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Ticket Resolution Summary Owner Reporter
#9 wontfix Extensions to CF grid mapping attributes to support coordinate reference system properties cf-conventions@… pbentley

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.

#39 fixed Error in check of units attribute for coordinate variables cf-checker@… ros

3.1 of the convention states that "The units attribute is required for all variables that represent dimensional quantities."

Yet a Netcdf file like the following does not generate errors indicating that the units attribute is missing for any of the coordinate variables.

       longitude = 144 ;
       latitude = 91 ;
       press = 10 ;
       species = 6 ;
       rec = UNLIMITED ; // (12 currently)
       word = 13 ;

       float concentrations(rec, species, press, latitude, longitude) ;
             concentrations:long_name = "CFC concentrations" ;
             concentrations:Units = "pptv" ;
       float longitude(longitude) ;
       float latitude(latitude) ;
       float press(press) ;
       float species(species) ;
             species:long_name = "species" ;
             species:coord_labels = "species" ;
             species:selection_category = "NULL" ;
       float rec(rec) ;
             rec:long_name = "month" ;
             rec:coord_labels = "month" ;
             rec:selection_category = "NULL" ;
       char const_labels(species, word) ;
             const_labels:selection_category = "NULL" ;
       char month(rec, word) ;
             month:selection_category = "NULL" ;
#26 fixed Enhance CF flag definitions to support bit fields russ gregr

1. Title

Bit field enhancement to CF Flags definition

2. Moderator


3. Requirement

CF ought to provide a flag expression for multiple conditions, typically Boolean (binary), that describes one or more status conditions associated with a data variable.

4. Initial Statement of Technical Proposal

A new CF flag attribute, named "flag_masks", would enhance the current CF flags capabilities to describe multiple, independent status conditions using bit fields to define unique conditions or status codes.

5. Benefits

Bit field flag attributes are best suited for describing data variables possessing a number of status conditions that typically occur independently of each other.

Bit field attributes would simplify the description of status flags that don't conveniently map to a unique set of mutually exclusive status codes, currently defined with flag_values attributes.

For example, when describing a precipitation measurement within a particular geo-spatial grid, a bit field flag value may be defined to contain four possible binary status conditions, occupying the four least significant bits, whose values would be defined as follows: 1 to indicate no sensor coverage at the grid location, 2 to indicate observation impairment at that grid location, 4 to indicate mixed-phase precipitation at the grid location, 8 to indicate snow precipitation at the grid location.

6. Status Quo

Current flag values could be defined for every OR'ed combination of bit settings that define all possible status conditions, but the result would be inefficient compared to simple bit field definitions.

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