[CF-metadata] axis attribute

Jim Biard jbiard at cicsnc.org
Tue Apr 4 14:49:27 MDT 2017


Jonathan,

But was the axis attribute intended for use on 2D auxiliary coordinate 
variables? Perhaps that was before my time, but I don't recall seeing 
any discussion where that use was advocated.

Jim


On 4/4/17 4:30 PM, Jonathan Gregory wrote:
> Dear David and Jim
>
> Before CF 1.6, the axis attribute was allowed only for (Unidata) coordinate
> variables (i.e. the 1D ones whose name equals their dimension name). In CF 1.6
> it was generalised to be allowed for auxiliary coordinate variables, as
> described in the preamble of sect 5. I wasn't really in favour of this change,
> but the majority was.
>
> Best wishes
>
> Jonathan
>
> ----- Forwarded message from Jim Biard <jbiard at cicsnc.org> -----
>
>> Date: Fri, 31 Mar 2017 12:44:11 -0400
>> From: Jim Biard <jbiard at cicsnc.org>
>> CC: CF Metadata <cf-metadata at cgd.ucar.edu>
>> Subject: Re: [CF-metadata] CF compliant tripolar grid representation
>> User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.12; rv:45.0)
>> 	Gecko/20100101 Thunderbird/45.8.0
>>
>> David,
>>
>> Yes. I think the wording could stand to be clearer. What I wonder is
>> what use is there for identifying a 2D grid of latitude values as
>> being an axis? I do a lot of satellite swath imagery and have worked
>> with polar stereographic data, and latitude is not an axis of my
>> measurement variable grid in either case.
>>
>> I think part of the confusion arises from a somewhat unclear
>> definition of coordinate. I tend to use the phrase "true coordinate"
>> for one that is1-D, has a variable name equal to its dimension name,
>> is monotonic, has no fill values, etc, versus "auxiliary coordinate"
>> for one that doesn't meet one or more of those requirements. I
>> generally assume that true coordinates are being referred to when I
>> see the word coordinate in the Conventions unless it's made clear
>> that is not the case (as in Section 5 paragraph 6). With that
>> reading, the coordinate type and dimension type are one in the same
>> in Section 4 paragraph 2, since only true coordinate variables are
>> being discussed.
>>
>> Grace and peace,
>>
>> Jim
>>
>> On 3/31/17 12:28 PM, David Hassell wrote:
>>> Hi Jim,
>>>
>>> I agree you with in spirit, but the conventions do say that the
>>> axis attribute as being there to identify the *coordinate* type,
>>> rather than the *dimension* type (section 4, paragraph 2). Perhaps
>>> the wording here could be tightened up to say dimension type? I
>>> wonder how the axis attribute has been used over the last 6 years
>>> since 1.6 was released?
>>>
>>> All the best,
>>>
>>> David
>>>
>>> On 31 March 2017 at 17:04, Jim Biard <jbiard at cicsnc.org
>>> <mailto:jbiard at cicsnc.org>> wrote:
>>>
>>>     David,
>>>
>>>     As I read the Conventions, the axis attribute is to be applied to
>>>     coordinate variables (Section 4. Coordinate Types and Section 5.
>>>     Coordinate systems) to indicate that this variable can be treated
>>>     as representing an dimensional axis of corresponding variable
>>>     grids. Section 5 paragraph 6 talks about how it is still possible
>>>     to figure out that an auxiliary coordinate variable is a
>>>     spatiotemporal dimension of the if the axis attribute is not
>>>     present. I don't think a 2D auxiliary coordinate variable can be
>>>     considered to be a dimensional axis, can it?
>>>
>>>     Grace and peace,
>>>
>>>     Jim
>>>
>>>
>>>     On 3/31/17 11:52 AM, David Hassell wrote:
>>>>     Hello ​Sébastien and Jim,
>>>>
>>>>         You are right to feel weird about identifying 2D lat and lon
>>>>         as Y and X axes. The axis attribute should never be applied
>>>>         to 2D variables. It is only valid for 1D "true" coordinate
>>>>         variables.
>>>>
>>>>     ​The axis attribute can be attached to auxiliary coordinate
>>>>     variables with any number of dimensions. I would agree, though,
>>>>     that attaching the axis=X attribute to a 2-d longitude auxiliary
>>>>     coordinate variable is likely to confuse. The axis attribute's
>>>>     purpose is merely to make identification easier, but as long
>>>>     there are units of degrees_east (mandatory) and a standard name
>>>>     of longitude (optional), humans and software alike should be happy.
>>>>
>>>>     All the best,
>>>>
>>>>     David
>>>>
>>>>
>>>>     --     David Hassell
>>>>     National Centre for Atmospheric Science
>>>>     Department of Meteorology, University of Reading,
>>>>     Earley Gate, PO Box 243, Reading RG6 6BB
>>>>     Tel: +4
>>>>     ​4 ​
>>>>     118 378 5613
>>>>     http://www.met.reading.ac.uk/
>>>     --     CICS-NC <http://www.cicsnc.org/> Visit us on
>>>     Facebook <http://www.facebook.com/cicsnc> 	*Jim Biard*
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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
>>> http://www.met.reading.ac.uk/
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Facebook <http://www.facebook.com/cicsnc> 	*Jim Biard*
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NOAA National Centers for Environmental Information <http://ncdc.noaa.gov/>
/formerly NOAA’s National Climatic Data Center/
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