However, the heights obtained from GPS are in a different height system than those historically obtained with geodetic leveling. GPS data can be readily processed to obtain ellipsoidal height, h. This is height above or below a simple ellipsoid model of the Earth. Geodetic leveling gives rise to a height called orthometric height, H, often known as height above mean sea level. These are the heights found on topographic maps, stamped on markers, or stored in innumerable digital and paper data sets. To transform between these height systems, one requires geoid height, N. These height systems are related by the equation: h = H + N.
In the conterminous United States, geoid heights range from a low of -52.8 meters in the Atlantic (magenta) to a high of -7.7 meters (red) in the Rocky Mountains.
The G96SSS model is a gravimetric geoid model for the conterminous United States, suitable for scientific investigations. Geoid heights are referred to the GRS80 ellipsoid, and the computations were performed in the ITRF94(1996.0) reference frame. The G96SSS model is not suitable for direct conversion between NAD 83 GPS ellipsoid heights and NAVD 88 orthometric heights. It is necessary to subtract 12.0 cm from the G96SSS values to obtain the geoid undulation between the best-fit global geopotential surface and the GRS80 ellipsoid (when both are expressed in a tide-free system).
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