However, as the benefits of a global system outweigh the greater accuracy, the global WGS 84 datum has become widely adopted. OSGB36, for example, is a better approximation to the geoid covering the British Isles than the global WGS 84 ellipsoid. As you may know, people have search hundreds times for their favorite readings like this Geodesy Introduction To Geodetic Datum And Geodetic Systems, but end up in infectious downloads. This phenomenon is called datum shift.īecause Earth is an imperfect ellipsoid, local datums can give a more accurate representation of some specific area of coverage than WGS 84 can. Thank you for reading Geodesy Introduction To Geodetic Datum And Geodetic Systems. Using local datums, the disparity on the ground between a point having the same horizontal coordinates in two different datums could reach kilometers if the point is far from the origin of one or both datums. Because the ellipsoid or geoid differs between datums, along with their origins and orientation in space, the relationship between coordinates referred to one datum and coordinates referred to another datum is undefined and can only be approximated. Then the coordinates of other places are measured from the nearest control point through surveying. Even GPS requires a predefined framework on which to base its measurements, so WGS 84 essentially functions as a datum, even though it is different in some particulars from a traditional standard horizontal or vertical datum.Ī standard datum specification (whether horizontal or vertical) consists of several parts: a model for Earth's shape and dimensions, such as a reference ellipsoid or a geoid an origin at which the ellipsoid/geoid is tied to a known (often monumented) location on or inside Earth (not necessarily at 0 latitude 0 longitude) and multiple control points that have been precisely measured from the origin and monumented. Astronomical and chronological methods have limited precision and accuracy, especially over long distances. Before GPS, there was no precise way to measure the position of a location that was far from universal reference points, such as from the Prime Meridian at the Greenwich Observatory for longitude, from the Equator for latitude, or from the nearest coast for sea level.
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