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 <[email protected]
<mailto:[email protected]>> 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
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National Centre for Atmospheric Science
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National Centre for Atmospheric Science
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Tel: +44 118 378 5613
http://www.met.reading.ac.uk/
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*Research Scholar*
Cooperative Institute for Climate and Satellites NC <http://cicsnc.org/>
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NOAA National Centers for Environmental Information <http://ncdc.noaa.gov/>
/formerly NOAA’s National Climatic Data Center/
151 Patton Ave, Asheville, NC 28801
e: [email protected] <mailto:[email protected]>
o: +1 828 271 4900
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