[CF-metadata] axis attribute

Karl Taylor taylor13 at llnl.gov
Tue Apr 4 16:41:22 MDT 2017


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/
>>>>     --     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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> /formerly NOAA’s National Climatic Data Center/
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