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
>>
>>
>
>
>
>
--
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* Nan Galbraith (508) 289-2444 *
* Upper Ocean Processes Group Mail Stop 29 *
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Received on Thu Feb 18 2010 - 07:36:05 GMT
