SRID

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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 European Petroleum Survey Group (EPSG). (NOTE: As of 2005 the EPSG SRID values are now maintained by the International Association of Oil & Gas Producers (OGP) Surveying & Positioning Committee).

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, Microsoft SQL Server, MySQL, Oracle, Teradata, PostgreSQL and SQL Anywhere), 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 (WKT) string definition of the coordinate system (SRTEXT, above). From the Well Known Text Wikipedia page, “A WKT string for a spatial reference system describes the datum, geoid, coordinate system, and map projection of the spatial objects”. Here are two common coordinate systems with their EPSG SRID value followed by their well known text:

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.

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