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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. Some newer datums are bound to the [[center of mass]] of the Earth.
This combination of mathematical model and physical binding mean that anyone using the same datum will obtain the same location measurement for the same physical location. However, two different datums will usually yield different location measurements for the same physical location, which may appear to differ by as much as several hundred meters; this not because the location has moved, but because the reference system used to measure it has 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 [[WGS84]] will be different than a UTM coordinate based on [[NAD27]] for the same location. Converting coordinates from one datum to another requires a [[Geographic coordinate conversion#Datum transformations|datum transformation]] 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 local, meaning that they represent an ellipsoid best-fit to only a portion of the Earth. Examples of global datums include [[World Geodetic System]] (WGS{{nbsp}}84, also known as EPSG:4326<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>), the default datum used for the [[Global Positioning System]],{{NoteTag|WGS 84 is the default datum used in most GPS equipment, but other datums can be selected.}} and the [[International Terrestrial Reference System and Frame]] (ITRF), used for estimating [[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> The distance to Earth's center can be used both for very deep positions and for positions in space.<ref name="OSGB"/>
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