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 (commonly, though erroneously, called 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 Caribbean area, geoid heights range from a low of -71.0 meters over the Puerto Rico trench (magenta) to a high of 17.1 meters (red) in Costa Rica.
The CARIB97 model is a high resolution gravimetric geoid height model covering the region 9-28N, 86-58W. It was computed in a cooperative effort with the National Imagery and Mapping Agency (NIMA). All computations were performed in the ITRF94(1996.0) reference frame. The geoid heights are relative to a geocentric GRS-80 reference ellipsoid. With respect to the permanent tide of the Earth, the CARIB97 geoid model is in the non-tidal system. No attempt was made to incorporate GPS on levelled benchmarks for this model (this makes it different than GEOID96), and therefore CARIB97 is not suitable for direct conversion between NAD83 GPS ellipsoid heights and NAVD88 orthometric heights.
The CARIB97 geoid model has been designed specifically for the Caribbean Sea. Due to data coverage and computational issues, one will find offsets between CARIB97, and either GEOID96 or G96SSS in regions of overlap.
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