[CF-metadata] grid cells with a varying number of cell bounds

Jonathan Gregory j.m.gregory at reading.ac.uk
Fri Sep 8 09:24:56 MDT 2017


Dear Karl

I agree, other views are needed; it would be especially useful to hear from
Dave Blodgett and other proposers of the simple geometries. Although I agree
that what you describe is not necessarily inefficient (and in the case you
mention it would be more efficient than the simple geometries), and it's not
a large change to the existing convention, I do think it *is* a change, since
we don't normally permit missing data in boundary variables.

I don't have a strong preference between the methods. What discomforts me is
introducing two methods for doing the same thing. If we did that, we should
give some clear guidance to data-writers about which to choose.

Of course, there is no suggestion that the simple geometries approach should
replace the existing one for cells with polygons that all have the same number
of vertices. Consistent with the last paragraph, I suggest that we should
recommend the use of the existing convention for that case, rather than the
simple geometries convention.

Best wishes

Jonathan

----- Forwarded message from Karl Taylor <taylor13 at llnl.gov> -----

> Date: Fri, 8 Sep 2017 07:12:06 -0700
> From: Karl Taylor <taylor13 at llnl.gov>
> To: cf-metadata at cgd.ucar.edu
> Subject: Re: [CF-metadata] grid cells with a varying number of cell bounds
> User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.11; rv:52.0)
> 	Gecko/20100101 Thunderbird/52.2.1
> 
> Dear Jonathan and all,
> 
> Regarding the options for representing polygon cells with varying
> number of sides (see below), I agree that the "simple geometries"
> proposal can indeed handle this case. ( It of course could also
> handle the case when all grid cells have the same number of sides.)
> 
> I think, however, it might be a mistake to rule out the alternative
> method of storing data on these grids using the old method with cell
> bounds described by their vertices (without the need for
> "containers").  Certainly, I would want to retain the current
> treatment  when dealing with cells having the  *same* number of
> sides.  I also favor an option to use our current method (which is
> however incompletely described, as I've pointed out) to handle grids
> with cells of different shapes.  All we would need to make the
> current method well-described is to specify that the bounds could
> include "missing_values" when the number of grid cell vertices were
> fewer than the maximum specified in the dimension statement (and the
> missing_values would be the last ones in the bounds dimension).
> 
> I would point out that for at least one icosahedral grid (see
> Heikes, R., and D. A. Randall, 1995a: Numerical integration of the
> shallow-water equations on a twisted icosahedral grid. Part I: Basic
> design and results of tests. /Mon. Wea. Rev.,/*123,* 1862–1880), all
> the grid cells except 12 are hexagons.  The 12 exceptions come from
> the "corners" of a cube and are pentagons. This means that the cell
> bounds would contain only 12 missing_values, so not a significant
> problem with wasting storage.
> 
> If we decide to eliminate the "old" option for handling these grid
> cells, we should:
> 
> 1) first poll the modeling groups to see if anyone knows of any
> attempts to use the current CF conventions to store data on such
> grids.  If not, then changing the standard won't have any real
> consequences on legacy datasets.
> 
> 2)  Provide an example in the proposed section 7.5 illustrating how
> an icosahedral grid like the one referenced above would be stored. I
> must say I think few would be able to quickly read section 7.5 and
> be able to successfully store CF-compliant data on such grids;  the
> current method, on the other hand, is easy to understand.
> 
> I have no problem allowing the new method to be used, but wonder if
> most users wouldn't find it easier in the case under consideration
> here, to interpret datasets stored with the older method?  Hope to
> hear other views on this.
> 
> best regards,
> Karl
> 
> 
> >>I certainly always envisaged the possibility of cells of different
> >>shapes, and we imply that this might be the case in the description
> >>of cell bounds with the explicit mention of "maximum":
> >>
> >>/"A boundary variable will have one more dimension than its
> >>associated coordinate or auxiliary coordinate variable./ The
> >>additional dimension should be the most rapidly varying one, and its
> >>size is the maximum number of cell vertices."
> >That's true. Nonetheless we didn't provide for it later on in the document!
> >
> >Well, now we have a choice. Dave Blodgett's proposal has been debated and
> >revised over many months, and I support it in its current form. That is a
> >more general solution, which allows not only mixtures of different sorts of
> >polygons, but also sets of discontiguous polygons (for each cell), polygons
> >with holes in them, and lines. It does not require missing data to be stored
> >in the bounds, because it has a count of how many vertices belong to each cell.
> >Permitting missing data in polygon bounds in sect 7.1 would be a different
> >way of describing a mixture of different sorts of polygons. Providing more
> >than one method to do this would introduce unnecessary complexity. How do you
> >and others think we should choose?
> >
> >
> 

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----- End forwarded message -----



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