[gdal-dev] netCDF signed nodata value

[Simon Lyngby Kokkendorff]

Hello list,

   Just observed a puzzling behaviour of the netCDF driver. When creating a
netCDF dataset of Byte type, the netCDF driver seems to interpret the
nodata value as a signed byte.
 For example a nodata value assigned with -a_nodata 200, will become -56.
Of course one can work around this by simply casting the nodata value (as
e.g. returned by band.GetNoDataValue()) to an unsigned byte.

A simple example:

>gdalinfo --version
GDAL 1.11.2, released 2015/02/10
>gdal_translate -of netCDF -a_nodata 200 something.tif something.nc
>gdalinfo something.nc
Driver: netCDF/Network Common Data Format
Files: something.nc
Size is 10, 10
Coordinate System is `'
Origin = (0.000000000000000,10.000000000000000)
Pixel Size = (1.000000000000000,-1.000000000000000)
Metadata:
  Band1#_FillValue=-56
  Band1#_Unsigned=true
  Band1#long_name=GDAL Band Number 1
  Band1#valid_range={0,255}
  lat#long_name=latitude
  lat#standard_name=latitude
  lat#units=degrees_north
  lon#long_name=longitude
  lon#standard_name=longitude
  lon#units=degrees_east
  NC_GLOBAL#Conventions=CF-1.5
  NC_GLOBAL#GDAL=GDAL 1.11.2, released 2015/02/10
  NC_GLOBAL#history=Mon Oct 26 23:58:09 2015: GDAL CreateCopy( something.nc,
... )
Corner Coordinates:
Upper Left  (   0.0000000,  10.0000000)
Lower Left  (   0.0000000,   0.0000000)
Upper Right (  10.0000000,  10.0000000)
Lower Right (  10.0000000,   0.0000000)
Center      (   5.0000000,   5.0000000)
Band 1 Block=10x1 Type=Byte, ColorInterp=Undefined
  NoData Value=-56
  Metadata:
    _FillValue=-56
    _Unsigned=true
    long_name=GDAL Band Number 1
    NETCDF_VARNAME=Band1
    valid_range={0,255}

Cheers,
Simon Kokkendorff, DMI
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