Measuring Land Subsidence Using GPS: Ellipsoid Height versus Orthometric Height

Global positioning system (GPS) technology has been frequently used to monitor geological hazards associated with ground deformations, such as long-term landslides and subsidence. When GPS data are processed, they yield ellipsoid heights, which are the distances above a smooth ellipsoid surface. However, orthometric heights are often used in practical surveying and engineering applications. Orthometric height is a physical quantity that refers to the surface of the geoid. In this study, a more practical alternative definition of orthometric height is used. This approximation is the one commonly implemented in practical surveying and engineering applications to compute relative orthometric height values. This well-known procedure computes orthometric heights by combining GPS-measured ellipsoid height and a geoid model. Any type of orthometric height is a physically based quantity. GPS alone, which is a geometric technique, cannot directly measure orthometric heights. This study investigates the ver...