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 > >> [email protected] > >> 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 _______________________________________________ CF-metadata mailing list [email protected] http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
