These conventions generalize and extend the COARDS conventions [COARDS]. A major design goal has been to maintain backward compatibility with COARDS. Hence applications written to process datasets that conform to these conventions will also be able to process COARDS conforming datasets. We have also striven to maximize conformance to the COARDS standard so that datasets that only require the metadata that was available under COARDS will still be able to be processed by COARDS conforming applications. But because of the extensions that provide new metadata content, and the relaxation of some COARDS requirements, datasets that conform to these conventions will not necessarily be recognized by applications that adhere to the COARDS conventions. The features of these conventions that allow writing netCDF files that are not COARDS conforming are summarized below.
COARDS standardizes the description of grids composed of independent latitude, longitude, vertical, and time axes. In addition to standardizing the metadata required to identify each of these axis types COARDS restricts the axis (equivalently dimension) ordering to be longitude, latitude, vertical, and time (with longitude being the most rapidly varying dimension). Because of I/O performance considerations it may not be possible for models to output their data in conformance with the COARDS requirement. The CF convention places no rigid restrictions on the order of dimensions, however we encourage data producers to make the extra effort to stay within the COARDS standard order. The use of non-COARDS axis ordering will render files inaccessible to some applications and limit interoperability. Often a buffering operation can be used to miminize performance penalties when axis ordering in model code does not match the axis ordering of a COARDS file.
COARDS addresses the issue of identifying
dimensionless vertical coordinates, but does
not provide any mechanism for mapping the
dimensionless values to dimensional ones that
can be located with respect to the earth's
surface. For backwards compatibility we continue
to allow (but do not require) the
attribute of dimensionless vertical coordinates to take the
values "level", "layer", or "sigma_level." But we
recommend that the
attributes be used to identify the appropriate
definition of the dimensionless vertical
Section 4.3.2, “Dimensionless Vertical Coordinate”).
The CF conventions define attributes which enable the description of data properties that are outside the scope of the COARDS conventions. These new attributes do not violate the COARDS conventions, but applications that only recognize COARDS conforming datasets will not have the capabilities that the new attributes are meant to enable. Briefly the new attributes allow:
Identification of quantities using standard names.
Description of dimensionless vertical coordinates.
Associating dimensions with auxiliary coordinate variables.
Linking data variables to scalar coordinate variables.
Associating dimensions with labels.
Description of intervals and cells.
Description of properties of data defined on intervals and cells.
Description of climatological statistics.
Data compression for variables with missing values.