[CF-metadata] identification of vector components

Hedley, Mark mark.hedley at metoffice.gov.uk
Tue Apr 24 10:42:15 MDT 2012


Dear Jonathan et al.

I appreciate the concerns and I agree that the current situation provides the information required, however I do not feel comfortable with the way it is done: I think it is open to ambiguity and I am concerned that there are multiple standard names for what I consider the same phenomenon.

My perspective is that standard name should be independent of coordinate definitions: I think this can be achieved without impacting existing datasets and I think it would be a progressive step, adding clarity to interpretation of vector quantities.

> Can similar tests be used with Grib conversion? I assume that Grib must provide sufficient metadata to indicate whether the grid is lon-lat.

Yes and your assumption is correct, there is sufficient information to deliver interoperability, but implementing this is adding complexity which I feel is not necessary and may cause future issues maintaining the special cases.  One of the reasons I raise GRIB is that it treats vector component as one phenomenon, as I have described, not two as is done in CF currently.

all the best
mark

p.s. I am away on holiday from today, so I will pick up this discussion on my return

-----Original Message-----
From: cf-metadata-bounces at cgd.ucar.edu on behalf of Jonathan Gregory
Sent: Tue 24/04/2012 16:01
To: cf-metadata at cgd.ucar.edu
Subject: Re: [CF-metadata] identification of vector components
 
Dear Mark

I see your point of view, and when we discussed this on the phone it did not
sound like a large issue to me. I agree with you that X/Y and lon/lat are
related ideas in CF. However, it appears there are some concerns.

As you say, as a data-producer, where the vector component is aligned to the
data grid you have to use a different standard name according to whether the
grid is lon-lat or not. However, there are other reasons why you have to be
aware of this too. Specifically, to write a CF-compliant file, if the grid
is not lon-lat you have to provide 2D lon and lat aux coord vars. If the grid
is lon-lat you won't provide these. The program writing the data must be aware
of the distinction, and can use it to decide which standard names to use.

If the file is correctly written then, as a data-consumer, you can decide
whether eastward/northward would be identical with x/y simply by inspecting the
standard_names of the coordinate variables.

Can similar tests be used with Grib conversion? I assume that Grib must provide
sufficient metadata to indicate whether the grid is lon-lat.

Best wishes

Jonathan
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