[CF-metadata] Feedback requested on proposed CF Simple Geometries

Jonathan Gregory j.m.gregory at reading.ac.uk
Wed Feb 1 07:57:47 MST 2017


Dear Chris

Thanks for your comments on my comments. Here are replies to a subset!

> > Your aim is to
> > describe the network alone.
...
>  > You would like to have SOMETHING alone in the file, just to
> > describe the network itself. CF doesn't do this at present (domain without
> > data),
> 
> I don't see a conflict here -- if you can describe the network (geometry)
> then you can associate data with it (UGRID used indexes into cells, nodes,
> etc, this should be equally applicable)
> 
> isn't a set of coordinate variables essentially do that? i.e. you can
> define a rectangular grid -- even if there is no data on it. And you can
> certainly do that with UGRID, which is another standard, but I don't think
> it conflicts with CF.

There isn't a conflict, I agree, but it's not currently possible in CF. That
is because the data variable has all the coordinates attached to it, so you
can't have coordinates without data. Of course it could easily be done, for
instance by providing a dummy data variable which identified the dimensions
but was itself a scalar - that's been discussed before, but no-one's proposed
yet to add it to the convention. It's not a conceptual difficulty, but it is
an addition to the data model.

> >   data:
> >     SOMETHING=2, 4, 3;
> >     lon=0, 1,  0, -1, -2, -3,  2, 3, 4;
> >     lat=51, 52,  51, 50, 50, 49,  55, 55, 56;
> I'm confused about what this is.

There are three linestrings. The SOMETHING variable says how many nodes each
has, and the lon and lat variables are the coordinates of those nodes.

> > For the sake of applications which can
> > read CF but don't understand simple geometries, it might be a good idea in
> > addition to provide a "representative" location for each timeseries, as
> > representive_lat(station) and representative_lon(station), which could for
> > instance be the mean of the node coordinates for each geometry.
> 
> We do that in UGRID, too -- I think it's even required (and called
> coordinates, actually). It may make little sense with complex geometries,
> but it can be handy.

Yes. It is required in CF as well, and the attribute is named coordinates;
I think ugrid follows this.

> The stream network example would be a good one. also things like political
> boundaries -- they tend to be complex polygons with shared vertices.

There's a shared vertex at the confluence of two streams, but I guess those
are a fairly small fraction of the total number of points. With political
boundaries, I agree that most points (not coastlines) will appear twice.

Best wishes

Jonathan



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