Improvements to the global system included the Astrogeoid of Irene Fischer and the astronautic Mercury datum. Since the Army and Air Force systems agreed remarkably well for the NAD, ED and TD areas, they were consolidated and became WGS 60. By matching the relative astro-geodetic geoids of the selected datums with an earth-centered gravimetric geoid, the selected datums were reduced to an earth-centered orientation. The Army performed an adjustment to minimize the difference between astro-geodetic and gravimetric geoids. To determine their gravimetric orientation parameters, the Air Force used the mean of the differences between the gravimetric and astro-geodetic deflections and geoid heights (undulations) at specifically selected stations in the areas of the major datums. Air Force had each developed a world system by using different approaches to the gravimetric datum orientation method. (Every datum is relatively oriented with respect to different portions of the geoid by the astro-geodetic methods already described.) The sole contribution of satellite data to the development of WGS 60 was a value for the ellipsoid flattening which was obtained from the nodal motion of a satellite. In accomplishing WGS 60, a combination of available surface gravity data, astro-geodetic data and results from HIRAN and Canadian SHORAN surveys were used to define a best-fitting ellipsoid and an earth-centered orientation for each of initially selected datum. Therefore, a motivation, and a substantial problem in the WGS and similar work is to patch together data that were not only made separately, for different regions, but to re-reference the elevations to an ellipsoid model rather than to the geoid. The latter observational method is more suitable for global mapping. As a result, the elevations in the data are referenced to the geoid, a surface that is not readily found using satellite geodesy. Heritage surveying methods found elevation differences from a local horizontal determined by the spirit level, plumb line, or an equivalent device that depends on the local gravity field (see physical geodesy). The term datum as used here refers to a smooth surface somewhat arbitrarily defined as zero elevation, consistent with a set of surveyor's measures of distances between various stations, and differences in elevation, all reduced to a grid of latitudes, longitudes, and elevations. Army, Navy and Air Force were combined leading to the DoD World Geodetic System 1960 (WGS 60). In the late 1950s, the United States Department of Defense, together with scientists of other institutions and countries, began to develop the needed world system to which geodetic data could be referred and compatibility established between the coordinates of widely separated sites of interest. Western Cold War preparedness necessitated a standardised, NATO-wide geospatial reference system, in accordance with the NATO Standardisation Agreement.Need for global maps for navigation, aviation, and geography.The inability of the large geodetic systems, such as European Datum ( ED50), North American Datum (NAD), and Tokyo Datum (TD), to provide a worldwide geo-data basis.The lack of inter-continental geodetic information.International space science and the beginning of astronautics.Austria and Germany founded the Zentralbüro für die Internationale Erdmessung (Central Bureau of International Geodesy), and a series of global ellipsoids of the Earth were derived (e.g., Helmert 1906, Hayford 1910/ 1924).Ī unified geodetic system for the whole world became essential in the 1950s for several reasons: Helmert's famous book Mathematische und Physikalische Theorien der Physikalischen Geodäsie ( Mathematical and Physical Theories of Physical Geodesy). History Įfforts to supplement the various national surveying systems began in the 19th century with F.R. The standard is published and maintained by the United States National Geospatial-Intelligence Agency. The current version, WGS 84, defines an Earth-centered, Earth-fixed coordinate system and a geodetic datum, and also describes the associated Earth Gravitational Model (EGM) and World Magnetic Model (WMM). The World Geodetic System ( WGS) is a standard used in cartography, geodesy, and satellite navigation including GPS.
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