Determination of the geoid of central highlands in Sri Lanka

The available geoid undulations on the WGS84 ellipsoid at over two hundred GPS stations are interpolated using a least-squares surface fitting technique to determine the geoid of the central highlands in Sri Lanka. However, it is not possible to interpolate these points directly to prepare a detailed map of the geoid surface as the geoid separation varies intense due to the rugged local topography making the interpolation inaccurate. The gravity potential and subsequently the undulation of the local geoid due to the topography have been calculated separately using a topographic model and removed from the available geoid undulations. This model was created using information obtained from 1:50 000 digital topographic maps provided by the Survey department of Sri Lanka. The resulting geoid separations were interpolated and three surface polynomials were employed to determine the geoid using the least-squares surface fitting technique. To avoid possible artefacts in regions without observations, an area including central highlands was selected to determine the geoid. Finally, the geoid undulations due to the topography were added back to the Bouguer co-geoid represented by three mathematical surfaces to create a detailed map of the geoid of Sri Lanka. A local positive geoid surface superimposing a large negative regional surface has been obtained and the local maximum value of the geoid undulation is about –92.05 m in the vicinity of Piduruthalagala peak.