Hello David,
it looks to me as though the attempt to remove ambiguity has not been entirely successful. The statement in the definition of "coordinate variable" that "We use this term precisely as it is defined in the NUG section on coordinate variables<
http://www.unidata.ucar.edu/software/netcdf/docs/netcdf_data_set_components.html#coordinate_variables>" is clearly at odds with what is proposed in #104.
I find the terminology confusing. The adjective "auxiliary" is usually used as a qualification, e.g. an "auxiliary verb" is a kind of verb. The CF convention, however, saying that an "auxiliary coordinate" is not a kind of coordinate variable ... just a variable containing coordinate data. It may help clarify things if we use the term "standard coordinate variable" when referring to the NUG++ type of coordinate variable (which I think means something conforming to the NUG definition, or something like "time(time)" which is referenced in a coordinates attribute or a scalar variable referenced in a coordinates attribute, provided the scalar variable has an axis attribute -- though I'm unclear about the intention regarding scalar coordinates).
regards,
Martin
________________________________
From: David Hassell <david.hassell at ncas.ac.uk>
Sent: 27 February 2018 08:46
To: Juckes, Martin (STFC,RAL,RALSP)
Subject: Re: [CF-metadata] Use of axis attribute in an auxillary coordinate
Hi Martin,
I think the reason for the change was to remove any ambiguity on how "zero-dimensional variables referenced by the coordinates attribute" should be interpreted. See
https://cf-trac.llnl.gov/trac/ticket/104 for many details ...
The third paragraph in section 5 is indeed misleading. It is modified, somewhat belatedly, a couple of paragraphs later ("Note that it is permissible, but optional, to list coordinate variables as well as auxiliary coordinate variables in the coordinates attribute"), but this point should be made earlier, and include scalar coordinate variables. Perhaps there are other examples like this.
Are there any other cases you have seen? Perhaps that defect defect ticket should now be compiled.
All the best,
David
On 26 February 2018 at 17:16, <martin.juckes at stfc.ac.uk<mailto:martin.juckes at stfc.ac.uk>> wrote:
Dear Jim, David,
Sorry for dropping this thread from last August.
I agree that the convention does not have to adhere rigidly to the NUG definition of a coordinate variable, but the text of version 1.7 implies that it does. David's suggested rewording of the definition of an auxiliary coordinate below would begin to address this, but there may be other places where a change is needed. E.g.
The current text says that "The value of the coordinates attribute is a blank separated list of the names of auxiliary coordinate variables." (3rd paragraph in section 5) -- I think you are saying that this is not the meaning you want?
I'm still not entirely clear what you are trying to achieve here .. the definition of auxiliary coordinate in version 1.7 looked fine to me (where it is clearly stated in relation to the NUG definition of a coordinate variable): I'm not objecting to your change, but it might help this conversation if you explained the rationale behind the change.
regards,
Martin
________________________________
Martin,
The height scalar variable you are referring to is numeric, right? It
may not be a coordinate variable in the pure NUG sense, but CF isn't
bound by exact adherence to NUG if it defines its terms, and David has
posted the relevant sections from the Conventions. It is clear to me
that those sections declare that a numeric scalar variable may serve as
a valid 'true' coordinate variable, /as opposed to/ an auxiliary
coordinate variable. The mechanism for relating a scalar coordinate
variable to a data variable is through the 'coordinates' attribute, but
that does not force it to be considered to be an auxiliary coordinate.
Martin is trying to find a way to clarify these concepts, since you seem
to find them unclear.
What would make it clearer to you that CF declares that the height
scalar variable mentioned in this discussion qualifies as a true or full
or complete (or whatever word you prefer) coordinate and is not
relegated to auxiliary coordinate status?
Grace and peace,
Jim
On 7/31/17 10:10 AM, martin.juckes at stfc.ac.uk<
http://stfc.ac.uk><
http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata> wrote:
>
> Why?
>
> *From:*David Hassell [mailto:david.hassell<mailto:david.hassell> at ncas.ac.uk<http://ncas.ac.uk><http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata>]
> *Sent:* 31 July 2017 14:50
> *To:* Juckes, Martin (STFC,RAL,RALSP)
> *Cc:* CF Metadata
> *Subject:* Re: [CF-metadata] Use of axis attribute in an auxillary
> coordinate
>
> Hi Martin,
>
> In #104 <https://cf-trac.llnl.gov/trac/ticket/104> perhaps we should
> have updated the definition of an auxiliary coordinate variable to
> reflect the clarification of scalar cooridnate variables. Something like:
>
>
> auxiliary coordinate variable
>
> Any netCDF variable that contains coordinate data, but is not a
> coordinate variable (in the sense of that term defined by the NUG and
> used by this standard - see below) */nor is functionally equivalent to
> one (such as a numeric scalar coordinate variable)/*. Unlike
> coordinate variables, there is no relationship between the name of an
> auxiliary coordinate variable and the name(s) of its dimension(s).
>
> This could be done with a defect ticket.
>
> All the best,
>
> Diavd
--
David Hassell
National Centre for Atmospheric Science
Department of Meteorology, University of Reading,
Earley Gate, PO Box 243, Reading RG6 6BB
Tel: +44 118 378 5613<tel:+44%20118%20378%205613>
http://www.met.reading.ac.uk/
Received on Tue Feb 27 2018 - 02:33:29 GMT
