Spatial reference system


A spatial reference system or coordinate reference system is a framework used to precisely measure locations on, or relative to, the surface of Earth as coordinates. It is thus the application of the abstract mathematics of coordinate systems and analytic geometry to geographic space. A particular SRS specification comprises a choice of Earth ellipsoid, horizontal datum, map projection, 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 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 codes 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.
The above refers to locations directly on the surface of the earth. Information on elevation may also be specified, via a vertical reference frame, so-called vertical CRS, or an integrated 3D CRS. Terminology in this area is evolving in line with increasing technical sophistication in measurement.

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
;Geocentric coordinate system
;Projected coordinate system
;Engineering coordinate system
;Vertical reference frame
;3D coordinate system
These standards acknowledge that standard reference systems also exist for time. 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 image, Linear referencing measurements along linear features, 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 coordinate system, an abstract framework for measuring locations. Like any mathematical coordinate system, its definition consists of a measurable space, an origin point, a set of axis vectors emanating from the origin, and a unit of measure.
  • A geodetic datum 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. 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, and one or more anchor points or control points, established locations 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:
EPSG codeNameEllipsoidHorizontal datumCS typeProjectionOriginAxesUnit of measure
GCS WGS 84GRS 80WGS 84ellipsoidal equator/prime meridianequator, prime meridiandegree of arc
UTM Zone 17N NAD 27Clarke 1866NAD 27cartesian Transverse Mercator: central meridian 81°W, scaled 0.9996500 km west of equator, 81°W meridianmetre
SPCS Tennessee Zone NAD 83 ftUSGRS 80NAD 83 cartesian Lambert Conformal Conic: center 86°W, 34°20'N, standard parallels 35°15'N, 36°25'N600 km grid west of center pointgrid east at center point, 86°W meridianUS survey foot

Examples by continent

Examples of systems around the world are:

Asia

A Spatial Reference System Identifier 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 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,
AUTH_SRID INTEGER,
SRTEXT CHARACTER VARYING

In spatially enabled databases, SRIDs are used to uniquely identify the coordinate systems used to define columns of spatial data or individual spatial objects in a spatial column. SRIDs are typically associated with a well-known text string definition of the coordinate system.
Here are two common coordinate systems with their EPSG SRID value followed by their WKT:
UTM, Zone 17N, NAD27 — SRID 2029:

PROJCS,
PARAMETER,
PARAMETER,
PARAMETER,
PARAMETER,
PARAMETER,
AUTHORITY,
AXIS,
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.