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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.

A coordinate instance may reference one coordinate reference system, the coordinate is then 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.

In CF NetCDF files coordinate reference systems are defined by the data variable, using the `grid_mapping` attribute. Where the `grid_mapping` attribute does not define explicit references between coordinates and a CRS, there exists and implicit reference from the X and Y coordinate variables to the `grid_mapping` target variable.

### Derived Coordinate

A derived coordinate links together explicit coordinate instances of a Field with a defined process for combining these to form a new coordinate.

Each input coordinate is named and a derivation formula is provided. At present derivation formulae are provided by named reference to Appendix D ~~and Appendix F~~ of the CF conventions for NetCDF files.

These transform constructs correspond to the functions of the CF-netCDF attributes `formula_terms`, which describes how to compute a vertical auxiliary coordinate variable from components (CF Appendix D)~~, and ~~.
`grid_mapping`, which can describe how to compute true latitude and longitude auxiliary coordinate variables from horizontal projection dimension coordinates (CF Appendix F)