Hi Lars,
Thanks for bringing this up. Here is the background, as I understand it, to
the data model description of global attributes.
The conventions do not state rules for combining attributes that appear
both globally and on variables; rightly so, because there is no generically
correct way to do it, even assuming that they are consistent. Which order
to you concatenate strings? What if the attributes are numbers? Bearing
this in mind, and noting that i) variable attributes take precedence
(whatever that means!) and ii) the data model has no concept of a file
(this is deliberate, as one of the aims of the data model is to transcend
the file format), leads to the data model saying that "*netCDF global file
attributes apply to every data variable in the file, except where they are
superseded by netCDF data variable attributes with the same name.*"
There was some of interesting discussion on this, around 6 years ago, in
Trac ticket #95 (starting at
https://cf-trac.llnl.gov/trac/ticket/95#comment:59). The current
discussion, and the one in the Trac ticket, highlights that there are
multiple interpretations of the conventions in common use, and that not all
of these interpretations are suitable for all users.
Even though the data model interpretation has to be as described, this does
not preclude a software implementation of the data model from adding extra
functionality to record the provenance of attributes, or even combing
global/variable attributes in a particular manner. Software does this sort
of thing all the time, for example in remembering netCDF variable names,
which are also not part of the data model. For example, the cf-python
library will note which field properties were global attributes in a file
that it has read, so when you write the field back to disk it can create
the same global attributes (assuming there that no inconsistencies have
been introduced after the read ...).
I hope this is useful,
All the best,
David
On Fri, 15 Mar 2019 at 12:48, B?rring Lars <Lars.Barring at smhi.se> wrote:
> Dear all,
>
> I have come across many CMIP5 files that have the same attribute [name]
> attached to the data variable as found in the globals.
>
> In particular it seems that CMOR was writing variable processing history
> attached to the variable, and more general file processing in the global
> history.
>
> I have also seen that "comment" sometimes occur both as global and as
> variable attribute.
>
> If such a duplication occurs, CF writes:
> "When an attribute appears both globally and as a variable attribute, the
> variable?s version has precedence."
>
> My interpretation of this is that if there are contradictory information
> then the variable's attribute has precedence, but otherwise it does not
> invalidate or overshadows what is in the global attribute.
>
> However, the data model presented in the GMD paper (
> https://www.geosci-model-dev.net/10/4619/2017/ page 4629, [page 11],
> under "4.1 The field construct") presents a more restrictive
> interpretation:
> "In the data model, we consider that netCDF global file attributes apply
> to every data variable in the file, except where they are superseded by
> netCDF data variable attributes with the same name."
>
> I understand this to mean that if the same attributes (e.g. comment or
> history) is present both as global and variable attribute, then the global
> attribute is overshadowed (as if it did not exist at all)?
>
> Is this correct? If so, is this the best solution?
>
> Would not concatenating the information contained in each (putting the
> variable text first) be a more appropriate way to resolve the situation
> (and thus specify in the data model)?
>
>
> Kind regards,
> Lars
>
> --
> Lars B?rring
>
> FDr, Forskare
> PhD, Research Scientist
>
> SMHI / Swedish Meteorological and Hydrological Institute
> Rossby Centre
> SE - 601 76 NORRK?PING
> http://www.smhi.se
>
> E-post / Email: lars.barring at smhi.se
> Tel / Phone: +46 (0)11 495 8604
> Fax: +46 (0)11 495 8001
> Bes?ksadress / Visiting address: Folkborgsv?gen 17
> _______________________________________________
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>
--
David Hassell
National Centre for Atmospheric Science
Department of Meteorology, University of Reading,
Earley Gate, PO Box 243, Reading RG6 6BB
Tel: +44 118 3785183
http://www.met.reading.ac.uk/
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