A spatial reference system (SRS) or coordinate reference system (CRS) is a framework used to precisely measure locations on the surface of the Earth as coordinates. It is thus the application of the abstract mathematics of

coordinate system In geometry, a coordinate system is a system that uses one or more numbers, or coordinates, to uniquely determine the position of the points or other geometric elements on a manifold such as Euclidean space. The order of the coordinates is sig ...

s and

analytic geometry In classical mathematics, analytic geometry, also known as coordinate geometry or Cartesian geometry, is the study of geometry using a coordinate system. This contrasts with synthetic geometry. Analytic geometry is used in physics and engineerin ...

to geographic space. A particular SRS specification (for example, " Universal Transverse Mercator WGS 84 Zone 16N") comprises a choice of Earth ellipsoid,

horizontal datum A geodetic datum or geodetic system (also: geodetic reference datum, geodetic reference system, or geodetic reference frame) is a global datum reference or reference frame for precisely representing the position of locations on Earth or other plan ...

, map projection (except in the

geographic coordinate system The geographic coordinate system (GCS) is a spherical or ellipsoidal coordinate system for measuring and communicating positions directly on the Earth as latitude and longitude. It is the simplest, oldest and most widely used of the various ...

), 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 transformations between different SRS. Although they date to the Hellenic Period, spatial reference systems are now a crucial basis for the sciences and technologies of Geoinformatics, including cartography,

geographic information systems A geographic information system (GIS) is a type of database containing geographic data (that is, descriptions of phenomena for which location is relevant), combined with software tools for managing, analyzing, and visualizing those data. In a br ...

surveying Surveying or land surveying is the technique, profession, art, and science of determining the terrestrial two-dimensional or three-dimensional positions of points and the distances and angles between them. A land surveying professional is ca ...

, remote sensing, and civil engineering. This has led to their standardization in international specifications such as the EPSG codes and ''ISO 19111:2007 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''.

Types of systems

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 The geographic coordinate system (GCS) is a spherical or ellipsoidal coordinate system for measuring and communicating positions directly on the Earth as latitude and longitude. It is the simplest, oldest and most widely used of the various ...

(or geodetic) :A

spherical coordinate system In mathematics, a spherical coordinate system is a coordinate system for three-dimensional space where the position of a point is specified by three numbers: the ''radial distance'' of that point from a fixed origin, its ''polar angle'' measu ...

measuring locations directly on the Earth (modeled as a sphere or ellipsoid) using latitude (degrees north or south of the

equator The equator is a circle of latitude, about in circumference, that divides Earth into the Northern and Southern hemispheres. It is an imaginary line located at 0 degrees latitude, halfway between the North and South poles. The term can als ...

) and longitude (degrees west or east of a prime meridian). ; Geocentric coordinate system (or Earth-centered Earth-fixed) :A three-dimensional

cartesian coordinate system A Cartesian coordinate system (, ) in a plane is a coordinate system that specifies each point uniquely by a pair of numerical coordinates, which are the signed distances to the point from two fixed perpendicular oriented lines, measured in t ...

that models the Earth as a three-dimensional object, measuring locations from a center point, usually the

center of mass In physics, the center of mass of a distribution of mass in space (sometimes referred to as the balance point) is the unique point where the weighted relative position of the distributed mass sums to zero. This is the point to which a force may ...

of the Earth, along x, y, and z axes aligned with the

and the prime meridian. This system is commonly used to track the orbits of satellites, because they are based on the center of mass. Thus, this is the internal coordinate system used by Satellite navigation systems such as GPS to compute locations using multilateration. ; Projected coordinate system (or planar, grid) Utm-latlon grid en

:A standardized

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 British National Grid, and

State Plane Coordinate System The State Plane Coordinate System (SPCS) is a set of 124 geographic zones or coordinate systems designed for specific regions of the United States. Each state contains one or more state plane zones, the boundaries of which usually follow county li ...

(SPCS). ;Engineering coordinate system (or local, custom) :A

(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

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. These standards acknowledge that standard reference systems also exist for measuring elevation using vertical datums and time (e.g. ISO 8601), which 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 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. To accomplish this, any coordinate reference system definition needs to be composed of several specifications: * A

, 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

, 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 coordinates (latitude and longitude). Examples include the World Geodetic System and the 1927 and 1983

North American Datum The North American Datum (NAD) is the horizontal datum now used to define the geodetic network in North America. A datum is a formal description of the shape of the Earth along with an "anchor" point for the coordinate system. In surveying, ca ...

. 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 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:

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 Transverse Mercator (JTM) (Arabic: نظام تربيع ميركاتور الأردني المستعرض) is a grid system created by the Royal Jordan Geographic Center (RJGC). This system is based on 6° belts with a Central Meridian of 37� ...

, Jordan

Europe

British national grid reference system The Ordnance Survey National Grid reference system (OSGB) (also known as British National Grid (BNG)) is a system of geographic grid references used in Great Britain, distinct from latitude and longitude. The Ordnance Survey (OS) devised the ...

, Britain * Lambert-93 (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), 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

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 A geographic information system (GIS) is a type of database containing Geographic data and information, geographic data (that is, descriptions of phenomena for which location is relevant), combined with Geographic information system software, sof ...

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 Db2 is a family of data management products, including database servers, developed by IBM. It initially supported the relational model, but was extended to support object–relational features and non-relational structures like JSON a ...

, IBM Informix,

Ingres Jean-Auguste-Dominique Ingres ( , ; 29 August 1780 – 14 January 1867) was a French Neoclassicism, Neoclassical Painting, painter. Ingres was profoundly influenced by past artistic traditions and aspired to become the guardian of academic ...

, 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 Well-known text (WKT) is a text markup language for representing vector geometry objects. A binary equivalent, known as well-known binary (WKB), is used to transfer and store the same information in a more compact form convenient for computer proc ...

(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"">quot;EPSG","9001".html" ;"title="metre",1, AUTHORITY["EPSG","9001"">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.

References

External links

spatialreference.org
– A website that defines spatial reference systems, in a variety of formats.
OpenGIS Specifications (Standards)

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)

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