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CoverNewton's Bulletin

Issue n. 3, January 2006   [pp. 61-70]

A gravimetric quasi-geoid evaluation in the Northern region of Algeria using EGM96 and GPS/Levelling

Muhammad Sadiq and Zulfiqar Ahmad

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The present study has been carried out to compare the geoid-quasigeoid separation in two areas of Pakistan with different topographic distributions. The height datum of Pakistan is based on the orthometric height system. Since Pakistan has a diversity of terrain distribution as regards the elevation from mean sea level due to vast expanse comprising both land and hilly areas, the emphasis of this study has been placed on the quantification of the geoid-quasigeoid separation term with respect to elevation distribution for future geoid determination. Bouguer gravity anomalies and digital terrain elevation were used to estimate the minimum/maximum separation between these two reference surfaces. This was also compared witha separation term C computed from EGM96 gravity anomalies. The statistics of the results in the two areas exhibit a one to one correspondence of EGM96 gravity anomalies with observed gravity data and digital elevation data. The area with high mountains (Kalat) has more offset between the two surfaces. The standard deviation of separation term is ~77 mm from observed and 62.5 mm from model data. In contrast with the low elevation area (Dadu), the standard deviation of the separation becomes as small as ~2 mm from observed data and ~7mm from gravity anomalies computed from the EGM96 model. The terrain correction has measurable effect on the standarddeviation in Kalat and is very insignificant in Dadu area. The difference of the separation term from observed data and the model can be related to assumption of the topographic density, local mass irregularities and inherent omission error of the EGM96 model. These results also show that the separation term C is significant in Pakistan and may be required to be incorporated in the final geoidal solution.