Editing Geodetic datum (section)

{{short description|Reference frame for measuring location}}
[[Image:Chicago City Datum.jpg|thumb|right|City of Chicago Datum Benchmark]]
{{Geodesy}}

A '''geodetic datum''' or '''geodetic system''' (also: '''geodetic reference datum''', '''geodetic reference system''', or '''geodetic reference frame''', or '''terrestrial reference frame''') is a global [[datum reference]] or [[reference frame]] for unambiguously representing the position of [[location]]s on [[Earth]] by means of either [[geodetic coordinates]] (and related [[vertical coordinate]]s) or [[geocentric coordinates]].<ref name="jensen">{{cite book |last1=Jensen |first1=John R. |last2=Jensen |first2=Ryan R. |title=Introductory Geographic Information Systems |date=2013 |publisher=Pearson |page=25}}</ref> 
Datums<ref group="note">The plural is not "data" in this case</ref> are crucial to any technology or technique based on spatial location, including [[geodesy]], [[navigation]], [[surveying]], [[geographic information system]]s, [[remote sensing]], and [[cartography]]. 
A '''horizontal datum''' is used to measure a [[horizontal position]], across the [[Earth]]'s surface, in [[latitude]] and [[longitude]] or another related coordinate system. A ''[[vertical datum]]'' is used to measure the elevation or depth relative to a standard origin, such as mean [[sea level]] (MSL). A '''three-dimensional datum''' enables the expression of both horizontal and vertical position components in a unified form.<ref name="v707">{{cite web | title=NOAA/NOS's VDatum: A tutorial on datums | website=NOAA/NOS's VDatum 4.7 | date=2014-03-14 | url=https://vdatum.noaa.gov/docs/datums.html | access-date=2024-08-11}}</ref> 
The concept can be generalized for other celestial bodies as in ''[[planetary datum]]s''.

Since the rise of the [[global positioning system]] (GPS), the [[ellipsoid]] and datum [[World Geodetic System | WGS 84]] it uses has supplanted most others in many applications. The WGS{{nbsp}}84 is intended for global use, unlike most earlier datums.
Before GPS, there was no precise way to measure the position of a location that was far from reference points used in the realization of local datums, such as from the [[Prime Meridian]] at the [[Royal Observatory, Greenwich | Greenwich Observatory]] for longitude, from the [[Equator]] for latitude, or from the nearest coast for sea level. Astronomical and chronological methods have limited precision and accuracy, especially over long distances. Even GPS requires a predefined framework on which to base its measurements, so WGS{{nbsp}}84 essentially functions as a datum, even though it is different in some particulars from a traditional standard horizontal or vertical datum.

A standard datum specification (whether horizontal, vertical, or 3D) consists of several parts: a model for Earth's shape and dimensions, such as a ''[[reference ellipsoid]]'' or a ''[[geoid]]''; an ''origin'' at which the ellipsoid/geoid is tied to a known (often monumented) location on or inside Earth (not necessarily at 0 latitude 0 longitude); and multiple [[Geodetic control network | control points]] or reference points that have been precisely measured from the origin and physically monumented. Then the coordinates of other places are measured from the nearest control point through [[surveying]]. Because the ellipsoid or geoid differs between datums, along with their origins and orientation in space, the relationship between coordinates referred to one datum and coordinates referred to another datum is undefined and can only be approximated. Using local datums, the disparity on the ground between a point having the same horizontal coordinates in two different datums could reach kilometers if the point is far from the origin of one or both datums. This phenomenon is called ''[[datum shift]]'' or, more generally, ''datum transformation'', as it may involve rotation and scaling, in addition to displacement.

Because Earth is an imperfect ellipsoid, local datums can give a more accurate representation of some specific area of coverage than WGS{{nbsp}}84 can. [[Ordnance Survey National Grid|OSGB36]], for example, is a better approximation to the [[geoid]] covering the British Isles than the global WGS{{nbsp}}84 ellipsoid.<ref>{{Cite web|url=http://desktop.arcgis.com/en/arcmap/10.3/guide-books/map-projections/geoid.htm|title=Geoid—Help |website=ArcGIS for Desktop|access-date=2017-01-23|url-status=live|archive-url=https://web.archive.org/web/20170202054013/http://desktop.arcgis.com/en/arcmap/10.3/guide-books/map-projections/geoid.htm|archive-date=2017-02-02}}</ref> However, as the benefits of a global system often outweigh the greater accuracy, the global WGS{{nbsp}}84 datum has become widely adopted.<ref>{{Cite web|url=http://desktop.arcgis.com/en/arcmap/10.3/guide-books/map-projections/datums.htm|title=Datums—Help |website=ArcGIS for Desktop|access-date=2017-01-23|url-status=live|archive-url=https://web.archive.org/web/20170202052131/http://desktop.arcgis.com/en/arcmap/10.3/guide-books/map-projections/datums.htm|archive-date=2017-02-02}}</ref>