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?
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