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.

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.