Terrain effects on geoid determination
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AbstractGravimetric reduction schemes play an important role on precise geoid determination, especially in rugged areas. The main theme of this research is to explore different gravimetric reduction schemes in the context of precise geoid determination, in addition to the usual Helmert's second method of condensation and residual terrain model (RTM). A numerical investigation is carried out in the rugged area of the Canadian Rockies to study gravimetric geoid solutions based on the Rudzki inversion scheme, Helmert's second method of condensation, RTM, and the topographic-isostatic reduction methods of Airy-Heiskanen (AH) and Pratt-Hayford (PH). The mathematical formulations of each of these techniques are presented. This study shows that the Rudzki inversion scheme, which had neither been used in practice in the past nor is it used at present, can become a standard tool for gravimetric geoid determination since the Rudzki geoid performs as well as the Helmert and RTM geoids (in terms of standard deviation and range of maximum and minimum values) when compared to the GPS-levelling geoid of the test area. Also, it is the only gravimetric reduction scheme which does not change the equipotential surface and thus does not require the computation of the indirect effect. In addition, this thesis investigated two important topics for precise geoid determination; the density and gravity interpolation effects on Helmert geoid determination and the terrain aliasing effects on geoid determination using different mass reduction schemes. The study of first topic shows that the topographic-isostatic gravimetric reduction schemes like the PH or AH models or the topographic reduction of RTM, should be applied for smooth gravity interpolation for precise Helmert geoid determination instead of the commonly used Bouguer reduction scheme. The density information should be incorporated not only for the computation of terrain corrections (TC), but also in all other steps of the Helmert geoid computational process. The study of the second topic suggests that a DTM grid resolution of 6" or higher is required for precise geoid determination with an accuracy of a decimetre or higher for any gravimetric reduction method chosen in rugged areas.
Bibliography: p. 106-114