Version 3 (modified by davidhassell, 5 years ago) (diff)


Please don't modify this page. It is an annex to ticket 107 comment 2.

A transform construct defines a mapping from one set of coordinates which can not geo-locate the field construct's data to another set of coordinates that can geo-locate the field construct's data. [added 05 Feb 2014]

A transform construct contains

  • A transform name, which indicates the nature of the transformation and implies the formula to be used. CF metadata does not explicitly record the formula; it is assumed that application software knows what to do.
  • An unordered collection of variables which correspond to the terms of the transformation formula. The variables may be scalar parameters, pointers to dimension or auxiliary coordinate constructs of the field construct, or pointers to other field constructs. The collection includes the input coordinates being mapped, but not the output coordinates.

A transform construct provides geo-locating metadata to all of the dimension and auxiliary coordinate constructs referenced by it.

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 describes how to compute true latitude and longitude auxiliary coordinate variables from horizontal projection dimension coordinates, or describes the figure of earth for true latitude and longitude coordinate variables (CF Appendix F).

The transform name is the standard_name of a vertical coordinate variable with formula_terms, and the grid_mapping_name of a grid_mapping variable. The scalar parameters are scalar data variables (which should have units if dimensional) named by formula_terms, and attributes of grid_mapping variables (for which the units are specified by the transform construct). The role of each term in the formula of the transform construct is identified by its keyword in a formula_terms attribute, or its attribute name in a grid_mapping variable.

Note that the transform construct for a CF-netCDF grid mapping of latitude_longitude is a special case in which the outputs are the same as the input coordinates.