GPS On Bench Marks for GEOID99

Last updated April 5, 2000

(changes since last update are shown in RED)

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.

Accurate transformation between ellipsoidal and orthometric heights requires a high resolution geoid height model of comparable accuracy. An example of such a model is GEOID99, computed at the National Geodetic Survey. Geoid model computations are intricate processes, and rely primarily upon gravity and digital elevation models. GPS on leveled bench mark data provide a powerful tool in checking geoid computations, as well as in the removal of long wavelength geoid errors. In addition, many investigators use GPS on bench marks to test global geopotential models, such as EGM96.

The GPS on bench mark data set for the United States has been developed as a product to support research on geoid and geopotential models. It consists of 6415 points occupied by GPS receivers on leveled bench marks. Of these, 246 points have been flagged as outliers, leaving 6169 data points (plotted below). Details on the processing and cleansing of the data can be found in the documentation. Note that two different forms of the data set are provided (NAD83 and ITRF96(1997.0)). The only difference is in the choice of the reference frame used to express the GPS latitude, longitude, and ellipsoid height.

Due to a change in the adopted ITRF97(1997.0) parameters, the ITRF96(1997.0) are considered a distinct system and no longer equivalent. Click here for further details about the difficulty in defining the origin of the GPS on bench mark data set.

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