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

martin.juckes at stfc.ac.uk martin.juckes at stfc.ac.uk
Sun Jul 30 00:50:37 MDT 2017


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 
> <mailto:jbiard at 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
>     <mailto:martin.juckes at 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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