GEOID96

Now Available: GEOID03

The impact of the Global Positioning System (GPS) on geodetic control surveys has been immense. In the past we have relied upon line-of-sight instrumentation to develop our coordinates. With GPS, ground station intervisibility is no longer required, and we can survey with much longer lines. Different instruments and survey techniques were used to measure horizontal and vertical coordinates, leading to two different networks with little overlap. GPS, on the other hand, is a three-dimensional system.

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 -51.6 meters in the Atlantic (magenta) to a high of -7.2 meters (red) in the Rocky Mountains.

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