(For example, in Great Britain, the national vertical datum, Ordnance Datum Newlyn, is based on what was mean sea level at Newlyn in Cornwall between 19).Ī different principle is used when choosing a datum for nautical charts. For this reason, a country will choose the mean sea level at one specific point to be used as the standard “sea level” for all mapping and surveying in that country. It will not remove the effects of local gravity strength, and so the height of MSL, relative to a geodetic datum, will vary around the world, and even around one country. This allows an average sea level to be determined, with the effects of waves, tides, and short-term changes in wind and currents removed. This is determined by measuring the height of the sea surface over a long period (preferably around 18 years, to account for all the astronomical effects that contribute to tide levels). In common usage, elevations are often cited in height above sea level, although what “sea level” actually means is a more complex issue than might at first be thought: the height of the sea surface at any one place and time is a result of numerous effects, including waves, wind and currents, atmospheric pressure, tides, topography, and even differences in the strength of gravity due to the presence of mountains etc.įor the purpose of measuring the height of objects on land, the usual datum used is Mean Sea Level. 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.
DEFINE 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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