[gdal-dev] gdal_merge and the treatment of nodata value

Andreas Neumann a.neumann at carto.net
Wed Sep 3 10:53:55 EDT 2008

Thanks for having a look at my problem. I am adding more information:

I am merging a number of rgb tiff files (scanned city maps), but they
don't align perfectly at the outer edges. This means that on the edges of
the resulting output file, where the tiff files don't align, I get borders
with nodata values.

To display the data, after it was merged, I use either Quantum GIS or UMN
Mapserver. Both display the values as black. However, I want the borders
to appear as white.


> Andreas,
> I can't answer this without knowing more about your image configuration
> and what applications you are using to judge that the nodata is white.
> Does the image have a colormap?
> What is the nodata value?
> What is the pixel type?
> What are typical image values?
> If the image does not have a colormap, and the nodata value is larger
> than the typical image values, then most applications that don't treat
> nodata values specially will just treat them as white or a more intense
> value than the rest of the image values.
> If the image has a colormap then there may be options available to alter
> the colormap, though this will depend on the file format, and perhaps
> you capacity to write a small script.
> Best regards,
> --
> ---------------------------------------+--------------------------------------
> I set the clouds in motion - turn up   | Frank Warmerdam,
> warmerdam at pobox.com
> light and sound - activate the windows | http://pobox.com/~warmerdam
> and watch the world go round - Rush    | Geospatial Programmer for Rent

Andreas Neumann
Böschacherstrasse 6, CH-8624 Grüt/Gossau, Switzerland
Email: a.neumann at carto.net, Web:
* http://www.carto.net/ (Carto and SVG resources)
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