A study on the evaluation of the geoid-quasigeoid separation term over Pakistan with a solution of first and second order height terms

Abstract

An attempt has been made to evaluate the geoid-quasigeoid separation term over Pakistan by using solutions of terms involving first and second order terrain heights. The first term, involving the Bouguer anomaly, has a significant value and requires being incorporated in any case for determination of the geoid from the quasigoidal solution. The results of the study show that the second term of separation, which involves the vertical gravity anomaly gradient, is significant only in areas with very high terrain elevations and reaches a maximum value of 2-3 cm. The integration radius of 18 km for the evaluation of the vertical gravity anomaly gradient was found to be adequate for the near zone contribution in the case of the vertical gravity anomaly gradient. The Earth Gravity Model EGM96 height anomaly gradient terms were evaluated to assess the magnitude of the model dependent part of the separation term. The density of the topographic masses was estimated with the linear operator of vertical gravity anomaly gradient using the complete Bouguer anomaly data with an initial arbitrary density of 2.67 g/cm3 to study the effect of variable Bouguer density on the geoid-quasigeoid separation. The density estimates seem to be reasonable except in the area of very high relief in the northern parts. The effect of variable density is significant in the value of the Bouguer anomaly-dependent geoid-quasigeoid separation and needs to be incorporated for its true applicability in the geoid-quasigeoid separation determination. The geoid height (N) was estimated from the geoid-quasigeoid separation term plus global part of height anomaly and terrain-dependant correction terms. The results were compared with the separation term computed from EGM96-derived gravity anomalies and terrain heights to estimate its magnitude and the possible amount of commission and omission effects. Copyright © The Society of Geomagnetism and Earth.