Editing Geodetic datum (section)

== Discussion and examples ==
{{see also|Reference ellipsoid}}

A geodetic reference datum is a known and constant surface which is used to describe the location of unknown points on Earth. Since reference datums can have different radii and different center points, a specific point on Earth can have substantially different coordinates depending on the datum used to make the measurement. There are hundreds of locally developed reference datums around the world, usually referenced to some convenient local reference point. Contemporary datums, based on increasingly accurate measurements of the shape of Earth, are intended to cover larger areas. The most common reference Datums in use in [[North America]] are NAD{{nbsp}}27, NAD{{nbsp}}83, and [[WGS 84]].

The [[North American Datum]] of 1927 (NAD{{nbsp}}27) is "the horizontal control datum for the United States that was defined by a location and azimuth on the Clarke spheroid of 1866, with origin at (the survey station) [[Meades Ranch Triangulation Station|Meades Ranch (Kansas)]]." ... The geoidal height at Meades Ranch was assumed to be zero, as sufficient gravity data was not available, and this was needed to relate surface measurements to the datum. "Geodetic positions on the North American Datum of 1927 were derived from the (coordinates of and an azimuth at Meades Ranch) through a readjustment of the triangulation of the entire network in which Laplace azimuths were introduced, and the Bowie method was used."<ref>{{cite web | url=http://www.ngs.noaa.gov/faq.shtml#WhatDatum | title=National Geodetic Survey - Frequently Asked Questions FAQ }}</ref> NAD{{nbsp}}27 is a local referencing system covering North America.

The North American Datum of 1983 (NAD 83) is "The horizontal control datum for the United States, Canada, Mexico, and Central America, based on a geocentric origin and the Geodetic Reference System 1980 ([[GRS{{nbsp}}80]]). "This datum, designated as NAD{{nbsp}}83…is based on the adjustment of 250,000 points including 600 satellite Doppler stations which constrain the system to a geocentric origin." NAD{{nbsp}}83 may be considered a local referencing system.

WGS{{nbsp}}84 is the [[World Geodetic System]] of 1984. It is the reference frame used by the [[United States Department of Defense|U.S. Department of Defense]] (DoD) and is defined by the [[National Geospatial-Intelligence Agency]] (NGA) (formerly the Defense Mapping Agency, then the National Imagery and Mapping Agency). WGS{{nbsp}}84 is used by the DoD for all its mapping, charting, surveying, and navigation needs, including its [[GPS]] "broadcast" and "precise" orbits. WGS{{nbsp}}84 was defined in January 1987 using Doppler satellite surveying techniques. It was used as the reference frame for broadcast GPS Ephemerides (orbits) beginning January 23, 1987. At 0000 GMT January 2, 1994, WGS{{nbsp}}84 was upgraded in accuracy using GPS measurements. The formal name then became WGS{{nbsp}}84 (G730), since the upgrade date coincided with the start of GPS Week 730. It became the reference frame for broadcast orbits on June 28, 1994. At 0000 GMT September 30, 1996 (the start of GPS Week 873), WGS{{nbsp}}84 was redefined again and was more closely aligned with [[International Earth Rotation Service]] (IERS) frame [[International Terrestrial Reference System|ITRF]] 94. It was then formally called WGS{{nbsp}}84 (G873). WGS{{nbsp}}84 (G873) was adopted as the reference frame for broadcast orbits on January 29, 1997.<ref>{{cite web|url=http://www.ngs.noaa.gov/faq.shtml#WGS84|website=National Geodetic Survey |title=Frequently Asked Questions |url-status=live|archive-url=https://web.archive.org/web/20111019024226/https://www.ngs.noaa.gov/faq.shtml#WGS84|archive-date=2011-10-19}}</ref> Another update brought it to WGS{{nbsp}}84 (G1674).

The WGS{{nbsp}}84 datum, within two meters of the NAD{{nbsp}}83 datum used in North America, is the only world referencing system in place today. WGS{{nbsp}}84 is the default standard datum for coordinates stored in recreational and commercial GPS units.

Users of GPS are cautioned that they must always check the datum of the maps they are using. To correctly enter, display, and to store map related map coordinates, the datum of the map must be entered into the GPS map datum field.

=== Examples ===
Examples of map datums are:
* [[WGS 84]], 72, 66 and 60 of the [[World Geodetic System]]
* [[NAD 83]], the [[North American Datum]] which is very similar to WGS{{nbsp}}84
* [[North American Datum#North American Datum of 1927|NAD 27]], the older [[North American Datum]], of which NAD{{nbsp}}83 was basically a readjustment [http://www.ngs.noaa.gov/TOOLS/Nadcon/Nadcon.html]
* [[OSGB36]] of the [[Ordnance Survey]] of [[Great Britain]]
* [[European Terrestrial Reference System 1989|ETRS89]], the European Datum, related to [[International Terrestrial Reference System and Frame|ITRS]]
* [[ED50]], the older European Datum
* [[GDA94]], the Australian Datum<ref>{{cite web |last=Craven |first=Alex |title=GDA94 : Frequently Asked Questions |url=http://www.geoproject.com.au/gda.faq.html |url-status=live |archive-url=https://web.archive.org/web/20160815023907/http://www.geoproject.com.au/gda.faq.html |archive-date=2016-08-15 |website=Geoproject Solutions }}</ref>
* [[JGD2011]], the Japanese Datum, adjusted for changes caused by [[2011 Tōhoku earthquake and tsunami]]<ref>{{cite web|url=http://club.informatix.co.jp/?p=998|title=日本測地系2011（JGD2011）とは？ - 空間情報クラブ|date=2015-08-20|website=club.informatix.co.jp|url-status=live|archive-url=https://web.archive.org/web/20160820032137/http://club.informatix.co.jp/?p=998|archive-date=2016-08-20}}</ref>
* [[Tokyo97]], the older Japanese Datum<ref>{{cite web|url=http://www.gsi.go.jp/sokuchikijun/tky2jgd.html|title=座標変換ソフトウェア TKY2JGD｜国土地理院|website=www.gsi.go.jp|url-status=live|archive-url=https://web.archive.org/web/20171105201418/http://www.gsi.go.jp/sokuchikijun/tky2jgd.html|archive-date=2017-11-05}}</ref>
* [[KGD2002]], the Korean Datum<ref>{{cite journal | bibcode = 2007AGUSM.G33B..03Y | title=The Korean Datum Change to a World Geodetic System | journal=AGU Spring Meeting Abstracts | year=2007 |author1=Yang, H. |author2=Lee, Y. |author3=Choi, Y. |author4=Kwon, J. |author5=Lee, H. |author6=Jeong, K. | volume=2007 | pages=G33B–03 }}</ref>
* [[TWD67]] and [[TWD97]], different datum currently used in Taiwan.<ref>{{cite web|url=http://www.sunriver.com.tw/grid_tm2.htm|title=大地座標系統與二度分帶座標解讀 - 上河文化|last=台灣地圖夢想家-SunRiver|website=www.sunriver.com.tw|url-status=live|archive-url=https://web.archive.org/web/20160820192537/http://www.sunriver.com.tw/grid_tm2.htm|archive-date=2016-08-20}}</ref>
* [[BJS54]] and [[XAS80]], old geodetic datum used in China<ref>[http://www.chinaminingmagazine.com/ch/reader/view_abstract.aspx?file_no=20141431&flag=1 Analysis of Conversion Method and Map Merging from BJS54 XA80 Surveying and Mapping Results to CGCS2000] {{webarchive|url=https://web.archive.org/web/20160918005031/http://www.chinaminingmagazine.com/ch/reader/view_abstract.aspx?file_no=20141431&flag=1 |date=2016-09-18 }}</ref>
* [[Restrictions on geographic data in China#GCJ-02|GCJ-02]] and [[Baidu Maps#Coordinate system|BD-09]], Chinese encrypted geodetic datum.
* [[PZ-90|PZ-90.11]], the current geodetic reference used by [[GLONASS]]<ref>{{cite web|url=http://www.glonass-iac.ru/en/content/news/?ELEMENT_ID=721|title=The transition to using the terrestrial geocentric coordinate system "Parametry Zemli 1990" (PZ-90.11) in operating the GLObal NAvigation Satellite System (GLONASS) has been implemented|website=www.glonass-iac.ru|url-status=live|archive-url=https://web.archive.org/web/20150907014714/https://www.glonass-iac.ru/en/content/news/?ELEMENT_ID=721|archive-date=2015-09-07}}</ref>
* [[GTRF|Galileo Terrestrial Reference Frame (GTRF)]], the geodetic reference used by [[Galileo (satellite navigation)|Galileo]]; currently defined as ITRF2005<ref name=GNSS>{{cite web|url=http://www.unoosa.org/documents/pdf/icg/activities/2008/icg3/45.pdf|title=Use of international references for GNSS operations and applications|website=unoosa.org|url-status=live|archive-url=https://web.archive.org/web/20171222052312/http://www.unoosa.org/documents/pdf/icg/activities/2008/icg3/45.pdf|archive-date=2017-12-22}}</ref>
* [[CGCS2000]], or [[CGS-2000]], the geodetic reference used by [[BeiDou Navigation Satellite System]]; based on ITRF97<ref name=GNSS /><ref>Handbook of Satellite Orbits: From Kepler to GPS, Table 14.2</ref><ref>[http://www.beidou.gov.cn/attach/2013/12/26/20131226b8a6182fa73a4ab3a5f107f762283712.pdf BeiDou Navigation Satellite System Signal In Space Interface Control Document, Open Service Signal (Version 2.0)] {{webarchive|url=https://web.archive.org/web/20160708094247/http://www.beidou.gov.cn/attach/2013/12/26/20131226b8a6182fa73a4ab3a5f107f762283712.pdf |date=2016-07-08 }} section 3.2</ref>
* [[International Terrestrial Reference Frame]]s (ITRF88, 89, 90, 91, 92, 93, 94, 96, 97, 2000, 2005, 2008, 2014), different realizations of the [[International Terrestrial Reference System|ITRS]].<ref>{{cite web |url=http://www.quickclose.com.au/stanaway07pres.pdf |title=Archived copy |access-date=2016-08-19 |url-status=live |archive-url=https://web.archive.org/web/20170126050914/http://www.quickclose.com.au/stanaway07pres.pdf |archive-date=2017-01-26 }}</ref><ref>{{cite web|url=http://itrf.ensg.ign.fr/general.php|title=General concepts|website=itrf.ensg.ign.fr|url-status=live|archive-url=https://web.archive.org/web/20081204020715/http://itrf.ensg.ign.fr/general.php|archive-date=2008-12-04}}</ref>
* [[Hong Kong Principal Datum]], a vertical datum used in Hong Kong.<ref>{{cite web|url=  http://georepository.com/datum_5135/Hong-Kong-Principal-Datum.html|title=  Vertical Datum used in China – Hong Kong – onshore|url-status=  live|archive-url=  https://web.archive.org/web/20121113103618/http://georepository.com/datum_5135/Hong-Kong-Principal-Datum.html|archive-date=  2012-11-13}}</ref><ref>{{cite web|url=http://www.geodetic.gov.hk/data/pdf/explanatorynotes.pdf|title=Explanatory Notes on Geodetic Datums in Hong Kong|website=geodetic.gov.hk|url-status=dead|archive-url=https://web.archive.org/web/20161109121152/http://www.geodetic.gov.hk/data/pdf/explanatorynotes.pdf|archive-date=2016-11-09|access-date=2016-08-19}}</ref>
* [[SAD69]] - South American Datum 1969