Geodetic parameters derived from gravity data across Nigeria

Physical geodetic survey of the Nigeria landmass was undertaken with the aim of furnishing engineers and scientists with vital geodetic information which have not been adequately available but required for the development of a standard national economic framework. The basic data used for the study were the absolute gravity field values, and the recorded coordinates and elevations at the Primary Gravity Network of Nigeria (PGNN) base stations. The absolute gravity data were processed using the appropriate mathematical and computational procedures to deduce geodetic information which include geoid undulation, height above the geoid, deflections of the vertical, distance from the earth’s centre of mass to the reference ellipsoid and geoid, and the gravity potential value at the geoid. Comparison was also made between the WGS’84 and the geoid in the study area. Geoid undulations were found to range from 16.53 to 28.35 m and the deflections of the vertical ranged between 0.738685 to 10.28328 arc second. Height above the topography ranged from -16.38 m in Warri to +1257.13 in Jos. The distance from the earth’s centre of mass to the WGS’84 reference ellipsoidal surface was maximum at Oron (6377988.04 m) and least at Illela (6376941.31 m). Similarly, the distance from the earth’s centre of mass to the geoid was also maximum at Oron (6378008.39 m) and minimum in Illela (6376961.659 m). The interpolation of the distances from the earth’s centre of mass to the spheroidal surface at each base station gave the definition of the WGS’84 ellipsoid over the continental Nigeria landmass while the interpolation of the distances from the earth’s centre of mass to the geoidal surface gave the definition of the geoid over the continental Nigeria landmass. The average gravity potential at the geoid was evaluated as 62375050.41 m²s^-2. The geoid over the continental Nigeria landmass was found to be comparably smooth as the WGS’84 ellipsoid over the region though they differ in terms of vertical separation. The quantitative difference in mGal corresponding to the vertical separation between the WGS’84 reference ellipsoid and the geoid was computed as 6.8042 mGal. The values of the deflections of the vertical at each station of the PGNN gave the angular correction to be applied when combining satellite-based ellipsoidal measurements and geoid-based physical geodetic measurement. Based on the significant range of the deflections of the vertical across the continental area of Nigeria, it was concluded that the physical structure of the lithosphere beneath the study area must be highly complex and heterogeneous.