[CF-metadata] Geometries in NetCDF Update

David Blodgett dblodgett at usgs.gov
Thu Mar 16 13:19:43 MDT 2017

Dear Jonathan,

Thank you for looking things over.

Makes sense that we would add a new ‘geometry’ attribute that would point at the geometry container variable and be in both the node coordinates and the data variables.

Yes, the 0 or 1 is meant to be inside or outside. We thought that would be specified in the document. I hadn’t thought of it as a true/false, but 0 and 1 are nice because they can be treated that way. We could also use O and I, but I’m hesitant to use characters, where an integer will suffice. We will choose a suitable name that allows the variable to be interpreted as a true/false. Your suggestion is a good starting point, I’ll see how Tim feels about it.

I spent a lot of brain cycles trying to figure out how a “count”ed contiguous ragged array of nodes could be made to apply to a grid. Would you have a multi-dimension count variable? I guess I’d need to see the cdl structure to understand that case. I’m stuck on the “count” needing to be on the instance dimension. 

Tim and I will reconcile some of this in the README as well as a more detailed wiki writeup in that GitHub repository then submit the Trac ticket with a proposed section for the spec unless others have input we should consider?


- Dave

> On Mar 16, 2017, at 12:28 PM, Jonathan Gregory <j.m.gregory at reading.ac.uk> wrote:
> Dear Dave
> Thanks for this
>> https://github.com/twhiteaker/netCDF-CF-simple-geometry/blob/master/README.md
> I think your explanation of what is needed and why is very clear, and I like
> the present form of the proposal.
> You asked me what I'd expect the bounds attribute to point to. This is my only
> reservation about the current design. We agree that the simple geometries are
> a kind of complicated bounds specification. However, we don't have to use the
> existing bounds att with a new purpose because of that. It currently points to
> a variable which must be numeric and have particular dimensions; hence one
> could tell reliably if it was a simple geometry instead if we insist that the
> geometry container variable must be a scalar, for example. So this does work,
> but to make it work would require changing existing software, which otherwise
> would think this new convention is an error. That wouldn't be friendly.
> Therefore I would suggest *not* using the bounds att of the aux coord vars, but
> instead giving them a new att e.g. geometry, to point to the geometry container
> variable. We can make a rule that it's an error to have both a geometry and a
> bounds att, like we do with climatology and bounds att.
> I'd also suggest requiring the data variable to have the same geometry att.
> That's because CF is generally "centred" on the data variable. If you've found
> the data variable, it's convenient to go straight from there to the geometry
> spec, rather than having to go via the aux coords. However I can see that you
> also want to attach it to the coordinates, since you might want to describe a
> geometry without a data variable at all but with nominal coordinates (although,
> at present, CF always expects there to be data variables).
> Arising from your discussion with Dan, I suppose you could say that if there
> is no part_node_count it must imply there is only one part per instance i.e.
> you can omit this attribute if it's not needed.
> I assume that 0 means inside and 1 outside for the polygon_ring_type. If it's
> a binary choice like that, maybe it would be more self-explanatory to call it
> outside, or something else which indicates the convention.
> I would note that this new convention is applicable for data on a grid too
> i.e. on a polyhedron which is composed of more than one type of polygon. Its
> usefulness is not limited to DSGs.
> Best wishes
> Jonathan
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