[gdal-dev] Fillvalue set incorrectly on gdal_translate to NetCDF for Byte

[Julian Zeidler]

Am 27.09.2017 um 12:27 schrieb Even Rouault:
>
> On mercredi 27 septembre 2017 10:00:23 CEST Julian Zeidler wrote:
>
> > Hi all,
>
> >
>
> > I noticed, what I believe is an Error when converting a Byte Image from
>
> > geotiff to Netcdf. In the newest Gdal Version (trunk from Monday),  it
>
> > reports the correkt NoData Value (in older gdal Version  i tested this
>
> > 1.11 this was also incorrekt) but it reports the _FillValue as -1 while
>
> > reporting that the Dataset is _Unsigned. This looks like the FillValue
>
> > is written as a signed byte (For Na=254 it reports fillvalue=-2).
>
> >
>
> > Is this difference between noData and _FillValue intentional? And is
>
> > there any way to "fix" the FillValue in my Netcdf ?
>
> >
>
> > I ran  gdal_translate example.tif example.nc -of netCDF
>
> >
>
> > The tiff reports the (korrekt) NaValue as :
>
> >
>
> > Band 1 Block=2000x4 Type=Byte, ColorInterp=Gray
>
> >   NoData Value=255
>
> >
>
> > While in the NetCDF I get :
>
> >
>
> > Band 1 Block=2000x1 Type=Byte, ColorInterp=Undefined
>
> >   NoData Value=255
>
> >   Metadata:
>
> >     grid_mapping=transverse_mercator
>
> >     long_name=GDAL Band Number 1
>
> >     NETCDF_VARNAME=Band1
>
> >     valid_range={0,255}
>
> >     _FillValue=-1
>
> >     _Unsigned=true
>
> Julian,
>
> this is an artifact of the old netCDF3 format (which is the default 
> when reating a file with GDAL), which doesn't support unsigned byte, 
> but signed byte. So GDAL has to use hacks to put unsigned byte to 
> signed byte.
>
> It sets a Band1:_Unsigned = "true" metadata, and then on reading, it 
> knows that it must interpret signed as unsigned. The _FillValue=-1 
> just reflects that. That is what you will get if you use the "ncdump" 
> utility. I'm ambivalent if GDAL should "fix" it or not.
>
> Recent versions of GDAL do fix the NoData value since that's what 
> users should use. The _FillValue is more an internal implementation 
> detail.
>
> As netCDF 4 is quite widespread now, you could also just do
>
> gdal_translate example.tif example.nc -of netCDF -co FORMAT=NC4
>
> and then everything will be unsigned.
>
> Even
>
> -- 
>
> Spatialys - Geospatial professional services
>
> http://www.spatialys.com
>
Hi Even,

Thanks for the quick reply.

I was aktually running it with -co FORMAT=NC4, but on Version 2.11.3, 
where there was no difference with or without this option. So i 
simplified it when testing on trunk. My Bad. Will migrate to a more 
recent Version.

Thanks

Julian

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