[gdal-dev] gdal, netCDF and projection

Mon Mar 16 12:51:44 EDT 2009

Roger André wrote:
> Hi all,
> 
> Just a couple quick netCDF related questions:
> 
> 1)  Is there any way to include projection information in a netCDF
> file so that gdal can properly read it?  It appears that
> georeferencing is handled correctly, but not projection info.  This
> causes problems with the whole "north/south origin shift" between the
> netCDF and other raster formats when gdal is used to convert the .nc
> file into a GeoTIFF.  Most notably, I'm not sure how to assign the
> data a projection - either for display in MapServer, or to reproject
> it to EPSG:4326:  See example below.
> 
> File created with ArcGIS 9.3:
> $ gdalinfo fwdir_bas.nc
> Driver: netCDF/Network Common Data Format
> Files: fwdir_bas.nc
> Size is 42000, 42000
> Coordinate System is `'
> Origin = (65.000000000000000,5.000000000028436)
> Pixel Size = (0.000833333333333,0.000833333333333)
> Metadata:
>   NC_GLOBAL#Conventions=CF-1.0
>   NC_GLOBAL#Source_Software=ESRI ArcGIS
>   Fwdir_bas#long_name=Fwdir_bas
>  Fwdir_bas#esri_pe_string=GEOGCS["GCS_WGS_1984",DATUM["D_WGS_1984",SPHEROID["WGS_1984",6378137.0,298.257223563]],PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]]
>   Fwdir_bas#coordinates=lon lat
>   Fwdir_bas#units=Degree
>   Fwdir_bas#missing_value=
>   lon#long_name=longitude coordinate
>   lon#standard_name=longitude
>   lon#units=degrees_east
>   lat#long_name=latitude coordinate
>   lat#standard_name=latitude
>   lat#units=degrees_north

Roger,

The above example is not projected as far as I can see.  It is
geographic.  So I'm not sure what this was intended to be an
example of.

If I use gdal_translate to convert a utm GeoTIFF file to
Netcdf I get a gdalinfo report like below.

Driver: netCDF/Network Common Data Format
Files: out.nc
Size is 512, 512
Coordinate System is:
PROJCS["NAD27 / UTM zone 11N",
     GEOGCS["NAD27",
         DATUM["North_American_Datum_1927",
             SPHEROID["Clarke 1866",6378206.4,294.9786982139006,
                 AUTHORITY["EPSG","7008"]],
             AUTHORITY["EPSG","6267"]],
         PRIMEM["Greenwich",0],
         UNIT["degree",0.0174532925199433],
         AUTHORITY["EPSG","4267"]],
     PROJECTION["Transverse_Mercator"],
     PARAMETER["latitude_of_origin",0],
     PARAMETER["central_meridian",-117],
     PARAMETER["scale_factor",0.9996],
     PARAMETER["false_easting",500000],
     PARAMETER["false_northing",0],
     UNIT["metre",1,
         AUTHORITY["EPSG","9001"]],
     AUTHORITY["EPSG","26711"]]
Origin = (440720.000000000000000,3751320.000000000000000)
Pixel Size = (60.000000000000000,-60.000000000000000)
Metadata:
   NC_GLOBAL#Conventions=CF-1.0
   NC_GLOBAL#AREA_OR_POINT=Area
   Band1#grid_mapping=transverse_mercator
   Band1#long_name=GDAL Band Number 1
   transverse_mercator#Northernmost_Northing=3.75132e+06
   transverse_mercator#Southernmost_Northing=3.7206e+06
   transverse_mercator#Easternmost_Easting=471440
   transverse_mercator#Westernmost_Easting=440720
   transverse_mercator#spatial_ref=PROJCS["NAD27 / UTM zone 
11N",GEOGCS["NAD27",DATUM["North_American_Datum_1927",SPHEROID["Clarke 
1866",6378206.4,294.9786982139006,AUTHORITY["EPSG","7008"]],AUTHORITY["EPSG","6267"]],PRIMEM["Greenwich",0],UNIT["degree",0.0174532925199433],AUTHORITY["EPSG","4267"]],PROJECTION["Transverse_Mercator"],PARAMETER["latitude_of_origin",0],PARAMETER["central_meridian",-117],PARAMETER["scale_factor",0.9996],PARAMETER["false_easting",500000],PARAMETER["false_northing",0],UNIT["metre",1,AUTHORITY["EPSG","9001"]],AUTHORITY["EPSG","26711"]]
   transverse_mercator#GeoTransform=440720 60 0 3.75132e+06 0 -60
   transverse_mercator#grid_mapping_name=transverse_mercator
   transverse_mercator#latitude_of_projection_origin=0.000000e+00
   transverse_mercator#central_meridian=-1.170000e+02
   transverse_mercator#scale_factor_at_central_meridian=9.996000e-01
   transverse_mercator#false_easting=5.000000e+05
   transverse_mercator#false_northing=0.000000e+00
Corner Coordinates:
Upper Left  (  440720.000, 3751320.000) (117d38'28.21"W, 33d54'8.47"N)
Lower Left  (  440720.000, 3720600.000) (117d38'20.79"W, 33d37'31.04"N)
Upper Right (  471440.000, 3751320.000) (117d18'32.07"W, 33d54'13.08"N)
Lower Right (  471440.000, 3720600.000) (117d18'28.50"W, 33d37'35.61"N)
Center      (  456080.000, 3735960.000) (117d28'27.39"W, 33d45'52.46"N)
Band 1 Block=512x1 Type=Byte, ColorInterp=Undefined
   NoData Value=0
   Metadata:
     NETCDF_VARNAME=Band1

It is my understanding that with some limitations GDAL
is producing NetCDF files conforming to the CF-1.0
conventions, including those for coordinate systems.
We also insert the spatial_ref item with the GDAL WKT
string for more accurate preservation of the coordinate
system.

I just tried removing the spatial_ref from the netcdf
file and I see that GDAL does *not* successfully read back
the coordinate system definition.  Grr.  Well, that is
presumably a bug and should be corrected.

I have filed a ticket on this:

   http://trac.osgeo.org/gdal/ticket/2893

and assigned it to Chaitanya to work on since Denis does not really
have time to maintain the netcdf driver anymore.

> 2)  Is there any continuing work being done in the gdal community with
> regards to the GALEON project?  As far as I can see, all gdal posts
> related to this are from 2005 - 2006, yet recent articles in some
> industry rags suggest that GALEON is still on the OGC's radar.  I'd be
> interested in working on this with anyone else who already is to help
> resolve some of the remaining hiccups that are left in getting netCDF
> files to work more easily with gdal.  Feel free to contact me off-list
> regarding this.

I have not been following the GALEON work much lately though I'm still
on a related mailing list I believe.  I am keen on getting our netcdf
support into good enough shape that we can use it for serving and
consuming netcdf over WCS smoothly.   I just can't spend much of my
time on it.

Best regards,
-- 
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