL04C) is consistently superior to other models in the Algerian region. Its standard deviation fit with GPS/levelling data is 30 cm and 27.5 cm before and after fitting. Hence, we strongly recommend the use of this new model in the computation of the new gravimetric geoid model for Algeria.