[CF-metadata] Use of axis attribute in an auxillary coordinate

David Hassell david.hassell at ncas.ac.uk
Mon Jul 31 03:48:10 MDT 2017


Hi Martin,

The terminology entry has also been updated:

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). Unlike coordinate variables, there is no
relationship between the name of an auxiliary coordinate variable and the
name(s) of its dimension(s).
scalar coordinate variable

A scalar variable (i.e. one with no dimensions) that contains coordinate
data. Depending on context, it may be functionally equivalent either to a
size-one coordinate variable (Section 5.7, "Scalar Coordinate Variables"
<http://cfconventions.org/cf-conventions/cf-conventions.html#scalar-coordinate-variables>)
or to a size-one auxiliary coordinate variable (Section 6.1, "Labels"
<http://cfconventions.org/cf-conventions/cf-conventions.html#labels>
and Section
9.2, "Collections, instances, and elements"
<http://cfconventions.org/cf-conventions/cf-conventions.html#collections-instances-elements>
).


​The phrase ​"and used by this standard" allows, I think, the scalar
coordinate variable to not be an auxiliary coordinate in the right context
(i.e. when it's numeric).

Does that make sense?

Thanks, David


On 31 July 2017 at 09:55, <martin.juckes at stfc.ac.uk> wrote:

> Hi David,
>
> But it is not a NUG coordinate variable has, by definition, the same name
> as a dimension (see http://www.unidata.ucar.edu/
> software/netcdf/docs/netcdf_data_set_components.html#coordinate_variables
> ) ... so height, in the example below, is not a NUG coordinate variable. It
> is an auxiliary coordinate variable, if you follow the definitions we have
> in the CF convention.
>
> Martin
> ________________________________
> From: David Hassell [david.hassell at ncas.ac.uk]
> Sent: 31 July 2017 09:33
> To: Juckes, Martin (STFC,RAL,RALSP)
> Cc: CF Metadata
> Subject: Re: [CF-metadata] Use of axis attribute in an auxillary coordinate
>
> Hi Martin,
>
> Because it's numeric, I would say that it acting as a coordinate variable
> in the NUG sense.
>
> All the best,
>
> Daivd
>
> On 31 July 2017 at 09:18, <martin.juckes at stfc.ac.uk<mailto:
> martin.juckes at stfc.ac.uk>> wrote:
> Hello David,
>
> yes, so "height" in the example I gave is clearly an auxiliary coordinate,
> right?
>
> cheers,
> Martin
>
> ________________________________
> From: David Hassell [david.hassell at ncas.ac.uk<mailto:
> david.hassell at ncas.ac.uk>]
> Sent: 31 July 2017 09:03
> To: Juckes, Martin (STFC,RAL,RALSP)
> Cc: CF Metadata
> Subject: Re: [CF-metadata] Use of axis attribute in an auxillary coordinate
>
> Hello Martin,
>
> This definition was tightened up in ticket #104<https://cf-trac.llnl.gov/
> trac/ticket/104>. An arbitrary scalar coordinate can be either, but not
> both at the same time. At version 1.7, section 5.7 (
> http://cfconventions.org/cf-conventions/cf-conventions.
> html#scalar-coordinate-variables) now says:
>
>   "A numeric scalar coordinate variable has the same information content
> and can be used in the same contexts as a size one numeric coordinate
> variable. Similarly, a string-valued scalar coordinate variable has the
> same meaning and purposes as a size one string-valued auxiliary coordinate
> variable"
>
> Thanks,
>
> David
>
> On 30 July 2017 at 07:50, <martin.juckes at stfc.ac.uk<mailto:
> martin.juckes at stfc.ac.uk><mailto:martin.juckes at stfc.ac.uk<mailto:
> martin.juckes at stfc.ac.uk>>> wrote:
> Dear All,
>
> I'm afraid I'm not undrestanding how you are distinguishing between
> "auxillary" and other coordinates. The terminology section of the CF
> Convention says that a variable is an auxillary coordinate if it contains
> coordinate data and is not an NUG coordinate. The definition of a scalar
> coordinate states that a scalar coordinate can be either an auxillary
> coordinate or a coordinate variable .. so the "height" variable here is
> both a scalar coordinate and an auxillary coordinate. It is clearly not a
> NUG coordinate variable.
>
>
> regards,
> Martin
>
> ------------------------------------------------------------
> ----------------------------------------------------------------
>
> David,
>
> I'm still not convinced of the utility of axis for coordinate variables
> that aren't true coordinate variables, but this case doesn't fit that
> one, does it? In this case isn't height a true (scalar) coordinate
> variable? Shouldn't this pass the checker, regardless?
>
> Jim
>
>
> On 5/24/17 3:54 PM, David Hassell wrote:
> > Hi Jim, Martin,
> >
> > I agree - "height" in this case is not an auxiliary coordinate
> > variable, rather a scalar coordinate variable (because it doesn't span
> > any of the dimensions of "tas").
> >
> > I also agree that the conformance document needs changing to allow the
> > "axis" attribute on auxiliary coordinate variables - this was accepted
> > in CF-1.6, I think.
> >
> > Thanks,
> >
> > Daivd
> >
> >
> >
> > On 24 May 2017 at 19:59, Jim Biard <jbiard at cicsnc.org<
> http://cicsnc.org><http://cicsnc.org>
> > <mailto:jbiard<mailto:jbiard><mailto:jbiard<mailto:jbiard>> at
> cicsnc.org<http://cicsnc.org><http://cicsnc.org>>> wrote:
> >
> >     Martin,
> >
> >     We just had some discussion about the proper use of the axis
> >     attribute, but this seems to me like it might be a flaw in the
> >     checker. As a scalar coordinate, height can only be associated
> >     with the tas variable via the coordinates attribute (per section
> >     5.7), but I don't think that makes it an auxiliary coordinate,
> >     does it?
> >
> >     What do other people think? Chime in!
> >
> >     Grace and peace,
> >
> >     Jim
> >
> >
> >     On 5/23/17 10:50 AM, martin.juckes at stfc.ac.uk<http://stfc.ac.uk><
> http://stfc.ac.uk>
> >     <mailto:martin.juckes<mailto:martin.juckes><mailto:martin.juckes
> <mailto:martin.juckes>> at stfc.ac.uk<http://stfc.ac.uk><http://stfc.ac.uk>>
> wrote:
> >>     Hello All,
> >>
> >>     I'd just like to check one aspect of the conformanc document, which
> came to our attention when somebody ran the CF checker on some CMIP5. If
> you check using the convention version declared in the file, 1.4, it will
> raise an error if there is a scalar coordinate variable with the axis
> attribute set, e.g.
> >>
> >>     float tas(time,lat,lon);
> >>          ......
> >>          tas:coordinates = "height" ;
> >>     float height ;
> >>          ....
> >>          height: axis = "Z";
> >>
> >>     In this case "height" variable is, following the logic of section
> 1.2 of the convention, classed as an auxillary coordinate because it is not
> of the form "height (height) ; ".
> >>
> >>     The error message appears to relate to a line in the conformance
> document saying that "The axis attribute is not allowed for auxiliary
> coordinate variables." If the checker is asked to use a later version of
> the convention, the error message goes away, but the requirement is still
> there in the conformance document.
> >>
> >>     It looks to me as though it should be removed from the conformance
> document. The convention document says, in section 4. that "The methods of
> identifying coordinate types described in this section apply both to
> coordinate variables", referring to the use of the axis attribute, which
> appears to directly contradict the line of the conformance document cited
> above. But is there another part of the convention that requires some
> restriction on the use of the axis attribute?
> >>
> >>     This construction is widely used in CMIP data, so we should get
> this point cleared up.
> >>
> >>     regards,
> >>     Martin
> >>
> >>     _______________________________________________
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> > --
> > David HassellNational Centre for Atmospheric ScienceDepartment of
> > Meteorology, University of Reading, Earley Gate, PO Box 243, Reading
> > RG6 6BB Tel: +44 118 378 5613<tel:%2B44%20118%20378%205613>
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> --
> CICS-NC <http://www.cicsnc.org/> Visit us on Facebook
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> for Environmental Information <http://ncdc.noaa.gov/> /formerly NOAA’s
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> --
> 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:%2B44%20118%20378%205613>
> http://www.met.reading.ac.uk/
>
>
>
> --
> 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
> http://www.met.reading.ac.uk/
>



-- 
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
http://www.met.reading.ac.uk/
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