Editing Geographic coordinate system (section)

== Geodetic datum ==
{{Main|Geodetic datum}}
{{further|Figure of the Earth|Reference ellipsoid|Geographic coordinate conversion|Spatial reference system}}

In order to use the theoretical definitions of latitude, longitude, and height to precisely measure actual locations on the physical earth, a ''[[geodetic datum]]'' must be used. A ''horizonal datum'' is used to precisely measure latitude and longitude, while a ''[[vertical datum]]'' is used to measure elevation or altitude. Both types of datum bind a mathematical model of the shape of the earth (usually a [[reference ellipsoid]] for a horizontal datum, and a more precise [[geoid]] for a vertical datum) to the earth. Traditionally, this binding was created by a network of [[geodetic control network|control points]], surveyed locations at which monuments are installed, and were only accurate for a region of the surface of the Earth. Newer datums are based on a global network for satellite measurements ([[Satellite navigation|GNSS]], [[Very-long-baseline interferometry|VLBI]], [[Satellite laser ranging|SLR]] and [[DORIS (satellite system)|DORIS]]).

This combination of a mathematical model and physical binding ensures that users of the same datum obtain identical coordinates for a given physical point. However, different datums typically produce different coordinates for the same location (sometimes deviating several hundred meters) not due to actual movement, but because the reference system itself is shifted. Because any [[spatial reference system]] or [[map projection]] is ultimately calculated from latitude and longitude, it is crucial that they clearly state the datum on which they are based. For example, a [[Universal transverse mercator|UTM]] coordinate based on a [[WGS84]] realisation will be different than a UTM coordinate based on [[NAD27]] for the same location. Transforming coordinates from one datum to another requires a [[Geographic coordinate conversion#Datum transformations|datum transformation]] method such as a [[Helmert transformation]], although in certain situations a simple [[Translation (geometry)|translation]] may be sufficient.<ref name=Irish>{{cite web |url = http://www.osi.ie/GetAttachment.aspx?id=25113681-c086-485a-b113-bab7c75de6fa |title=Making maps compatible with GPS |publisher=Government of Ireland 1999 |access-date=15 April 2008 |archive-url = https://web.archive.org/web/20110721130505/http://www.osi.ie/GetAttachment.aspx?id=25113681-c086-485a-b113-bab7c75de6fa |archive-date=21 July 2011 |url-status=dead }}</ref>

Datums may be global, meaning that they represent the whole Earth, or they may be regional,<ref>{{cite web | publisher = Ordnance Survey | title = A guide to the coordinate systems in Great Britain | url = https://docs.os.uk/os-downloads/resources/a-guide-to-coordinate-systems-in-great-britain/the-shape-of-the-earth }}</ref> meaning that they represent an ellipsoid best-fit to only a portion of the Earth. Examples of global datums include the several realizations of [[WGS 84]] (with the 2D datum ensemble EPSG:4326 with 2 meter accuracy as identifier)<ref>{{Cite web|url=https://spatialreference.org/ref/epsg/4326/|title=WGS 84: EPSG Projection -- Spatial Reference|website=spatialreference.org|access-date=5 May 2020|archive-date=13 May 2020|archive-url=https://web.archive.org/web/20200513113544/https://spatialreference.org/ref/epsg/4326/|url-status=live}}</ref><ref>[https://epsg.org/crs_4326/WGS-84.html EPSG:4326]</ref> used for the [[Global Positioning System]],{{NoteTag|WGS 84 is the default datum used in most GPS equipment, but other datums and map projections can be selected.}} and the several realizations of the [[International Terrestrial Reference System and Frame]] (such as ITRF2020 with subcentimeter accuracy), which takes into account [[continental drift]] and [[crustal deformation]].<ref name=Bolstad>{{cite book |last=Bolstad |first=Paul |title=GIS Fundamentals |year=2012 |edition=5th |publisher=Atlas books |isbn=978-0-9717647-3-6 |page=102 |url=http://www.paulbolstad.net/5thedition/samplechaps/Chapter3_5th_small.pdf |access-date=27 January 2018 |archive-date=15 October 2020 |archive-url=https://web.archive.org/web/20201015162738/http://www.paulbolstad.net/5thedition/samplechaps/Chapter3_5th_small.pdf |url-status=dead }}</ref>

Datums with a regional fit of the ellipsoid that are chosen by a national cartographical organization include the [[North American Datum]]s, the European [[ED50]], and the British [[OSGB36]]. Given a location, the datum provides the latitude <math>\phi</math> and longitude <math>\lambda</math>. In the United Kingdom there are three common latitude, longitude, and height systems in use. WGS{{nbsp}}84 differs at Greenwich from the one used on published maps OSGB36 by approximately 112{{nbsp}}m. ED50 differs from about 120{{nbsp}}m to 180{{nbsp}}m.<ref name=OSGB/>

Points on the Earth's surface move relative to each other due to continental plate motion, subsidence, and diurnal [[Earth tide|Earth tidal]] movement caused by the [[Moon]] and the Sun. This daily movement can be as much as a meter. Continental movement can be up to {{nowrap|10 cm}} a year, or {{nowrap|10 m}} in a century. A [[weather system]] high-pressure area can cause a sinking of {{nowrap|5 mm}}. [[Scandinavia]] is rising by {{nowrap|1 cm}} a year as a result of the melting of the ice sheets of the [[quaternary glaciation|last ice age]], but neighboring [[Scotland]] is rising by only {{nowrap|0.2 cm}}. These changes are insignificant if a regional datum is used, but are statistically significant if a global datum is used.<ref name="OSGB">{{Citation |title=A guide to coordinate systems in Great Britain |date=2020 |series=D00659 v3.6 |access-date=17 December 2021|publisher=Ordnance Survey |url=https://www.ordnancesurvey.co.uk/documents/resources/guide-coordinate-systems-great-britain.pdf |archive-url=https://web.archive.org/web/20200402024515/http://www.ordnancesurvey.co.uk/documents/resources/guide-coordinate-systems-great-britain.pdf |archive-date=2020-04-02 |url-status=live }}</ref>