Use namespace tags to include other conventions in netcdf files without repeating them in CF
|Reported by:||benno||Owned by:||edavis|
Strictly speaking, this is not an enhancement to CF -- it is a way of encoding additional information in a netcdf files without extending CF. It certainly would affect discussions for extending CF, and hopefully will elicit some discussion in the CF community. In any case, creating a ticket was suggested.
My suggestions are based on an evolution of how metadata is encoded in netcdf files: rather than simply allowing multiple conventions in netcdf files by a comma-separated list in the Conventions attribute, specify prefixes so that attributes can be explicitly labelled as belonging to a different convention. In particular, were I to want to use attributes from the convention used by WCS to describe projections, I would use a conventions tag in the file like,
Conventions = "CF-1.0, wcs=http://www.opengis.net/wcs/1.1/"
means the default convention is CF-1.0 (i.e the usual usage), but any attribute starting 'wcs:' belongs to the convention defined by the namespace http://www.opengis.net/wcs/1.1/ (which is how wcs or any other XML convention standardizes its labels). This would change the WGS 84 example in ticket 18 slightly, for example, to
dimensions: lat = 18 ; // dummy values lon = 36 ; variables: double lat(lat) ; // conventional definition double lon(lon) ; // conventional definition float temp(lat, lon) ; temp:long_name = "temperature" ; temp:units = "K" ; temp:grid_mapping = "crs" ; temp:wcs\:gridCRS = "crs" ; int crs ; crs:grid_mapping_name = "lat_long_wgs1984" ; crs:wcs\:GridBaseCRS = "urn:ogc:def:crs:EPSG:6.0:4326" ; // Use EPSG ID 4979 for 3D CRS. ID 4326 refers to 2D CRS. crs:crs_name = "WGS 84" ; crs:geodetic_datum_name = "World Geodetic System 1984" ; crs:longitude_of_prime_meridian = 0.0 ; crs:ellipsoid_name = "WGS 84" ; crs:semi_major_axis = 6378137.0 ; crs:inverse_flattening = 298.257223563 ; Conventions = "CF-1.0, wcs=http://www.opengis.net/wcs/1.1/"
where gridCRS and GridBasCRS are the tags used in WCS to characterize projects. In particular, it declares 'crs' to be both a CF convention grid_mapping, and a WCS convention gridCRS, and it adds an WCS attribute to gridCRS called GridBaseCRS, which is part of the WCS convention.
This keeps the redundancy out of CF, but allows alternate specifications to be put in a netcdf file. It also makes use of standards that other people maintain, i.e. OGC maintains the wcs namespace and list of concepts.
There are a number of reasons for doing it this way, listing of which would probably hide the essence of the proposal and should be put elsewhere. The short version is redundant representations belong to different conventions, and this is a framework for writing down the mappings between the different conventions, and for tagging datasets with them. This would allow creation of a system that could deliver the metadata in alternate representations, so that WCS applications could use WCS-style information, and CF applications could use CF information, regardless of the actual source of the data being analyzed.
Note that recent and near-term changes to netcdf make this possible -- we could not have made this choice two years ago. But netcdf version 3.6 libraries support ':' characters in attributes, and version 3.7 will support it in ncgen/ncdump, i.e. the '\:' escapes shown above will work in CDL.
Change History (36)
comment:16 Changed 8 years ago by edavis
- Owner changed from cf-conventions@… to edavis
- Status changed from new to assigned