Hi Julien,
I agree the grid you describe is an example of what CF refers to as a
reduced grid. Generally the cells in a given latitude circle of a reduced
grid are all the same size, so the cell edges will not be aligned with the
cells in adjacent latitude bands that contain a different number of cells.
In addition to Jonathans suggestion of a bin_area variable, the vertices of
the bins can be attached to the lat/lon arrays using the bounds attribute.
That would look like this:
dimensions:
Data_Bins = 5965 ;
nver = 4;
variables:
short mean(Data_Bins) ;
mean:long_name = "Arithmetic mean of the geophysical variable for each bin" ;
mean:units = "mg m-3" ;
mean:coordinates="lat lon";
mean:cell_measures="area: bin_area";
float lat(Data_Bins);
lat:units="degrees_north";
lat:bounds="lat_bnds";
float lon(Data_Bins);
lon:units="degrees_east";
lon:bounds="lon_bnds";
float bin_area(Data_Bins);
bin_area:units="m2";
float lat_bnds(Data_Bins,nver);
float lon_bnds(Data_Bins,nver);
CF places the restriction that the vertices should be ordered
anticlockwise when viewed from above, but places no restriction on the
starting vertex.
Brian
On Sun, Mar 19, 2006 at 08:15:07PM +0000, Jonathan Gregory wrote:
> Dear Julien
>
> > Its characteristics are that on each row of latitude there is an integer
> > number of longitude bins. Poles always represented as 3 triangular bins.
> > Bins aren't aligned between rows.
> > In SEAWiFS/MODIS Level 3 HDF only bins (pixels) with data are stored,
> > and the geolocation information for each bin is only its indice in the
> > global integerized sinusoidal grid (first bin is on south, last on the
> > north).
> >
> > The only mecanisms I see in CF to do that are:
> >
> > - "5.3 Reduced horizontal grid": but:
>
> Yes, I think your case is most like this case.
>
> > Are bins always aligned ?
> You have to specify the latitude and longitude of each bin (grid point).
>
> > How can I know/compute the coverage of each pixel (its vertex) ?
> I am not sure what you mean. Do you mean the area of the grid box? This can
> be stored using cell_measures (CF 7.2).
>
> > Is this mechanism largely used ?
> > Are there free tools to see this kind of CF files ? (FERRET ?)
> I don't know whether Ferret will handle it. The intention of the convention
> is that a generic application will be able to interpret the lat and lon
> information without understanding the grid itself.
>
> dimensions:
> Data_Bins = 5965 ;
> variables:
> short mean(Data_Bins) ;
> mean:long_name = "Arithmetic mean of the geophysical variable for each bin" ;
> mean:units = "mg m-3" ;
> mean:coordinates="lat lon";
> mean:cell_measures="area: bin_area";
> float lat(Data_Bins);
> lat:units="degrees_north";
> float lon(Data_Bins);
> lon:units="degrees_east";
> float bin_area(Data_Bins);
> bin_area:units="m2";
>
> You would have to obtain the values of lat and lon by looking them up in the
> general grid by using the bin number, I imagine.
>
> You are right, there is no mechanism for describing your integerised sinusoidal
> grid at present. If you wish to include a description of the grid construction
> in the file, maybe it could be done by providing a new entry for Appendix F.
>
> Best wishes
>
> Jonathan
> _______________________________________________
> CF-metadata mailing list
> CF-metadata at cgd.ucar.edu
> http://www.cgd.ucar.edu/mailman/listinfo/cf-metadata
Received on Sun Mar 19 2006 - 14:08:54 GMT
