[CF-metadata] axis attribute

From: Jonathan Gregory <j.m.gregory>
Date: Wed, 22 Nov 2006 11:27:51 +0000

Dear All

I've argued so far in favour of restricting axis X and Y to 1D lon and lat
coord variables. Now I'd like to argue the alternative of generalising them
to be used with any 1D horizontal coord variables i.e. removing the exclusion
that they can't be used for rotated lon and lat.

For 1D lon and lat, the axis attribute is still redundant while we adhere to
COARDS, because such coord vars must be identifiable by units (and can
optionally also be identified by standard name). However for other horizontal
coordinates the axis attribute is not redundant if generalised. It tells you
that this axis is a horizontal axis. If you had some non-spatiotemporal
axes e.g. realization or land cover type, it might be useful to be told which
of the axes are the horizontal coordinates, since this is not easy to deduce
from the standard names without some intelligence. Is it useful to know?

I think the choice of X and Y is arbitrary. Both of them indicate horizontal,
and the only difference is a preferred choice for plotting, I imagine. Rotated
lon is not necessarily X, for instance, if the pole is tipped over a long way.

I would not favour allowing X and Y on 2D aux coord variables. I think that
in cases where there are 2D horizontal aux coord variables, the X and Y
designations still properly belong to the 1D axes e.g. the projection coords.
Because these axes are thus designated, you can tell which 2D coord vars are
horizontal; they have dimensions belonging to axes which are labelled with
X and Y. Perhaps we should make it mandatory to attach standard_names of
longitude and latitude to 2D auxiliary lon and lat variables so that they can
be easily identified as such.

Cheers

Jonathan
Received on Wed Nov 22 2006 - 04:27:51 GMT