Editing Geodetic datum (section)

== History ==
{{main | History of geodesy | Spherical Earth#History | Earth's circumference#History}}
{{seealso | History of navigation | History of longitude | History of latitude}}
[[File:Index chart of the Great Trigonometrical Survey of India (vectorized).svg|thumb|The Great Trigonometrical Survey of India, one of the first surveys comprehensive enough to establish a geodetic datum.]]

The spherical nature of Earth was known by the ancient Greeks, who also developed the concepts of latitude and longitude, and the first astronomical methods for measuring them. These methods, preserved and further developed by [[Muslim]] and Indian astronomers, were sufficient for the global explorations of the 15th and 16th Centuries. 

However, the scientific advances of the [[Age of Enlightenment]] brought a recognition of errors in these measurements, and a demand for greater precision. This led to technological innovations such as the 1735 [[Marine chronometer]] by [[John Harrison]], but also to a reconsideration of the underlying assumptions about the shape of Earth itself. [[Isaac Newton]] postulated that the [[conservation of momentum]] should make Earth [[Equatorial bulge | oblate]] (wider at the  equator than the corresponding sphere), while the early surveys of [[Jacques Cassini]] (1720) led him to believe Earth was [[Spheroid | prolate]] (narrower at the equator). The subsequent French geodesic missions (1735-1739) to [[French Geodesic Mission to Lapland | Lapland]] and [[French Geodesic Mission | Peru]] corroborated Newton, but also discovered variations in gravity that would eventually lead to the [[geoid]] model.

A contemporary development was the use of the [[trigonometric survey]] to accurately measure distance and location over great distances. Starting with the surveys of [[Jacques Cassini]] (1718) and the [[Anglo-French Survey (1784–1790)]], by the end of the 18th century, survey control networks covered [[Cartography of France#Cassini maps | France]] and [[Principal Triangulation of Great Britain | the United Kingdom]]. More ambitious undertakings such as the [[Struve Geodetic Arc]] across Eastern Europe (1816-1855) and the [[Great Trigonometrical Survey]] of India (1802-1871) took much longer, but resulted in more accurate estimations of the shape of the [[Earth ellipsoid]]. The first triangulation across the United States was not completed until 1899.

The U.S. survey resulted in the [[North American Datum]] (horizontal) of 1927 (NAD{{nbsp}}27) and the Vertical Datum of 1929 (NAVD29), the first standard datums available for public use. This was followed by the release of national and regional datums over the next several decades. Improving measurements, including the use of early [[Satellite | satellites]], enabled more accurate datums in the later 20th century, such as [[North_American_Datum#North_American_Datum_of_1983|NAD 83]] in North America, [[ETRS89]] in Europe, and [[GDA94]] in Australia. At this time global datums were also first developed for use in [[satellite navigation]] systems, especially the [[World Geodetic System]] (WGS{{nbsp}}84) used in the U.S. [[global positioning system]] (GPS), and the [[International Terrestrial Reference System and Frame]] (ITRF) used in the European [[Galileo (satellite navigation) | Galileo]] system.