[CF-metadata] axis attribute

Jim Biard jbiard at cicsnc.org
Tue Apr 4 16:43:58 MDT 2017


Karl,

Don't allow the attribute on 2D variables. :) I feel like it's a pretty 
far stretch to get to your 2D example.

Jim


On 4/4/17 6:41 PM, Karl Taylor wrote:
> Hi all,
> I don't think the issue of 2-d auxiliary coordinates entered the 
> discussion leading to their allowance by CF 1.6 (but I only quickly 
> reviewed the discussion).  I think that allowing the axis attribute to 
> be attached to an auxiliary coordinate that is 1-d can be useful 
> (e.g., when a balloon records temperature as a function of time and we 
> want to record its lat and lon positions as a function of time; one 
> could conceivably want to plot the temperature as a function of 
> latitude and/or longitude, with one or the other of them providing the 
> positions along a coordinate axis).
>
> I agree that saying lat(x,y) is a "y axis" seems rather odd, but if 
> you consider each x,y pair to define an index, then it might be 
> tolerable to say they each could be regarded as parametric axes 
> defined as a function of an index.
>
> In both cases, of course, the axis values may not be monotonic, so 
> they wouldn't be considered coordinates axes themselves.
>
> It's really not a pretty situation.  Not sure what can be done about it.
>
> best regards,
> Karl
>
>
> On 4/4/17 1:49 PM, Jim Biard wrote:
>>
>> 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/
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>>>>>     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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