Jonathan,
I found Trac Ticket #8, which summarizes even earlier email list
discussions. That's 10 years ago. It seems pretty clear to me from that
ticket that the axis attribute was never intended to be applied to 2D
auxiliary "coordinates".
Grace and peace,
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 <jbi...@cicsnc.org> -----
Date: Fri, 31 Mar 2017 12:44:11 -0400
From: Jim Biard <jbi...@cicsnc.org>
CC: CF Metadata <cf-metadata@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 <jbi...@cicsnc.org
<mailto:jbi...@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
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