Hello Jonathan
I think the second need is the more important. This is the information I really want to capture.
> You suggest listing all the coord or aux coord vars in the cell_methods comment. Would it not be sufficient, and more economical, to give the name of the uncollapsed dimension(s)?
My wariness in this regard is linked to the scope of the variable. I need a variable to be in scope with respect to the data variable to maintain the referencing, particularly if the file is chopped up or reformed. The 'coords' from the input data are no longer in scope for the result data variable unless I add them explicitly; the mechanism available to do this is an ancillary variables.
Once I have a set of ancillary variables, it is easy for me to see that I can use these as part of a domain definition inside a cell method.
If I use the dimension as a short cut for this then I make this scoping less clear, and more difficult to test for. I prefer to be explicit that I am referencing variables which are in scope, even if that means I get a longer domain definition. The short hand of using the dimension name seems to me to be bringing issues of variable scope and variable management onto shakier ground, so I am not sure it is worth the benefit of simplified domain definition.
I have raised a ticket for this addition to the conventions, we can continue the discussion there:
https://cf-pcmdi.llnl.gov/trac/ticket/108
Having reread section 7.3.4 I am beginning to understand what you mean in your first point. However the text appears clear, the aim of using a standard_name as a cell_method name is to identify the dimension:
'''the "name" that appears in a "name: method" pair may be an appropriate standard_name (which identifies the dimension)'''
There is no standard_name which identifies the dimension in my example; I see that a coordinate could be added, but the contents of this would be arbitrary in my case.
> In the absence of these vars, the dimension name in the cell methods isn't informative.
I'm not sure I agree with this statement, i think it is a bit informative, and better than nothing. It provides basic cell_method definition which the domain can augment.
I agree that no change is required to the conventions to enable linking, but I am not convinced that the use of the standard_name syntax from 7.3.4 really helps here.
many thanks for your comments and thoughts
mark
________________________________________
From: Jonathan Gregory [j.m.gregory at reading.ac.uk]
Sent: 09 November 2013 21:07
To: Hedley, Mark
Cc: CF metadata
Subject: [CF-metadata] Cell methods when there are no coordinates
Dear Mark
Thanks for the clarifications in your last email. If I have understood this
correctly now, there are two needs, which might be distinct.
(1) Indicate that a collapsed (size-one) axis is an ensemble axis, although
it doesn't have a coord var or any aux coord vars. I can see that it might
well not have these vars because there might not be anything useful you could
put in them. In the absence of these vars, the dimension name in the cell
methods isn't informative. We could perhaps solve this by introducing a new
standard name such as ensemble_member_id (a string) or ensemble_member_number
(a number) or perhaps both. These could anyway be useful. These standard names
could appear in your cell_methods, following section 7.3.4, instead of the name
of the size-one dimension, indicating that the statistic applied to all the
available members of an ensemble, without needing any coord information. I
don't think this change would require an alteration to the convention.
(2) Point to the coordinate information which applied to the axis before the
collapse. This could be useful for any sort of collapsed axis, and in fact I
think we have discussed it before at some point in the last 15 years! I agree
with your suggestion that it would be logical to record this in cell_methods
as a standardised comment. An alternative would be to add an attribute to the
collapsed coord var, but you don't have those (as you say), and also the
collapse may apply to a combination of axes. You suggest listing all the coord
or aux coord vars in the cell_methods comment. Would it not be sufficient, and
more economical, to give the name of the uncollapsed dimension(s)?
Best wishes
Jonathan
Received on Tue Nov 12 2013 - 06:50:26 GMT
