Dear Tom
It would be a coordinate variable if it's location(location) and in that case
its values have to be unique and monotonic because generic applications
may make that assumption when plotting or doing computations with them. If
these are really labels then those operations don't really make sense anyway.
In that case it would be more suitable to store the IDs in an auxiliary
coordinate variable, which doesn't have the same name as the dimension, and
is listed in the coordinates attribute of the data variable. Values in aux
coord variables do not have to be unique or monotonic or even numeric. It is
not mandatory to include a coordinate variable (in the Unidata sense).
Best wishes
Jonathan
----- Forwarded message from Tom Evans <Tom.Evans at niwa.co.nz> -----
> Date: Mon, 8 Jul 2019 01:51:19 +0000
> From: Tom Evans <Tom.Evans at niwa.co.nz>
> To: "cf-metadata at cgd.ucar.edu" <cf-metadata at cgd.ucar.edu>
> Subject: [CF-metadata] Indices or Labels as Coordinate Variables
>
> I have a question regarding coordinate variables:
>
> I am working with time-series data representing hydrological conditions at fixed locations in a stream network. The values are generated by a model at regular time intervals, and I believe that the data will fit well into the timeSeries feature type described in the ?Discrete Sampling Geometries? chapter of the CF conventions. For example, we would put all the discharge values into a single 2D array:
>
> double flow(time, location)
>
> The dimension ?location? here is the ?instance dimension? described in the convention.
>
> I would like to use an integer variable named ?location? as a coordinate variable to go along with the location dimension. I think this would provide a handy way for post-processing programs to locate a time series in our model result files. The Best Practices guidance on the Unidata website, though, says that coordinate variables ?must be strictly monotonic? and the order of the IDs in my location variable is arbitrary. All of the location values are unique, but the location numbers are essentially numerical labels ? location 1524 is distinct from location 2817, but neither is greater than the other in a way that means anything to the model. Location IDs do not consistently increase or decrease traveling downstream, for example.
>
> So, is the guidance that coordinate variable should strictly increase or decrease relevant to my case? I?ve built some sample files and examined them using Panoply, and in Python using xarray. I haven?t seen any problems with using non-monotonic integer ?ID numbers? as coordinate variables, but that ?must? in the guidance troubles me. If my locations are identified by arbitrary numbers, do I run the risk of scrambling the links between my time series and their identifiers?
>
> Thanks
> Tom Evans
>
>
>
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>
> Dr Tom Evans
> Software Developer
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>
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----- End forwarded message -----
Received on Mon Jul 08 2019 - 06:42:21 BST
