[Gdal-dev] Mosaicing with gdalwarp

Frank Warmerdam warmerdam at pobox.com
Wed Mar 21 22:13:37 EDT 2007

Stephen Woodbridge wrote:
> I'm not sure what the nodata value is and these were tiled out of CCM 
> mrsid files. I seem to remember that there is some option like nearblack 
> that can be used to set the nodata values. Is there a gdal tool that 
> will help me with this?


You should be able to inspect the files with something like OpenEV to
inspect the exact pixel values in nodata areas.

If the files have suffered some "nodata damage" due to lossy compression
in mrsid format, you can use the nearblack program (in gdal/apps) to
convert values near black to exactly black (or near white to exactly white).
The utility is described on the web site, but is not in GDAL 1.4.0, so you
might want to grab source from subversion or use FWTools.

If the nodata values aren't set in the files you can just indicate the
values on the gdalwarp commandline.  If you want to embed the nodata values
in the files, use the -a_nodata flag to gdal_translate.

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    | President OSGeo, http://osgeo.org

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