Re: GMU WCS Server supports netCDF output

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Frank Warmerdam wrote:

On 8/25/05, yliu5@xxxxxxx <yliu5@xxxxxxx> wrote:


   getCoverage (9.6M bytes)
http://data.laits.gmu.edu/cgi-bin/tsunami/wcs100?version=1.0.0&service=WCS&request=getCoverage&coverage=/data4/TSUNAMI/L71131056_05620041229_B10.hdf:Grid:L71131056_05620041229_B10:L71131056_05620041229_B10&crs=EPSG:32647&bbox=23400,534000,267000,747000&resx=300&resy=300&format=netCDF



Yang Liu,

The ncdump of this result looks like:

netcdf wcs100 {
dimensions:
        xdim = 813 ;
        ydim = 711 ;
variables:
        double xdim(xdim) ;
                xdim:standard_name = "projection_x_coordinate" ;
                xdim:units = "m" ;
        double ydim(ydim) ;
                ydim:standard_name = "projection_y_coordinate" ;
                ydim:units = "m" ;
        int projection ;
                projection:grid_mapping_name = "transverse_mercator" ;
                projection:scale_factor_at_central_meridian = 0.9996 ;
                projection:longitude_of_central_meridian = 99 ;
                projection:latitude_of_projection_origin = 0 ;
                projection:false_easting = 500000 ;
                projection:false_northing = 0 ;
        double lon(ydim, xdim) ;
                lon:standard_name = "longitude" ;
                lon:units = "degrees_east" ;
                lon:long_name = "longitude" ;
        double lat(ydim, xdim) ;
                lat:standard_name = "latitude" ;
                lat:units = "degrees_north" ;
                lat:long_name = "latitude" ;
        byte L71131056_05620041229_B10(ydim, xdim) ;
                L71131056_05620041229_B10:long_name
"L71131056_05620041229_B10" ;
                L71131056_05620041229_B10:coordinates = "lat lon" ;
                L71131056_05620041229_B10:grid_mapping = "projection" ;

// global attributes:
                :institution = "LAITS, GMU" ;
                :Conventions = "CF-1.0" ;
...

Is the set of projection parameters standard? Currently the GDAL NetCDF
driver does not seem to support your coordinate system conventions. In fact, I find it odd that you list projection parameters but you don't 
give any bounds or other information about the image in that projection.
I think we need a way of encoding the projected origin and offset
vector information in the netcdf result.  You obviously have it for
generating the coverage description. 
Best regards,



Hi Frank:
Actually the above is correct for the CF conventions, see Appendix F (the projection parameters were originally taken from FGDC).
The xdim and ydim coordinates specify the coordinates in projection space, this is more general than a projected origin and offset vector, allowing non-uniform spacing. For this data, the coordinates are uniformly spaced (.6 km), and the projected origin and bounds are easy to compute if you know how to construct the projection function. If not, then you have to fall back on using the lat,lon coordinates which are very often curvilinear.
I guess the point is that the above is how we are used to seeing coordinate systems represented in these kinds of datasets. To summarize, from my experiencce anyway:
1. The x,y coordinates are orthogonal, but not necessarily uniformly spaced, in some projection. Typically the units are "km on the projection plane".
2. The projection function is typically parameterized, rather than being a predefined "well-known" EPSG. In practice, there are a dozen or so projections that are used. Standards like CF specify the names and meanings of the projection and its parameters.
3. Knowing the projection and the coordinates fully specifies the georeferencing. I personally prefer this to having the 2D lat,lon coordinates. 



John.
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