Spatial Reference Identifier (SRID)

A spatial reference system identifier is a code to easily reference a spatial reference system (SRS). An SRS contains parameters about projection, ellipsoid, and datum. It can be defined using the Open Geospatial Consortium's (OGC) well-known text (WKT) representation. The SRS for the geographic WGS84 reference system is as follows:

    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"]]

The last line contains the number 4326; this is the SRID uniquely identifying this SRS. The long form should also contain the authority, that is EPSG:4326, but you will often find it indicated only by the number.

EPSG is the acronym for European Petroleum Survey Group. Several European Oil companies founded it in 1986 to collect and maintain geodetic information. In 2005, EPSG was absorbed by OGP (an international forum for Oil and Gas producers) that formed the OGP Geomatics Committee. The committee maintains the registry and publishes it as a public web interface or a downloadable database.

It is very important that you know what SRID your data is in. Without it, you can't represent data on a map without the risk of great errors.

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Spatial Reference Identifier (SRID)