Dear Tim
CF standard names require canonical units, so it would not be possible to have
a standard name for a data variable whose elements did not all have the same
units. I suspect that in various places we have tacitly assumed that units is
a scalar string, as you say.
Your question is a bit like other ones which have come up about how we might
group various data variables that belong together, such as elements of a
vector, which also might not have the same units (if in a radial coordinate
system, for example) and have different standard names. We haven't agreed a way
to do that, but there have been ideas about using another variable to contain a
list of variable names. Maybe you could use a method like that.
Best wishes
Jonathan
----- Forwarded message from Timothy Patterson <Timothy.Patterson at eumetsat.int> -----
> Date: Wed, 6 Apr 2016 17:07:33 +0000
> From: Timothy Patterson <Timothy.Patterson at eumetsat.int>
> To: "cf-metadata at cgd.ucar.edu" <cf-metadata at cgd.ucar.edu>
> Subject: [CF-metadata] Multiple units attribute for an array of polynomial
> coefficients
>
> I have a set of measurement data to which I fit a polynomial and store the coefficients in a single netCDF file along with other information.
>
> The degree of the polynomial is not always the same, so for flexibility I planned to store the coefficients in a single array variable rather than as separate variables so that I have one format that handles any degree of polynomial.
>
> As I'm fitting measurement data, the coefficients have associated units. If I stored the data as individual variables, I could attach these values via a "units" attribute, but I'm storing the values in an array. However, an attribute can be a vector of values so in theory it could be possible to define a vector "units" attribute of the same length as my array dimension and thereby associate a different unit value to each coefficient.
>
> I think this probably violates the CF conventions and that when the conventions state - "The value of the units attribute is a string" - that I should interpret that as meaning a single string and not a vector of strings.
>
> Is there a better and CF-compliant way to handle this, or should I just resign myself to the fact that if I want the flexibility of an array variable, the users will have to work out the units for themselves?
>
> Thanks!
>
> Tim
>
>
> _________________________________________________________________________________________
> Dr. Tim Patterson
> Instrument Data Simulation Expert
> Product Engineering/Test Data Coordination
> MTG Programme
> GEO Division
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Received on Thu Apr 07 2016 - 08:33:43 BST
