[gdal-dev] NetCDF generated by GDAL not compliant with CF-1.0 for geographical grids

[Reinoud Bokhorst]

Quoting Frank Warmerdam <warmerdam at pobox.com>:

> Reinoud Bokhorst wrote:
>> Hi,
>> I have a WCS that reads and writes NetCDF using GDAL. The source   
>> NetCDF has a regular lat/lon geographical grid (no projection). The  
>>  NetCDF that is generated following a WCS request (parameters:   
>> BBOX=0.0,55.0,5.0,60.0&SRS=epsg:4326&RESX=0.5&RESY=0.5) has the   
>> following header (ncdump -c):
>>
>> netcdf my_netcdf {
>> dimensions:
>>        x = 10 ;
>>        y = 10 ;
>> variables:
>>        char GDAL_Geographics ;
>>                GDAL_Geographics:Northernmost_Northing = 55. ;
>>                GDAL_Geographics:Southernmost_Northing = 60. ;
>>                GDAL_Geographics:Easternmost_Easting = 5. ;
>>                GDAL_Geographics:Westernmost_Easting = 0. ;
>>                GDAL_Geographics:spatial_ref = "GEOGCS[\"WGS
>> 84\",DATUM[\"WGS_1984\",SPHEROID[\"WGS
>> 84\",6378137,298.257223563,AUTHORITY[\"EPSG\",\"7030\"]],TOWGS84[0,0,0,0,0,0,0],AUTHORITY[\"EPSG\",\"6326\"]],PRIMEM[\"Greenwich\",0,AUTHORITY[\"EPSG\",\"8901\"]],UNIT[\"degree\",0.01745329251994328,AUTHORITY[\"EPSG\",\"9122\"]],AUTHORITY[\"EPSG\",\"4326\"]]"
>> ;
>>                GDAL_Geographics:GeoTransform = "0 0.5 -0 55 0 0.5 " ;
>>                GDAL_Geographics:grid_mapping_name = "Geographics
>> Coordinate System" ;
>>                GDAL_Geographics:long_name = "Grid_latitude" ;
>>        float Band1(y, x) ;
>>                Band1:_FillValue = 0.f ;
>>                Band1:grid_mapping = "GDAL_Geographics" ;
>>                Band1:long_name = "GDAL Band Number 1" ;
>>
>> // global attributes:
>>                :Conventions = "CF-1.0" ;
>> data:
>> }
>>
>>
>> However, it seems that the grid_mapping_name "Geographics   
>> Coordinate System" is not a valid grid_mapping_name in the CF-1.0   
>> convention (only 8 are mentioned in the convention). In CF-1.0 a   
>> geographic grid should be represented like this:
>>
>>
>> netcdf my_netcdf_cf1.0 {
>> dimensions:
>>        y = 10 ;
>>        x = 10 ;
>> variables:
>>        float y(y) ;
>>                y:standard_name = "latitude" ;
>>                y:units = "degrees_north" ;
>>        float x(x) ;
>>                x:standard_name = "longitude" ;
>>                x:units = "degrees_east" ;
>>        float Band1(y, x) ;
>>                Band1:_FillValue = 0.f ;
>>                Band1:long_name = "GDAL Band Number 1" ;
>>
>> // global attributes:
>>                :Conventions = "CF-1.0" ;
>> data:
>>
>> y = 55, 55.5, 56, 56.5, 57, 57.5, 58, 58.5, 59, 59.5 ;
>>
>> x = 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5 ;
>> }
>>
>>
>> Where only the 'units' attribute is required to indicate lat / lon.  
>>  I am not sure if in this case the y and x arrays should start at   
>> 55.0N and 5.0E respectively or at 55.25N and 5.25E.
>
> Reinoud,
>
> I would encourage you to file this as a ticket.  Unfortunately the netcdf
> driver is not currently being well maintained so I don't know if/when the
> problem will be addressed.

Okay, I will do that. In the meantime I had a look at the source code  
and it doesn't seem so complicated to fix. I'll try that.

Regards,
Reinoud






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