View source for Spatial reference system

{{Short description|System to specify locations on Earth}}
{{Geodesy}}

A '''spatial reference system''' ('''SRS''') or '''coordinate reference system''' ('''CRS''') is a framework used to precisely measure locations on the surface of Earth as coordinates. It is thus the application of the abstract mathematics of [[coordinate system]]s and [[analytic geometry]] to geographic space. A particular SRS specification (for example, "[[Universal Transverse Mercator]] [[WGS 84]] Zone 16N") comprises a choice of [[Earth ellipsoid]], [[Geodetic datum|horizontal datum]], [[map projection]] (except in the [[geographic coordinate system]]), origin point, and unit of measure. Thousands of coordinate systems have been specified for use around the world or in specific regions and for various purposes, necessitating [[geographic coordinate conversion|transformations]] between different SRS.

Although they date to the [[Hellenistic period]], spatial reference systems are now a crucial basis for the sciences and technologies of [[Geoinformatics]], including [[cartography]], [[geographic information systems]], [[surveying]], [[remote sensing]], and [[civil engineering]]. This has led to their standardization in international specifications such as the [[EPSG code]]s<ref name="epsg">{{cite web |title=Using the EPSG geodetic parameter dataset, Guidance Note 7-1 |url=https://epsg.org/guidance-notes.html |website=EPSG Geodetic Parameter Dataset |publisher=Geomatic Solutions |access-date=15 December 2021 |archive-date=15 December 2021 |archive-url=https://web.archive.org/web/20211215215824/https://epsg.org/guidance-notes.html |url-status=live }}</ref> and ''ISO 19111:2019 Geographic information—Spatial referencing by coordinates'', prepared by [[ISO/TC 211]], also published by the [[Open Geospatial Consortium]] as ''Abstract Specification, Topic 2: Spatial referencing by coordinate''.<ref name="ogc as">{{Cite web|url=https://docs.ogc.org/as/18-005r5/18-005r5.html|title=OGC Abstract Specification Topic 2: Referencing by coordinates Corrigendum|website=Open Geospatial Consortium|access-date=2018-12-25|archive-date=2021-07-30|archive-url=https://web.archive.org/web/20210730043924/http://docs.ogc.org/as/18-005r5/18-005r5.html|url-status=live}}</ref>

==Types of systems==
[[File:ECEF.svg|alt=Earth Centered, Earth Fixed coordinates|thumb|upright=1.3|Earth centered, Earth fixed coordinates in relation to latitude and longitude.]]
The thousands of spatial reference systems used today are based on a few general strategies, which have been defined in the EPSG, ISO, and OGC standards:<ref name="epsg" /><ref name="ogc as" />
;[[Geographic coordinate system]] (or geodetic)
:A [[spherical coordinate system]] measuring locations directly on the Earth (modeled as a [[sphere]] or [[Geographic ellipsoid|ellipsoid]]) using [[latitude]] (degrees north or south of the [[equator]]) and [[longitude]] (degrees west or east of a [[prime meridian]]).
;[[Geocentric coordinate system]] (or Earth-centered Earth-fixed) 
:A three-dimensional [[cartesian coordinate system]] that models the Earth as a three-dimensional object, measuring locations from a center point, usually the [[center of mass]] of the Earth, along x, y, and z axes aligned with the [[equator]] and the [[prime meridian]]. This system is commonly used to track the orbits of [[satellite]]s, because they are based on the center of mass. Thus, this is the internal coordinate system used by [[Satellite navigation]] systems such as [[Global Positioning System|GPS]] to compute locations using [[True-range multilateration|multilateration]].
;[[Projected coordinate system]] (or planar, grid) [[File:Utm-latlon grid en.svg|thumb|Layout of a UTM coordinate system.]]
:A standardized [[cartesian coordinate system]] that models the Earth (or more commonly, a large region thereof) as a plane, measuring locations from an arbitrary origin point along x and y axes more or less aligned with the cardinal directions. Each of these systems is based on a particular [[Map projection]] to create a planar surface from the curved Earth surface. These are generally defined and used strategically to minimize the distortions inherent to projections. Common examples include the [[Universal transverse mercator]] (UTM) and national systems such as the [[Ordnance Survey National Grid|British National Grid]], and [[State Plane Coordinate System]] (SPCS).
;Engineering coordinate system (or local, custom)
:A [[cartesian coordinate system]] (2-D or 3-D) that is created bespoke for a small area, often a single engineering project, over which the curvature of the Earth can be safely approximated as flat without significant distortion. Locations are typically measured directly from an arbitrary origin point using [[surveying]] techniques. These may or may not be aligned with a standard projected coordinate system. [[Local tangent plane coordinates]] are a type of local coordinate system used in aviation and marine vehicles.
;Vertical reference frame: a standard reference system for measuring [[elevation]] using [[vertical datum]]s, based on [[levelling]], a [[geoid]] model, or a [[chart datum]] (considering [[tides]]).

These standards acknowledge that standard reference systems also exist for [[Calendar date|time]] (e.g. [[ISO 8601]]). These may be combined with a spatial reference system to form a ''compound coordinate system'' for representing three-dimensional and/or spatio-temporal locations. There are also internal systems for measuring location within the context of an object, such as the rows and columns of pixels in a [[Raster graphics|raster image]], [[Linear referencing]] measurements along linear features (e.g., highway mileposts), and systems for specifying location within moving objects such as ships. The latter two are often classified as subcategories of engineering coordinate systems.

==Components==
The goal of any spatial reference system is to create a common reference frame in which locations can be measured precisely and consistently as coordinates, which can then be shared unambiguously, so that any recipient can identify the same location that was originally intended by the originator.<ref name=OSGB>{{Citation |title=A guide to coordinate systems in Great Britain |date=2020 |page=11|series=D00659 v2.3 |access-date=2021-12-16 |publisher=Ordnance Survey |url=https://www.ordnancesurvey.co.uk/documents/resources/guide-coordinate-systems-great-britain.pdf |archive-url=https://web.archive.org/web/20150924061607/http://www.ordnancesurvey.co.uk/docs/support/guide-coordinate-systems-great-britain.pdf |archive-date=24 September 2015 |url-status=dead }}</ref> To accomplish this, any coordinate reference system definition needs to be composed of several specifications:
* A [[coordinate system]], an abstract framework for measuring locations. Like any mathematical coordinate system, its definition consists of a measurable space (whether a plane, a three-dimension void, or the surface of an object such as the Earth), an origin point, a set of axis vectors emanating from the origin, and a unit of measure.
* A [[geodetic datum]] (horizontal, vertical, or three-dimensional) which binds the abstract coordinate system to the real space of the Earth. A horizontal datum can be defined as a precise reference framework for measuring [[Geographic coordinate system|geographic coordinates]] (latitude and longitude). Examples include the [[World Geodetic System]] and the 1927 and 1983 [[North American Datum]]. A datum generally consists of an estimate of the shape of the Earth (usually an ellipsoid), and one or more ''anchor points'' or ''control points'', established locations (often marked by physical monuments) for which the measurement is documented.
* A definition for a projected CRS must also include a choice of [[map projection]] to convert the spherical coordinates specified by the datum into cartesian coordinates on a planar surface.
 
Thus, a CRS definition will typically consist of a "stack" of dependent specifications, as exemplified in the following table:
{| class="wikitable"
|-
! EPSG Code !! Name !! Ellipsoid !! Horizontal Datum !! CS Type!! Projection  !! Origin !! Axes !! Unit of Measure
|-
| [https://epsg.org/crs_4326/WGS-84.html 4326] || [[Geographic coordinate system|GCS]] WGS 84 || [[Geodetic Reference System 1980|GRS 80]] || [[World Geodetic System|WGS 84]] || ellipsoidal (lat, lon)|| N/A  || equator/prime meridian || equator, prime meridian || degree of arc
|-
| [https://epsg.org/crs_26717/NAD27-UTM-zone-17N.html 26717] || [[Universal transverse mercator|UTM]] Zone 17N NAD 27 || Clarke 1866 || [[North American Datum|NAD 27]] || cartesian (x,y)|| Transverse Mercator: central meridian 81°W, scaled 0.9996  || 500&nbsp;km west of (81°W, 0°N) || equator, 81°W meridian || meter
|-
| [https://epsg.org/crs_6576/NAD83-2011-Tennessee-ftUS.html 6576] || [[State plane coordinate system|SPCS]] Tennessee Zone NAD 83 (2011) ftUS || [[Geodetic Reference System 1980|GRS 80]] || [[North American Datum|NAD 83]] (2011 epoch) || cartesian (x,y)|| Lambert Conformal Conic: center 86°W, 34°20'N, standard parallels 35°15'N, 36°25'N  || 600&nbsp;km grid west of center point || grid east at center point, 86°W meridian || US survey foot
|}

==Examples by continent==
Examples of systems around the world are:

===Asia===
*[[Global Navigation Grid Code|Chinese Global Navigation Grid Code]], China
*[[Israeli Cassini Soldner]], Israel
*[[Israeli Transverse Mercator]], Israel
*[[Jordan Transverse Mercator]], Jordan

===Europe===
*[[British national grid reference system]], Britain
* Lambert-93 [[:fr:Projection conique conforme de Lambert|(fr)]], the official projection in [[Metropolitan France]]
*[[Hellenic Geodetic Reference System 1987]], Greece
*[[Irish grid reference system]], Ireland
*[[Irish Transverse Mercator]], Ireland
* SWEREF 99 [[:sv:SWEREF 99|(sv)]], Sweden

===North America===
*[[United States National Grid]], US

===Worldwide===
*[[Universal Transverse Mercator coordinate system]]
*[[Lambert conformal conic projection]]
*[[MapCode|International mapcode system]]
*[[Military Grid Reference System]]

==Identifiers{{anchor|Identifier}}==
{{Redirect|SRID|the polyhedron|Rhombicosidodecahedron}}

A '''Spatial Reference System Identifier''' ('''SRID''') is a unique value used to unambiguously identify projected, unprojected, and local spatial coordinate system definitions. These coordinate systems form the heart of all [[Geographic information system|GIS]] applications.

Virtually all major spatial vendors have created their own SRID implementation or refer to those of an authority, such as the [[EPSG Geodetic Parameter Dataset]].

SRIDs are the primary key for the [[Open Geospatial Consortium|Open Geospatial Consortium (OGC)]] '''spatial_ref_sys''' metadata table for the [[Simple Features|Simple Features for SQL Specification, Versions 1.1 and 1.2]],  which is defined as follows:
<syntaxhighlight lang=sql>
CREATE TABLE SPATIAL_REF_SYS
(
    SRID      INTEGER   NOT NULL PRIMARY KEY,
    AUTH_NAME CHARACTER VARYING(256),
    AUTH_SRID INTEGER,
    SRTEXT    CHARACTER VARYING(2048)
)
</syntaxhighlight>
In spatially enabled databases (such as [[IBM Db2]], [[IBM Informix]], [[Ingres (database)|Ingres]], [[Microsoft SQL Server]], [[MonetDB]], [[MySQL]], [[Oracle RDBMS]], [[Teradata]], [[PostGIS]], [[SQL Anywhere]] and [[Vertica]]), SRIDs are used to uniquely identify the coordinate systems used to define columns of spatial data or individual spatial objects in a spatial column (depending on the spatial implementation).  SRIDs are typically associated with a [[Well-known text representation of coordinate reference systems|well-known text]] (WKT) string definition of the coordinate system (SRTEXT, above).
Here are two common coordinate systems with their EPSG SRID value followed by their WKT:

UTM, Zone 17N, NAD27 — SRID 2029:
<syntaxhighlight lang="javascript">
PROJCS["NAD27(76) / UTM zone 17N",
    GEOGCS["NAD27(76)",
        DATUM["North_American_Datum_1927_1976",
            SPHEROID["Clarke 1866",6378206.4,294.9786982138982,
                AUTHORITY["EPSG","7008"]],
            AUTHORITY["EPSG","6608"]],
        PRIMEM["Greenwich",0,
            AUTHORITY["EPSG","8901"]],
        UNIT["degree",0.01745329251994328,
            AUTHORITY["EPSG","9122"]],
        AUTHORITY["EPSG","4608"]],
    UNIT["metre",1,
        AUTHORITY["EPSG","9001"]],
    PROJECTION["Transverse_Mercator"],
    PARAMETER["latitude_of_origin",0],
    PARAMETER["central_meridian",-81],
    PARAMETER["scale_factor",0.9996],
    PARAMETER["false_easting",500000],
    PARAMETER["false_northing",0],
    AUTHORITY["EPSG","2029"],
    AXIS["Easting",EAST],
    AXIS["Northing",NORTH]]
</syntaxhighlight>

[[WGS84]] — SRID 4326  
<syntaxhighlight lang=javascript>
GEOGCS["WGS 84",
    DATUM["WGS_1984",
        SPHEROID["WGS 84",6378137,298.257223563,
            AUTHORITY["EPSG","7030"]],
        AUTHORITY["EPSG","6326"]],
    PRIMEM["Greenwich",0,
        AUTHORITY["EPSG","8901"]],
    UNIT["degree",0.01745329251994328,
        AUTHORITY["EPSG","9122"]],
    AUTHORITY["EPSG","4326"]]
</syntaxhighlight>

SRID values associated with spatial data can be used to constrain spatial operations — for instance, spatial operations cannot be performed between spatial objects with differing SRIDs in some systems, or trigger coordinate system transformations between spatial objects in others.

==See also==
* [[Engineering datum]]
* [[Geodesy]]
* [[Geodetic datum]]
* [[Georeferencing]]
* [[Geographic coordinate systems]]
* [[Geographic information system]] ([[Geographic information system|GIS]]).
* [[Grid reference]]
* [[Linear referencing]]
* [[List of National Coordinate Reference Systems]]
* [[Terms of orientation]]

==References==
{{Reflist}}

==External links==
{{Wikidata property|P3037}}

* [https://spatialreference.org/ spatialreference.org] – A website with more than 13000 spatial reference systems, in a variety of formats.
* [http://www.opengeospatial.org/specs/?page=specs OpenGIS Specifications (Standards)] {{Webarchive|url=https://web.archive.org/web/20041213090611/http://www.opengeospatial.org/specs/?page=specs |date=2004-12-13 }}
* [http://www.opengeospatial.org/standards/sfc OpenGIS Simple Features Specification for CORBA (99-054)]
* [http://www.opengeospatial.org/standards/sfo OpenGIS Simple Features Specification for OLE/COM (99-050)]
* [http://www.opengeospatial.org/standards/sfs OpenGIS Simple Features Specification for SQL (99-054, 05-134, 06-104r3)]
* [http://www.gdal.org/ogr/ OGR] {{Webarchive|url=https://web.archive.org/web/20060422001312/http://www.gdal.org/ogr/ |date=2006-04-22 }} — library implementing relevant OGC standards
* [https://epsg.org/ EPSG.org] - Official EPSG Geodetic Parameter Dataset webpage. Search engine for EPSG defined reference systems.
* [http://www.epsg.io/ EPSG.io/] - Full text search indexing over 6000 coordinate systems
* [http://www.galdosinc.com/products/indicio-crs-registry Galdos Systems INdicio CRS Registry]

{{OGC}}
{{ISO standards}}

{{DEFAULTSORT:Spatial Referencing System}}
[[Category:Geographic coordinate systems]]
[[Category:Geographic information systems]]
[[Category:Geodesy]]
[[Category:ISO/TC 211]]
[[Category:Open Geospatial Consortium]]
[[Category:GIS file formats]]