[CF-metadata] Geometries in NetCDF Update

David Blodgett dblodgett at usgs.gov
Fri Mar 17 09:06:11 MDT 2017

Dear Jonathan,

OK. This tracks with how I was thinking. 

I was actually thinking this would be section 7.1.1. The rest of my comments about what I think we’ll do in the specification is here (https://github.com/twhiteaker/netCDF-CF-simple-geometry/issues/58 <https://github.com/twhiteaker/netCDF-CF-simple-geometry/issues/58>) for the record.

Thank you so much for all your time on this, I am very pleased with the outcome. 

- Dave

> On Mar 17, 2017, at 10:00 AM, Jonathan Gregory <j.m.gregory at reading.ac.uk> wrote:
> Dear Dave
> Yes, I think you would have a multi-dimensional node_count. I think that fits
> the design, because a DSG can always be thought of as though it were an
> orthogonal array, albeit sparsely filled, with an instance dimension and an
> element dimension. The instance dimension is a discrete axis in the sense of
> CF section 4.5. Instead of this single discrete axis, you could have several
> spatial axes.
> In fact, in the case I mentioned of a polyhedron, I imagine the data array
> might have to be 1D anyway, because you can't generally flatten a polyhedron
> into a rectangular 2D array of faces. So that would be formally just like the
> case of a DSG.
> Until/unless there is a use-case for the application of simple geometries
> outside DSGs we don't need to say anything about this, but I suggest that the
> possibility of its being needed means your new section doesn't belong in ch 9.
> Best wishes
> Jonathan
> ----- Forwarded message from David Blodgett <dblodgett at usgs.gov> -----
>> Date: Thu, 16 Mar 2017 14:19:43 -0500
>> From: David Blodgett <dblodgett at usgs.gov>
>> To: Jonathan Gregory <j.m.gregory at reading.ac.uk>
>> CC: cf-metadata at cgd.ucar.edu
>> Subject: Re: [CF-metadata]  Geometries in NetCDF Update
>> X-Mailer: Apple Mail (2.3259)
>> 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?
>> Regards,
>> - 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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> ----- End forwarded message -----
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