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### Coordinate Reference System

Coordinate reference systems provide the information to geolocate coordinates and thus fields in three dimensional space. 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, this is the system the coordinate is defined with respect to.

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** and the properties which are recognised for each of the coordinate reference systems 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.