A preliminary determination of a gravimetric geoid in Peninsular Malaysia

It is considered that precise gravimetric geoid determination is one of the main geodetic problems in Peninsular Malaysia for the near future, since the Global Positioning System (GPS) defined ellipsoidal coordinates require geoidal heights in practice. Geoidal heights can be determined by either a geopotential model alone, or in combination with local gravity and height data. The reference gravity field contributed by the geopotential model can be improved by a tailoring method. This is the main objective of the study; a tailored model called OSU89B-MM (Malaysian Model) complete to degree and order 360 was developed by fitting the pre-existing geopotential model OSU89B to the updated mean free-air anomalies of half degree blocks in the peninsular region. Numerical analyses indicate that the tailoring technique has improved the reference gravity field by about 50% for point anomalies (from 23 mgals to 11 mgals). Tests on absolute geoid heights in selected areas where GPS and levelling data were available have demonstrated about 10% improvement (1 cm to 2 cm) of the tailored model over OSU89B, whereby the long wavelength errors have been partly diminished. The gravimetric geoid height has been computed in three test areas (which were characterised by different types of topography, gravity density and coverage) by Least Squares Collocation (LSC) and Fast Fourier Transforms (FFT). The local height data were also utilised in a remove-restore procedure in order to study the gravitational influence of the topography, especially in test areas of rough terrain. Results show that the standard deviation of the absolute differences between the control GPS/levelling derived geoid heights and predicted gravimetric geoid heights have improved by 5 cm (from 11 cm to 6 cm) compared to the corresponding differences implied by OSU89B- MM. Work carried out in areas of high topographic relief (for which no GPS data were available) has nevertheless shown the terrain contribution to the geoid height to be as much as 0.5 m to 1.5 m