Orthometric Height (MSL)
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The height displayed on most consumer handheld GPS receivers is orthometric height, the height above mean sea level (MSL). It is straight forward to approximate MSL world wide by interpolation of the GEOID model (a lookup table in the receiver's firmware) and making the simple calculation:
GEOID03 is a refined model of the geoid in the United States, which supersedes the previous models. Also see: WGS 84 Earth Gravity Model below. WGS 84 Earth Gravitational Model  the Geospatial Sciences Division in NIMA is responsible for collecting, processing, and evaluating gravity data (freeair and Bouguer gravity anomalies). These data are then used to compute gravimetric quantities such as mean gravity anomalies, geoid heights, deflections of the vertical, and gravity disturbances. All of these quantities are used in World Geodetic System 1984 support, navigation systems, mapping projects, and different types of surveys. Vertical Datums, Elevations, and Heights  The zero surface to which elevations or heights are referred is called a vertical datum. Traditionally, surveyors and mapmakers have tried to simplify the task by using the average (or mean) sea level as the definition of zero elevation, because the sea surface is available worldwide. The mean sea level (MSL) is determined by continuously measuring the rise and fall of the ocean at "tide gauge stations" on seacoasts for a period of about 19 years. This averages out the highs and lows of the tides caused by the changing effects of the gravitational forces from the sun and moon which produce the tides. The mean sea level (MSL) then is defined as the zero elevation for a local or regional area. But what do you do for Mt. Elbert? Where is the MSL in Colorado? There is no tangible surface of the ocean from which to measure height. Height Modernization is a series of activities designed to advance and promote the determination of elevations by Global Positioning System (GPS) surveying, rather than by spirit leveling. It includes research and development activities seeking to improve the determination of geometric elevations by GPS surveys as well as activities seeking to improve the determination of geoid height. It includes activities leading to improved height determination both for the static surveyor and for the moving navigator.
© Copyright 2007  Samuel J. Wormley
