Editing World Geodetic System (section)

==={{anchor|WGS60}} WGS&nbsp;60===
In the late 1950s, the [[United States Department of Defense]], together with [[scientist]]s 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. Efforts of the U.S. Army, Navy and Air Force were combined leading to the DoD World Geodetic System 1960 (WGS&nbsp;60). 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 [[latitude]]s, [[longitude]]s, and [[elevation]]s. Heritage surveying methods found elevation differences from a local horizontal determined by the [[spirit level]], [[plumb-bob|plumb line]], or an equivalent device that depends on the local gravity field (see [[physical geodesy]]). As a result, the elevations in the data are referenced to the [[geoid]], a surface that is not readily found using [[satellite geodesy]]. The latter observational method is more suitable for global mapping.  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]].
[[Image:GRAVIMETRIC DATUM ORIENTATION.SVG|thumb|upright=1.75|Gravimetric datum orientation.
{{legend-line|solid cyan|Ellipsoid of [[Geodetic astronomy|astro-geodetically]] oriented datum}}
{{legend-line|solid lime|[[Geoid]]}}
{{legend-line|solid red|[[Gravimetry|Gravimetrically]]-oriented ellipsoid}}]]

In accomplishing WGS&nbsp;60, a combination of available surface [[gravity]] data, [[astro-geodetic]] data and results from HIRAN<ref>{{cite web|url=http://www.history.noaa.gov/stories_tales/ak7.html |title=NOAA History - Stories and Tales of the Coast & Geodetic Survey - Personal Tales/Earth Measurer/Aslakson Bio |website=History.noaa.gov |access-date=24 May 2017}}</ref> and Canadian [[SHORAN]] surveys were used to define a best-fitting [[ellipsoid]] and an earth-centered orientation for each initially selected datum. (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&nbsp;60 was a value for the [[ellipsoid]] flattening which was obtained from the nodal motion of a satellite.

Prior to WGS&nbsp;60, the U.S. Army and [[U.S. Air Force]] had each developed a world system by using different approaches to the gravimetric datum orientation method.  To determine their gravimetric orientation parameters, the Air Force used the mean of the differences between the gravimetric and astro-geodetic [[vertical deflection|deflections]] and geoid heights (undulations) at specifically selected stations in the areas of the major datums. The Army performed an adjustment to minimize the difference between astro-geodetic and [[gravimetric]] [[geoid]]s. 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. Since the Army and Air Force systems agreed remarkably well for the NAD, ED and TD areas, they were consolidated and became WGS&nbsp;60.