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

Jim Biard jbiard at cicsnc.org
Wed May 24 12:59:39 MDT 2017


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 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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