Hi Bob,
I agree that your interpretation was our original intent when the axis
attribute was included in CF. However I'm afraid we opened Pandora's Box
by allowing axis to be used for "generic spatial coordinates", as described
in the 3rd paragraph of section 4. In this relaxed interpretation it does
make sense to use axis="X" on a coordinate with the standard_name
projection_x_coordinate. In retrospect I'm starting to think that this was
not one of our better ideas :-) By allowing axis to refer to generic
coordinates you loose the ability to know that axis="X", when present, does
in fact identify a longitude coordinate. In its current state the only
information that can be derived from the axis attribute is that is provides
a hint to the processor about how a coordinate might be plotted.
It should probably be returned to it's original interpretation which you
described. That was also the opinion voiced by Karl, and Jonathan too (I
think, I've lost track of some of this thread).
Brian
On Mon, Nov 20, 2006 at 12:45:25PM -0800, Bob Drach wrote:
> CDAT uses the axis attribute to identify the spatio-temporal type of
> axis. (Since the axis attribute is optional, CDAT also checks the
> units). My interpretation of the standard is that, if the axis is a
> longitude, then axis="X" is appropriate, and this applies to
> auxiliary longitude coordinate axes as well as 1-D lon(lon)
> coordinate variables. However, it would not apply to
> projection_x_coordinate, if I understand that attribute correctly.
>
> Bob
>
>
> On Nov 19, 2006, at 6:05 PM, Simon Wood wrote:
>
> >Karl, Jonathan and others,
> >
> >>I'll reiterate what I said earlier (but I think somehow the email
> >>didn't get distributed). I'm in favor of eliminating the
> >>apparently contradictory statements. Of the two choices, perhaps
> >>least confusion would result from adopting Jonathan's suggestion:
> >>restrict X and Y to genuine longitude and latitude coordinates.
> >
> >By 'genuine longitude and latitude coordinates' would you intend to
> >exclude auxiliary (lat/lon) coordinates (eg lat(y,x) and lon(y,x)),
> >and allow only for datasets with lat lon dimensions (and hence lat
> >(lat) and lon(lon) coordinate variables)?
> >
> >I'm a bit confused since I originally interpreted CF 4 to mean that
> >the axis attribute as an extra way of identifying a coordinate
> >variable. I assumed it would be good to use it with auxiliary
> >coordinate variables for datasets using x,y dimensions and, say,
> >lambert conformal projection. But then by analogy to the
> >discussion about rotated pole projections on the earlier part of
> >this thread I got to thinking that maybe the axis attribute should
> >be associated with the x(x) and y(y) 'projection_x/y_coordinate'
> >variables instead (ie so it can be used as a hint to the plot
> >orientation).
> >
> >However, it seems to me that restricting the attribute to only be
> >used for 'proper' coordinate variables (ie lat(lat), lon(lon)) is a
> >bit drastic (and it doesn't really add much value to these
> >variables does it?). All thats really needed is to resolve the
> >ambiguity in intended usage (ie is it to hint at plot orientation
> >or to help identify coordinate variables including auxiliary
> >coordinate variables). Of course if both interpretations are in
> >common use then it gets a bit trickier...
> >
> >The reason I was intending to use the axis attribute was just in an
> >attempt to include as much metadata as possible so that (hopefully)
> >a wider range of data readers would be able to understand our data;
> >I guess if its just going to add confusion I won't use it. Does
> >anybody know of data readers that currently rely on the axis
> >attribute in any way?
> >
> >regards
> >
> >Simon
> >
> >--
> >Simon Wood
> >National Institute of Water and Atmospheric Research, Wellington, NZ
> >simon.wood at niwa.co.nz
> >http://www.niwa.co.nz
> >
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Received on Mon Nov 20 2006 - 14:25:33 GMT
