[CF-metadata] Integerized Sinusoidal Grid

[Brian Eaton]

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
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