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Using the GDAL Georeferencer QGIS 3.10.10 I have georeferenced an old map, so now it is rotated a bit. The NoData fields for the "frame" are now black (RBG = 0,0,0). I would like to get rid of the frame, but if I turn the RGB-Value of 0, 0, 0 to 100% transparency, some areas of the actual map, which are also black will become totally transparent too.

I vaguely remember solving this problem some years back by loading the georerenced TIFF into GIMP and filling the "frame" with some other colour that is not in my map image and then setting this colour to transparent in QGIS. I tried this this time, but the georeferencing is lost.

Does anyone know, how to remove the black frame without loosing the black pixel of my map?

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    Have you done the georeferencing already? You can change the black pixels of the map to nearly black prior to the georeferencing with a gdal command like gdal_translate -a_nodata 0 -scale 0 255 1 255 oldmap.png premap.tif. With the -a_nodata 0 option, georeferencing will know to assign the newly-missing pixels to be nodata.
    – Dave X
    Oct 30 '20 at 15:07
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    Use alpha channel instead of nodata. With gdalwarp gdal.org/programs/gdalwarp.html#gdalwarp use -dstalpha. Another option, that you mentioned, is to use an uncommon color for nodata, like totally blue -dstnodata 0 0 255.
    – user30184
    Oct 30 '20 at 15:46
  • Check out this question: gis.stackexchange.com/questions/336461/…
    – Kumbra
    Mar 30 at 21:09
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You need to set the NoData valeus to zero: Layer properties--> transparency --> set 'Additional no data value to' 0 --> apply/ok.

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  • I think that 0 was already nodata and that burns holes into the map.
    – user30184
    Oct 30 '20 at 15:52
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    As user30184 points out, this is not helping, it has the same effect than turning the black frame to 100% transparency. Oct 30 '20 at 17:31

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