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### Coordinate Reference System

Coordinate reference systems (CRS) provide the information required to geolocate coordinates and thus fields in three dimensional space with respect to the Earth. A coordinate reference system provides a geographic frame of reference which coordinate instances may be defined with respect to.

Geo-location in the third dimension of space, the vertical, is only provided with respect to the datum of the coordinate reference system.

A coordinate instance may reference one coordinate reference system, the coordinate is defined with respect to this referenced CRS.

Appendix F of the CF conventions for NetCDF files lists coordinate reference systems which may be defined using the controlled terminology name **grid_mapping_name**. The properties which are recognised for each of the coordinate reference systems are listed there.

The OGC Well Known Text (WKT) string is also supported within CF and may be used complementarily to entries in Appendix F to define a coordinate reference system. The attribute **crs_wkt** must be used for WKT strings. All uses of the **crs_wkt** attribute must be well formed well known text strings.