Spatial reference system

Aspatial reference system(SRS) orcoordinate 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 ofcoordinate systemsandanalytic geometryto geographic space. A particular SRS specification (for example, "Universal Transverse MercatorWGS 84Zone 16N ") comprises a choice ofEarth ellipsoid,horizontal datum,map projection(except in thegeographic 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, necessitatingtransformationsbetween different SRS.

Although they date to the Hellenic Period, spatial reference systems are now a crucial basis for the sciences and technologies ofGeoinformatics,includingcartography,geographic information systems,surveying,remote sensing,andcivil engineering.This has led to their standardization in international specifications such as theEPSG codesandISO 19111:2019 Geographic information—Spatial referencing by coordinates,prepared byISO/TC 211,also published by theOpen Geospatial ConsortiumasAbstract Specification, Topic 2: Spatial referencing by coordinate.

Types of systems

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:

Geographic coordinate system(or geodetic)

Aspherical coordinate systemmeasuring locations directly on the Earth (modeled as asphereorellipsoid) usinglatitude(degrees north or south of theequator) andlongitude(degrees west or east of aprime meridian).

Geocentric coordinate system(or Earth-centered Earth-fixed)

A three-dimensionalcartesian coordinate systemthat models the Earth as a three-dimensional object, measuring locations from a center point, usually thecenter of massof the Earth, along x, y, and z axes aligned with theequatorand theprime meridian.This system is commonly used to track the orbits ofsatellites,because they are based on the center of mass. Thus, this is the internal coordinate system used bySatellite navigationsystems such asGPSto compute locations usingmultilateration.

Projected coordinate system(or planar, grid)

Layout of a UTM coordinate system.

A standardizedcartesian coordinate systemthat 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 particularMap projectionto 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 theUniversal transverse mercator(UTM) and national systems such as theBritish National Grid,andState Plane Coordinate System(SPCS).

Engineering coordinate system (or local, custom)

Acartesian 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 usingsurveyingtechniques. These may or may not be aligned with a standard projected coordinate system.Local tangent plane coordinatesare a type of local coordinate system used in aviation and marine vehicles.

Vertical reference frame

a standard reference system for measuringelevationusingvertical datums,based onlevelling,ageoidmodel, or achart datum(consideringtides).

These standards acknowledge that standard reference systems also exist fortime(e.g.ISO 8601). These may be combined with a spatial reference system to form acompound coordinate systemfor 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 araster image,Linear referencingmeasurements 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.To accomplish this, any coordinate reference system definition needs to be composed of several specifications:

• Acoordinate 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.
• Ageodetic 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 measuringgeographic coordinates(latitude and longitude). Examples include theWorld Geodetic Systemand the 1927 and 1983North American Datum.A datum generally consists of an estimate of the shape of the Earth (usually an ellipsoid), and one or moreanchor pointsorcontrol 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 ofmap projectionto 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:

EPSG Code	Name	Ellipsoid	Horizontal Datum	CS Type	Projection	Origin	Axes	Unit of Measure
4326	GCSWGS 84	GRS 80	WGS 84	ellipsoidal (lat, lon)	N/A	equator/prime meridian	equator, prime meridian	degree of arc
26717	UTMZone 17N NAD 27	Clarke 1866	NAD 27	cartesian (x,y)	Transverse Mercator: central meridian 81°W, scaled 0.9996	500 km west of (81°W, 0°N)	equator, 81°W meridian	meter
6576	SPCSTennessee Zone NAD 83 (2011) ftUS	GRS 80	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 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

• 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),the official projection inMetropolitan France
• Hellenic Geodetic Reference System 1987,Greece
• Irish grid reference system,Ireland
• Irish Transverse Mercator,Ireland
• SWEREF 99(sv),Sweden

North America

• United States National Grid,US

Worldwide

• Universal Transverse Mercator coordinate system
• Lambert conformal conic projection
• International mapcode system
• Military Grid Reference System

Identifiers

"SRID" redirects here. For the polyhedron, seeRhombicosidodecahedron.

ASpatial 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 allGISapplications.

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

SRIDs are the primary key for theOpen Geospatial Consortium (OGC)spatial_ref_sysmetadata table for theSimple Features for SQL Specification, Versions 1.1 and 1.2,which is defined as follows:

CREATETABLESPATIAL_REF_SYS
(
SRIDINTEGERNOTNULLPRIMARYKEY,
AUTH_NAMECHARACTERVARYING(256),
AUTH_SRIDINTEGER,
SRTEXTCHARACTERVARYING(2048)
)

In spatially enabled databases (such asIBM Db2,IBM Informix,Ingres,Microsoft SQL Server,MonetDB,MySQL,Oracle RDBMS,Teradata,PostGIS,SQL AnywhereandVertica), 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 awell-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:

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
• Geodetic datum
• Georeferencing
• Geographic coordinate systems
• Geographic information system(GIS).
• Grid reference
• Linear referencing
• List of National Coordinate Reference Systems
• Terms of orientation

External links

• spatialreference.org– A website with more than 13000 spatial reference systems, in a variety of formats.
• OpenGIS Specifications (Standards)Archived2004-12-13 at theWayback Machine
• OpenGIS Simple Features Specification for CORBA (99-054)
• OpenGIS Simple Features Specification for OLE/COM (99-050)
• OpenGIS Simple Features Specification for SQL (99-054, 05-134, 06-104r3)
• OGRArchived2006-04-22 at theWayback Machine— library implementing relevant OGC standards
• EPSG.org- Official EPSG Geodetic Parameter Dataset webpage. Search engine for EPSG defined reference systems.
• EPSG.io/- Full text search indexing over 6000 coordinate systems
• Galdos Systems INdicio CRS Registry