wiki:markhDataModelDraft

Version 9 (modified by markh, 4 years ago) (diff)

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Cell Method

The cell methods construct describes the methods by which the data values of the field construct represent variation within their cells. It is an ordered list, because the methods specified are not necessarily commutative.

Each entry of the list specifies: one or more domain axes, or, CF standard names; and one method.

The variation is defined to be with respect to the named domain axes or with respect to the quantity defined by the standard names, depending on which is specified.

Appendix E of the CF conventions for NetCDF files lists method names which may be defined using the controlled terminology name cell_methods. Special methods indicate climatological time processing.

CF NetCDF

The cell measures construct corresponds to the cell_methods attribute of the data variable in CF-netCDF files.

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. Derivation formulae may be 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).