[gdal-dev] Accessing to a NetCDF subdataset with Python

[Even Rouault]

Le samedi 25 mai 2013 17:47:59, Paolo Corti a écrit :
> Hi list
> 
> I am with GDAL 1.10.
> I thought it was possible to access to a netCDF subdataset in the
> usual manner (using GetRasterBand), but it does not seem to work in my
> 
> local box:
> >>> from osgeo import gdal
> >>> from osgeo.gdalconst import *
> >>> gdal.AllRegister()
> >>> file_nc = 'NETCDF:"sresa1b_ncar_ccsm3_0_run1_200001.nc"'
> >>> ds = gdal.Open(file_nc, GA_ReadOnly)
> >>> ds.RasterCount
> 
> 0

If the netcdf file has only one variable with 2 dimensions, then you can 
directly open the dataset with the filename. If there are more than one 
variable with 2 dimensions, you need to specify the subdataset name 
(logically)

> 
> while, using the subdataset name access syntax, it is working:
> >>> from osgeo import gdal
> >>> from osgeo.gdalconst import *
> >>> gdal.AllRegister()
> >>> file_nc = 'NETCDF:"sresa1b_ncar_ccsm3_0_run1_200001.nc":area'
> >>> ds = gdal.Open(file_nc, GA_ReadOnly)
> 
> Warning 1: Latitude grid not spaced evenly.
> Seting projection for grid spacing is within 0.1 degrees threshold.
> 
> >>> ds.RasterCount
> 
> 1
> 
> Now, do I have a local problem, or the second way is the only way to
> access to a subdataset?

See above answer

> And in such a case, is the only way to programmatically access to the
> subdatasets by reading their name using GetSubdataset or there is a
> better way?

I can't think of a better way.

> 
> >>> ds.GetSubDatasets()
> 
> [('NETCDF:"sresa1b_ncar_ccsm3_0_run1_200001.nc":area', '[128x256] area
> (32-bit floating-point)'),
> ('NETCDF:"sresa1b_ncar_ccsm3_0_run1_200001.nc":lat_bnds', '[128x2]
> lat_bnds (64-bit floating-point)'),
> ('NETCDF:"sresa1b_ncar_ccsm3_0_run1_200001.nc":lon_bnds', '[256x2]
> lon_bnds (64-bit floating-point)'),
> ('NETCDF:"sresa1b_ncar_ccsm3_0_run1_200001.nc":msk_rgn', '[128x256]
> msk_rgn (32-bit integer)'),
> ('NETCDF:"sresa1b_ncar_ccsm3_0_run1_200001.nc":pr', '[1x128x256]
> precipitation_flux (32-bit floating-point)'),
> ('NETCDF:"sresa1b_ncar_ccsm3_0_run1_200001.nc":tas', '[1x128x256]
> air_temperature (32-bit floating-point)'),
> ('NETCDF:"sresa1b_ncar_ccsm3_0_run1_200001.nc":time_bnds', '[1x2]
> time_bnds (64-bit floating-point)'),
> ('NETCDF:"sresa1b_ncar_ccsm3_0_run1_200001.nc":ua', '[1x17x128x256]
> eastward_wind (32-bit floating-point)')]
> 
> >>> sds = gdal.Open(ds.GetSubDatasets()[0][0])
> 
> thanks in advance!
> p

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