[CF-metadata] CF compliant tripolar grid representation

Jim Biard jbiard at cicsnc.org
Fri Mar 31 10:44:11 MDT 2017


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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> -- 
> 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/

-- 
CICS-NC <http://www.cicsnc.org/> Visit us on
Facebook <http://www.facebook.com/cicsnc> 	*Jim Biard*
*Research Scholar*
Cooperative Institute for Climate and Satellites NC <http://cicsnc.org/>
North Carolina State University <http://ncsu.edu/>
NOAA National Centers for Environmental Information <http://ncdc.noaa.gov/>
/formerly NOAA’s National Climatic Data Center/
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e: jbiard at cicsnc.org <mailto:jbiard at cicsnc.org>
o: +1 828 271 4900

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