[CF-metadata] axis attribute

[Jonathan Gregory]

Dear Simon

> Reading this has made me realize I may have a similar confusion with 
> regards to other map projections eg Lambert conformal.  For these should 
> the 'axis=X' attribute be attached to the grid x coordinate variable 
> (x(x), ie the one we give the standard name 'projection_x_coordinate') 
> or the longitude auxiliary coordinate variable (lon(y,x))?  Or should it 
> not be used at all in this situation?

My reading of what the standard currently says is that you can use the axis
attribute if you want to, in order to suggest which axis is the plot x-axis,
but you don't have to, and it doesn't carry any specific meaning.

It does seem peculiar to me to allow it for the Lambert conformal but not for
the rotated pole, doesn't it! Also, it is really not much information. An
application could just assume the right-hand axis (in CDL notation) is the
one intended to be the x-axis.

I can't remember what advantage we saw in allowing X and Y to be used for
generic horizontal coordinates. At present I'd propose that we allow X and Y
only for genuine longitude and latitude. Any comments, anyone?

Cheers

Jonathan