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On 11/17/11 2:02 AM, Martin Raspaud wrote:
-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 On 26/10/11 17:44, Tom Rink wrote:Hi Martin, PeterHi Tom, Sorry for the late answer, I had to do a little investing of my onw on this subject...- - Are the longitudes and latitudes values different from what a "vertical perspective" with right parameters would provide ? If they do, why ? If they don't, why not include a grid-mapping definition ?We've defined a netcdf structure for the GOES-R ABI (Advanced Baseline Imager). For navigation, similar to SEVIRI, the ABI defines a fixed grid (FGF) of equal angle spaced view angles from an nominal stationary point in space. The grid can be represented as a cross product of 2 one dimension arrays of the north-south, east-west views angles in radians. The VerticalPerspective CF definition won't work here because it's a type of Map Projection which defines the transform x,y (kilometers for instance)<-> (Lon,Lat). Instead of two 1D arrays of view angles, a 2D array of computed map projection plane values for the FGF views would be required which defeats the purpose of defining the navigation via a transform algorithm. We didn't want rewrite metadata interpretation and visualization components in the software, so we defined a "grid_mapping_type" for the ABI FGF whose "projection_x,y_coordinate" could be radians instead of km. Effectively a projection (radians, raidians)<-> (lon,lat). We did this by extending, actually some modification were required, classes distributed in the ncIdv.jar. See attached netcdf header file. Another nice advantage to the FGF is that master lon/lat files don't change, and can be indexed directly if desired.
Hi Martin, Sorry for the late reply:
The projection you describe here, based on viewing angles, is the one defined as "geos" in proj.4, and comes actually from Eumetsat's description for the meteosat satellites, see here: http://remotesensing.org/geotiff/proj_list/geos.html Maybe we could make a proposal to include this projection in the CF conventions ? In the satellite world, this would make a lot of sense.
I think that's a great idea, since the FGF is based on the CGMS document reference above. I have no idea how to start such a proposal. Tom
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