Hi Nan
... but you couldn't plot this properly because you don't know that it's an ellipse ... which is why I would suggest a very slight modification to add that particular piece of metadata ... (or am I missing something)
Cheers
Bryan
On Thursday 18 February 2010 14:36:05 Nan Galbraith wrote:
> Four coordinates and an area is enough to define an ellipse; if
> these are more complex shapes, that's another problem.
>
> As long as these are ellipses, the existing convention should work; you'd
> give the n/s e/w extremes of the disc in lat(cell), lon(cell) and use
> cell_measures = "area: cell_area" after calculating the area from the
> lengths
> of the major & minor axes.
>
> Maybe my geometry is even rustier than I thought, otherwise this should
> work as it exists in the standard.
>
>
> > The vertices of the cells can be stored in the variable identified by
> > the bounds
> > attribute, but the cell perimeter is not uniquely defined by its
> > vertices (because
> > the vertices could, for example, be connected by straight lines, or,
> > on a sphere,
> > by lines following a great circle, or, in general, in some other way).
>
> - Nan
>
>
> > On Wednesday 17 February 2010 13:36:21 Thomas Lavergne wrote:
> >
> >> Dear all,
> >>
> >> I do not think someone reacted on my concern/question about non-polygonal cell boundaries. Maybe I am the only one with this issue or maybe this topic went un-noticed because of heavy load on the CF list at that time.
> >>
> >> I thus re-post my original message in hope that someone will comment on it (or point me to an archived thread that I did not yet see).
> >>
> >> Original post:
> >>
> >>
> >>> Hi everyone,
> >>>
> >>> I refer to Chapter 7 on "Data Representative of Cells", 7.1 "Cell
> >>> Boundaries".
> >>>
> >>> The specification of those boundaries seems to biased towards
> >>> polygonal boundaries (in the case of a 2D surface). This covers
> >>> certainly most of the needs but what happens if the cell is defined as
> >>> a disc of radius x km (with center at the coordinate value)?
> >>>
> >>> Of course, I can always give 10 to 10,000 vertices that will
> >>> approximate my disc but it does not sound very neat nor efficient. We
> >>> would have to somehow move away from listing the 'bounds' and start
> >>> describing the shape of the cell (disc, ellipse, rectangle, etc...).
> >>> Note that the concepts of "cell measures" and "cell methods" would
> >>> still perfectly hold.
> >>>
> >>> One example of such a dataset would be one where at each grid location
> >>> we report the mean/minimum/maximum temperature or pressure recorded by
> >>> any station found in a radius of, say, 30 km around the central
> >>> point.
> >>>
> >>> Another example is satellite data in swath projection where each
> >>> record is associated to a Field Of View, which is often approximated
> >>> as a an ellipse.
> >>>
> >>> Did someone give it a thought already?
> >>>
> >> _______________________________________________
> >> CF-metadata mailing list
> >> CF-metadata at cgd.ucar.edu
> >> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
> >>
> >>
> >
> >
> >
> >
>
>
--
Bryan Lawrence
Director of Environmental Archival and Associated Research
(NCAS/British Atmospheric Data Centre and NCEO/NERC NEODC)
STFC, Rutherford Appleton Laboratory
Phone +44 1235 445012; Fax ... 5848;
Web: home.badc.rl.ac.uk/lawrence
Received on Fri Feb 19 2010 - 07:09:41 GMT
