Coordinate systems are defined by identifiers. Identifiers have the format Type: Value. Depending on the type of identifier, the value has a different format:

EPSG

This type of identifier describes a coordinate system based on its EPSG code defined by the European Petroleum Survey Group. A coordinate code is defined using a code from a geographic system (GCS) or a projected system (PCS). The codes can be found at this address: http://www.epsg.org.

Example:

EPSG:4326(WGS84)

EPSG:23030(UTM 30N / ED50)

EPSG:32630(UTM 30N / WGS84)

EPSG:31463(DHDN / 3-degree Gauss zone 3)

PROJ4

Format used by the PROJ.4 library. More information can be found at this address: http://proj4.org/parameters.html.

Example:

PROJ4:+proj=utm +zone=30 +ellps=intl +units=m +no_defs

WKT

Identifiers in "Well-Known-Text" format of the Open Geoespatial Consortium (OGC). The specification of this format can be found in: OpenGIS Implementation Specification for Geographic information - Simple feature access - Part 1: Common architecture.

Example:

WKT:

PROJCS["ED50 / UTM zone 30N",

   GEOGCS["ED50",

      DATUM["European_Datum_1950",

         SPHEROID["International 1924",6378388,297,

            AUTHORITY["EPSG","7022"]],

         TOWGS84[-87,-98,-121,0,0,0,0],

         AUTHORITY["EPSG","6230"]],

      PRIMEM["Greenwich",0,

         AUTHORITY["EPSG","8901"]],

      UNIT["degree",0.01745329251994328,

         AUTHORITY["EPSG","9122"]],

      AUTHORITY["EPSG","4230"]],

   UNIT["metre",1,

      AUTHORITY["EPSG","9001"]],

   PROJECTION["Transverse_Mercator"],

   PARAMETER["latitude_of_origin",0],

   PARAMETER["central_meridian",-3],

   PARAMETER["scale_factor",0.9996],

   PARAMETER["false_easting",500000],

   PARAMETER["false_northing",0],

   AUTHORITY["EPSG","23030"]]

ESRI

These identifiers are a WKT dialect used by ESRI in ArcGIS. They can be obtained from the .PRJ files used by ArcGIS.