How much will an orthometric height measured with GPS and corrected with a geoid model vary in time (h-dot and N-dot)?
The Earth is a dynamic planet; it is always changing. Some changes can be quite drastic (e.g., landslide, earthquake) while others are can be more subtle (e.g., post-glacial rebound). When you have drastic events, the benchmarks can move significantly or be destroyed completely. In this case, you loose access to the vertical datum. On the other hand, the impact of a drastic event on the geoid is very small; thus, it is possible to take new GPS measurements and install new control stations immediately. Subtle changes are difficult to detect because they expend over a very large regions. These changes are usually not detected when conducting local relative surveys (e.g., levelling or differential GPS). However, GPS technique such as Precise Point Positioning (PPP) will show that the terrain can move by as much as 1 cm per year. This dynamic change of the topography will also bring a 10% change to the geoid. Thus, the long wavelength components of the geoid can change by approximately 1 c
