Sadiq, Muhammad2; Tscherning, Carl C.3; Ahmad, Zulfiqar2
1 Climate and Geophysics, The Niels Bohr Institute, Faculty of Science, Københavns Universitet2 unknown3 Climate and Geophysics, The Niels Bohr Institute, Faculty of Science, Københavns Universitet
This paper deals with the analysis of gravity anomaly and precise levelling in conjunction with GPS-Levelling data for the computation of a gravimetric geoid and an estimate of the height system bias parameter N-o for the vertical datum in Pakistan by means of least squares collocation technique. The long term objective is to obtain a regional geoid (or quasi-geoid) modeling using a combination of local data with a high degree and order Earth gravity model (EGM) and to determine a bias (if there is one) with respect to a global mean sea surface. An application of collocation with the optimal covariance parameters has facilitated to achieve gravimetric height anomalies in a global geocentric datum. Residual terrain modeling (RTM) technique has been used in combination with the EGM96 for the reduction and smoothing of the gravity data. A value for the bias parameter N-o has been estimated with reference to the local GPS-Levelling datum that appears to be 0.705 m with 0.07 m mean square error. The gravimetric height anomalies were compared with height anomalies obtained from GPS-Levelling stations using least square collocation with and without bias adjustment. The bias adjustment minimizes the difference between the gravimetric height anomalies with respect to residual GPS-Levelling data and the standard deviation of the differences drops from 35 cm to 2.6 cm. The results of this study suggest that N-o adjustment may be a good alternative for the fitting of the final gravimetric geoid as is generally done when using FFT methods.
Studia Geophysica Et Geodaetica, 2009, Vol 53, Issue 3, p. 375-388