A minimum of four GPS NAVSTAR satellites (more can be used in overdetermined PVT solutions) are used to determine three position coordinates and time. Position coordinates are computed by the receiver in Earth-Centered, Earth-Fixed X, Y, Z (ECEF XYZ) coordinates. Most receivers compute, convert (and store) positions in geodetic coordinates (datum WGS-84) latitude, longitude and height above the ellipsoid (HAE). Note - height calculations are independent of any user selected horizontal datum. 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:         Orthometric Height (hA MSL) = HAE (HA) - Geoid Undulation (NA)         Orthometric Height (hB MSL) = HAE (HB) - Geoid Undulation (NB)           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 (free-air 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.     Some Measurements of Accuracy USFS GPS Information Page & Receiver Performance Reports   FAA - Latest GPS PAN Report (PDF) FAA - Latest WAAS PAN Report (PDF) GPS SPS Performance Standard (Oct 2001) (PDF) GPS Accuracy Web Pages   from the National Geodetic Survey (NGS) GPS Accuracy Monitor   by Dennis Milbert Garmin Accuracy Report   by Wolfgang Rupprecht Circular Error Probable   by Logan Scott Garmins 25LP OEM Receiver Tested   by Storm Van Leeuwen S. GPS Accuracy Web Pages   by David L. Wilson Estimation of Trimble Scoutmaster DGPS Performance (1997)   by Sam Wormley GPS - Accuracy Statistics (PPS) (more)   GPS - perf_reports archive       © Copyright 2007 - Samuel J. Wormley   by swormley1@mchsi.com