Today, vertical measurements are used via GPS, laser, and satellite. In the past, datums were measured by survey control points using level bars and optical surface measurement tools. Vertical datums are either tidal, based on sea levels, or geodetic, based on the same ellipsoid models of the earth used for computing horizontal datums.
They are used as a reference for specifying heights. (February 2008)Ī vertical datum is used for measuring the elevations of points on the earth's surface. Unsourced material may be challenged and removed. Please help improve this article by adding citations to reliable sources. This section does not cite any references or sources. To correctly enter, display, and to store map-related coordinates, the datum of the map must be entered into the GPS map datum field. Users of GPS are cautioned that they must always check the datum of the maps they are using.
DEFINITION OF DATUM UPDATE
WGS 84 is valid through 2010 and is due for an update in the near future. Since its inception, WGS 84 has been updated twice, most recently in 2008, to increase its operability with GPS systems. In January 1987, it became the default standard datum for coordinates stored in recreational and commercial GPS units. WGS 84 is used by the Department of Defense for its mapping needs, including its GPS "broadcast" and "precise" orbits. Department of Defense and is defined by the National Geospatial-Intelligence Agency (NGA). It is the reference frame used by the U.S. This is a great advantage over NAD 27 and NAD 83, even though it is set up similarly to NAD 83. WGS 84 is unique in that it is designed to cover the entire world in its scope.
WGS 84 is the World Geodetic System of 1984. NAD 83 is not without error, but it has been found that NAD 27 differs from NAD 83 and true location by several hundred feet. NAD 83 is a great improvement over NAD 27, as it is a more accurate representation of the surface of the earth. It is based on a network of 250,000 points including 600 satellite Doppler stations. The North American Datum of 1983 (NAD 83) replaced NAD 27 in 1986 as the main horizontal datum for the United States, Canada, Mexico, and Central America. NAD 27 provided the best mathematical model of North America until the introduction of NAD 83, which largely made NAD 27 obsolete. NAD 27 is designed to represent North America and little beyond that scope. This datum gave Meade's Ranch a reference height of zero and projected the rest of North America using this azimuth. It is based on the Clarke spheroid of 1866, whose origin lies at Meade's Ranch, Kansas. The North American Datum of 1927 (NAD 27) is a local referencing system designed to accurately represent North America. One must pay extra careful attention to the datum they are referencing.
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