[CF-metadata] axis attribute

From: Jonathan Gregory <j.m.gregory>
Date: Mon, 13 Nov 2006 08:40:10 +0000

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
Received on Mon Nov 13 2006 - 01:40:10 GMT