Custom Query (124 matches)

1. Title

Proposed Extensions to CF Grid Mapping Attributes to support CRS Properties

2. Moderator

Jonathan Gregory

3. Requirement

Previous posts to the CF mailing list have identified the requirement for additional attributes that could be used to provide a fuller definition of the characteristics of the coordinate reference system (CRS) used by spatial coordinates within a netCDF file. This proposal attempts to define attributes for several commonly used CRS properties.

4. Initial Statement of Technical Proposal

Owing to the length of this proposal, the full specification text is included in the attached PDF document.

(NB: If it is considered more convenient, e.g. for discussion purposes, to upload the full text of this proposal into this Trac ticket, then the author is happy to do so.)

5. Benefits

Scope: potentially all producers and end-users of netCDF datasets could exploit the proposed new attributes.

New capabilities: the proposed new attributes would enable data producers to more accurately record the specific characteristics of the coordinate reference system (or systems) used to define spatial coordinates within netCDF files.

Example use-case 1: A data producer has collected meteorological observations using a sensor platform which uses, for example, a particular geodetic datum (e.g. WGS 84, NAD 83, OSGB 36) to record spatial coordinates. It is desirable for this piece of CRS metadata, and others like it, to be recorded in appropriate netCDF CF attributes.

Example use-case 2: A climate data center wishes to convert a legacy dataset to netCDF format and make it available over the internet. The legacy dataset is based upon an unusual or customised coordinate reference system (e.g. transverse mercator projection using, say, the Clarke 1880 ellipsoid). As before, these CRS details need to be encoded in agreed, standardised CF attributes.

6. Status Quo

The author is not aware of alternative CF attributes or mechanisms that could be used to encode the desired additional CRS properties.

In the CF 1.4 document some URLs referencing the UCAR Unidata site result in "404 Not Found" errors or use deprecated paths for which permanent replacements are now available. Following is a list of external URLs that should be replaced, each followed immediately by its recommended replacement. References to the NetCDF Fortran Users Guide have been replaced by references to the corresponding text in the language-neutral NetCDF Users Guide. Unidata commits to supporting the replacement URLs through future web site and documentation reorganizations, so they will continue to be valid.

http://www.unidata.ucar.edu/packages/netcdf/guidef/guidef-15.html#HEADING15-0
http://www.unidata.ucar.edu/netcdf/docs/netcdf.html#NetCDF-Utilities

http://www.unidata.ucar.edu/packages/netcdf/guidef/guidef-7.html#HEADING7-67
http://www.unidata.ucar.edu/netcdf/docs/netcdf.html#Variables

http://www.unidata.ucar.edu/packages/netcdf/conventions.html
http://www.unidata.ucar.edu/netcdf/conventions.html

http://www.unidata.ucar.edu/packages/netcdf/guidef/guidef-13.html#HEADING13-12
http://www.unidata.ucar.edu/netcdf/docs/netcdf.html#Attribute-Conventions

http://www.unidata.ucar.edu/packages/netcdf/guidef/guidef-12.html#HEADING12-1381
http://www.unidata.ucar.edu/netcdf/docs/netcdf.html#NetCDF-Classic-Format

http://www.unidata.ucar.edu/packages/netcdf/guidef/
http://www.unidata.ucar.edu/netcdf/docs/netcdf.html

http://www.unidata.ucar.edu/packages/netcdf/index.html
http://www.unidata.ucar.edu/netcdf/index.html

The format for the "ocean_sigma_coordinate" formula_terms attribute should be

formula_terms = "sigma: var1 eta: var2 depth: var3"

I checked the CF-conventions 1.0 doc from 2004 and it's correct there, but even version 1.0 of the docbook

​http://cf-pcmdi.llnl.gov/trac/browser/cf-conventions/trunk/docbooksrc/appendix-d.xml?rev=1

has it wrong, repeating instead the "ocean_s_coordinate" formula_terms.