[CF-metadata] Pre-proposal for "charset"

Jonathan Gregory j.m.gregory at reading.ac.uk
Tue Mar 7 10:03:15 MST 2017


Dear Chris

> We need to be "clear" about what we mean by "the intent is clear". I think
> that much of the point of CF is to be as explicit as possible, -- i.e. the
> reader of a CF file should not have to know anything about how given data
> tends to be used in order to determine what data type an array should be
> (or what shape it should be).

Yes, I agree with that. However, if you're reading a CF file, you aren't
just reading plain variables. If you're using/writing software which knows
how to interpret the file following the CF convention, it should know what
the "intent" is, in a CF context, of each of the variables of interest.
For example, you know that an auxiliary coordinate variable of char data must
be a vector of strings, and the trailing or only dimension is the max string
length. If you came across this variable when scanning all the variables in
a netCDF file, with no interest in CF, you wouldn't know that it was an array
of strings, but if you are using it as a CF aux coord var, you do know that,
so I don't think any further signal is needed - it would be redundant.

Best wishes

Jonathan

----- Forwarded message from Chris Barker <chris.barker at noaa.gov> -----

> Date: Mon, 6 Mar 2017 11:16:35 -0800
> From: Chris Barker <chris.barker at noaa.gov>
> To: Jonathan Gregory <j.m.gregory at reading.ac.uk>
> CC: "cf-metadata at cgd.ucar.edu" <cf-metadata at cgd.ucar.edu>
> Subject: Re: [CF-metadata] Pre-proposal for "charset"
> 
> On Mon, Mar 6, 2017 at 9:47 AM, Jonathan Gregory <j.m.gregory at reading.ac.uk>
> wrote:
> 
> > Yes, we can reopen the ticket. I think the _Encoding for char is a good
> > idea,
> > especially if it's an NUG convention.
> 
> 
> so let's do that part at least.
> 
> > Are there any files out in the wild that DO use ND arrays of NC_CHAR that
> > > are not intended to be interpreted as a (N-1)D array of Strings?
> >
> > That is the question. In particular, since this the CF convention we're
> > talking about, are there any char arrays which are part of CF,
> 
> 
> indeed.
> 
> 
> > where the
> > intent is not clear?
> >
> We need to be "clear" about what we mean by "the intent is clear". I think
> that much of the point of CF is to be as explicit as possible, -- i.e. the
> reader of a CF file should not have to know anything about how given data
> tends to be used in order to determine what data type an array should be
> (or what shape it should be).
> 
> I saw this an an author of sometimes generic tools -- the tool should be
> able to read the file, and produce the appropriate native array for the
> task at hand, without knowing something like: "ahh, this is the ID of a
> Acme-ocean-widget -- those use char IDs -- so this must be a char" --
> Humans can do that -- software can't (not easily anyway!)
> 
> And clearly specifying whether a char array is a char array or a string
> array will better unify netcdf3 and netcdf4.
> 
> netcdf4 can be explicit about it -- netcdf3 can't -- so it'd be nice if CF
> could fill that gap.
> 
> Now that I think about it, this really should be a netcdf convention --
> like _FillValue, but this is a CF list....
> 
> -CHB
> 
> -- 
> 
> Christopher Barker, Ph.D.
> Oceanographer
> 
> Emergency Response Division
> NOAA/NOS/OR&R            (206) 526-6959   voice
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> 
> Chris.Barker at noaa.gov

----- End forwarded message -----



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