Hi Jonathan,
Here is the note that resulted in the wording of CF sec 4.1:
On Wed, 12 Dec 2001, Jonathan Gregory wrote:
>
> Returning to the use of latitude and longitude for things which aren't really.
> I am happy with what we decided, but another possible pitfall has occurred to
> me. In the case of a rotated pole, it would be very natural for a data-writer
> to use standard-looking lat and lon axes for the *rotated* lat and lon. If we
> don't want this, we ought to specifically forbid it, I would suggest, by
> adding notes in 4.1 and 4.2 e.g.
>
> Coordinates of latitude with respect to a rotated pole should be given units
> of "degrees", not "degrees_north" or equivalents, because applications which
> use the units to identify axes would have no means of distinguishing such an
> axis from real latitude, and might draw incorrect coastlines, for instance.
> It would also not generally be appropriate to attach an axis attribute to a
> rotated-latitude coordinate variable. Such a variable can be identified by
> a standard_name of "grid_latitude".
I believe that the reason for excluding the use of "axis" is that the
default interpretation of axis="X" is to identify the longitude axis.
Allowing axis="X" to denote either the rotated latitude or longitude axes
is a recipe for confusion and unlikely to be helpful to any application.
Brian
On Mon, Nov 13, 2006 at 08:40:10AM +0000, Jonathan Gregory wrote:
> Dear all
>
> Jeff Cole has asked a question about the interpretation of the axis attribute
> in current version of CF. It's not clear to me either, so I wonder if others
> can remember or clarify the intention.
>
> In CF 4 we say
>
> "Because identification of a coordinate type by its units is complicated by
> requiring the use of an external software package [UDUNITS], we provide two
> optional methods that yield a direct identification. The attribute axis may be
> attached to a coordinate variable and given one of the values X, Y, Z or T
> which stand for a longitude, latitude, vertical, or time axis respectively."
>
> "Coordinate types other than latitude, longitude, vertical, and time are
> allowed. To identify generic spatial coordinates we recommend that the axis
> attribute be attached to these coordinates and given one of the values X, Y or
> Z. We attach no specific meaning to the axis values in this case, but note
> that they may provide a useful hint to an application that plots spatially
> oriented data."
>
> Then CF 4.1 says
>
> "Coordinates of latitude with respect to a rotated pole should be given units
> of degrees, not degrees_north or equivalents, because applications which use
> the units to identify axes would have no means of distinguishing such an axis
> from real latitude, and might draw incorrect coastlines, for instance. It
> would also not generally be appropriate to attach an axis attribute to a
> rotated-latitude coordinate variable."
>
> We don't understand the last exclusion. Why shouldn't rotated-latitude coord
> variables have axis="X"? To attach this would seem to "provide a useful hint
> to an application" about which to plot as the x-axis, and seems inconsistent
> with the recommendation to use the axis attribute for spatial axes even when
> they aren't lat, lon and vertical.
>
> Best wishes
>
> Jonathan
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Received on Mon Nov 13 2006 - 10:49:54 GMT
