Spatial reference system
Geodesy 

A spatial reference system (SRS) or coordinate reference system (CRS) is a coordinatebased local, regional or global system used to locate geographical entities. A spatial reference system defines a specific map projection, as well as transformations between different spatial reference systems. Spatial reference systems are defined by the OGC's Simple Feature Access using wellknown text representation of coordinate reference systems, and support has been implemented by several standardsbased geographic information systems. Spatial reference systems can be referred to using a SRID integer, including EPSG codes defined by the International Association of Oil and Gas Producers. It is specified in ISO 19111:2007 Geographic information—Spatial referencing by coordinates, prepared by ISO/TC 211, also published as OGC Abstract Specification, Topic 2: Spatial referencing by coordinate.^{[1]}
Components
In this Abstract Specification,^{[1]} a coordinate reference system shall be composed of one coordinate system and one datum.
A coordinate system is a set of mathematical rules for specifying how coordinates are to be assigned to points, such as: affine, cylindrical, Cartesian, ellipsoidal, linear, polar, spherical, vertical, etc.
A datum is a set of parameters that define the position of the origin, the scale, and the orientation of a coordinate system.
The main subtypes of coordinate reference system are: geodetic, vertical, engineering, and image; additional subtypes are: derived, projected, and compound.
Examples
Some systems are:
 Universal Transverse Mercator coordinate system
 British national grid reference system
 Chinese Global Navigation Grid Code
 Hellenic Geodetic Reference System 1987
 Irish grid reference system
 Irish Transverse Mercator
 Israeli Transverse Mercator
 Israeli Cassini Soldner
 Jordan Transverse Mercator
 Lambert conformal conic projection
 International mapcode system
 Military Grid Reference System
 United States National Grid
Identifiers
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 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 (OGC) spatial_ref_sys metadata table for the Simple Features for SQL Specification, Versions 1.1 and 1.2, which is defined as follows:
CREATE TABLE SPATIAL_REF_SYS
(
SRID INTEGER NOT NULL PRIMARY KEY,
AUTH_NAME CHARACTER VARYING(256),
AUTH_SRID INTEGER,
SRTEXT CHARACTER VARYING(2048)
)
In spatially enabled databases (such as IBM DB2, IBM Informix, Ingres, Microsoft SQL Server, 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 wellknown 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:
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]]
WGS84 — SRID 4326
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"]]
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
 Georeferencing
 Geographic coordinate systems
 Geographic information system (GIS).
 Grid reference
 List of National Coordinate Reference Systems
References
 ↑ ^{1.0} ^{1.1} "Abstract Specifications  OGC". www.opengeospatial.org. Retrieved 20181225.
External links
Wikidata has the property:

 spatialreference.org – A website that defines spatial reference systems, in a variety of formats.
 OpenGIS Specifications (Standards)
 OpenGIS Simple Features Specification for CORBA (99054)
 OpenGIS Simple Features Specification for OLE/COM (99050)
 OpenGIS Simple Features Specification for SQL (99054, 05134, 06104r3)
 OGR — library implementing relevant OGC standards
 EPSG Geodetic Parameter Registry  search engine for EPSG defined reference systems
 EPSG.io/  Full text search indexing over 6000 coordinate systems
 Galdos Systems INdicio CRS Registry