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

Jonathan Gregory j.m.gregory at reading.ac.uk
Wed Sep 6 09:06:10 MDT 2017


Dear Karl

> Consider a grid consisting of both triangular and rectangular grid
> cells with n total grid cells.  In this case the cell_bounds for
> longitude could be dimensioned (n, 4) because there are a maximum of
> 4 cell vertices.
> 
> My question is, for the triangular grid cells do we *require* the
> 4th element of the bounds to be set to missing_value?  If not, how
> does software decide how many vertices each grid cell has?

As Chris says, I don't think we envisaged a mixture of different sorts of
polygons. The text at the end of sect 7.1 speaks of "p-sided cells" with
"p the number of vertices of the cells". It doesn't suggest that p could
be multivalued. We could clarify that with a defect ticket.

The proposal by Dave Blodgett and others for simple geometries
https://cf-trac.llnl.gov/trac/ticket/164
https://github.com/dblodgett-usgs/cf-conventions/blob/7768e33e7edff459482e8ef8057ea6b8e015c9eb/ch07.adoc
does allow for a mixture of different polygons to define the cells of a grid.

> Also, in H.2.3 examples H.4 and H.5, missing_value is defined for
> the time coordinate, time(time).  I don't think coordinates are
> allowed to include missing values.  Is this a "defect" in our
> document?

No. It's not normally allowed, but it is permitted for unused elements in
discrete sampling geometries. See section 9.6.

Best wishes

Jonathan



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