GPS On Benchmarks

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 GEOID96, computed at the National Geodetic Survey. Geoid model computations are intricate processes, and rely primarily upon gravity and digital elevation models. GPS on leveled benchmark 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 benchmarks to test global geopotential models, such as EGM96.

The GPS benchmark data set for the United States has been developed as a product to support research on geoid and geopotential models. The data set consists of 5379 points occupied by GPS receivers on leveled benchmarks. 211 points have been flagged as outliers, leaving 5187 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. The only difference is in the choice of the reference frame used to express the GPS latitude, longitude, and ellipsoid height.

Got a geoid question?
Contact us in the USA at 301-713-3202 or at dennis.milbert@noaa.gov or dru.smith@noaa.gov .

Have a data problem?
Contact our height specialist, Renee Shields, in the USA at 301-713-3198x129 or at renee.shields@noaa.gov .

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