ORTHOMETRIC HEIGHTS FROM GLOBAL POSITIONING SYSTEM

It is generally accepted that observations from both Global Positioning System (GPS) surveys and geodetic leveling are capable of producing high-precision results in terms of ellipsoidal and orthometric height differences, respectively. It follows that a knowledge of the geoidal model to the specified degree of accuracy is the essential element in determining orthometric heights using GPS observations. This paper describes a mathematical procedure that improves any given geoidal model-e.g., GEOID4 and GEOID90-by combining geoidal heights with existing vertical network data and GPS observations. The evaluation of their relationship results in finding the best fit at the selected vertical control stations, then modeling it by using various regression surfaces involving linear or nonlinear equations, thus providing computational means to upgrade geoidal height estimates and, therefore, to improve GPS-derived orthometric heights as well. Current results indicate that not all problems have been resolved, and more experimentation is required to develop specific guidelines to achieve reliable solutions repeatedly.