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I am a vectors guy mostly, but for a current project I need to work with a bunch of old scanned paper maps (ww2 bomb damage maps for London, if your interested!)

We scanned and georeferenced the maps, and now want to create a tiled composite layer for serving on a website. We will cut out the borders obviously, thats not the issue here.

Problem is, that there are unsightly visual colour and brightness differences between map sheets. I am a bit at a loss as to how to equalise them so to give a coherent visual look. I read up on histogram equalising, but my current toolbox (Manifold GIS, GDAL, GeoServer) doesnt seem to have the required functionality.

Example of 4 scans already georeferenced:

Example of 4 scans already georeferenced

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Do you have access to Adobe Photo Shop? –  Mapperz Oct 12 '12 at 13:36
    
No, but GIMP ... –  petzlux Oct 12 '12 at 13:41
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2 Answers 2

This can be done in GIMP http://www.gimp.org/

You need to create a custom colour palette - this will read each image loaded and best match to the pixels forcing the colours to become a near as match.

You can also restrict contrast and brightness in a similar way

http://gimp.open-source-solution.org/manual/gimp-tool-brightness-contrast.html

http://docs.gimp.org/en/gimp-palette-dialog.html

For more advance detail you can use the Equalize function with the histogram

http://docs.gimp.org/en/gimp-layer-equalize.html

This can be automated with the script fu macro function.

http://www.squidoo.com/gimp-how-to-write-a-script-fu-macro

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Another way is to use photo (panorama) stitching software. Hugin is an example of a free&open project that does just that and it comes bundled with enblend that can take care of brightness differences. No need to muck with palletes, just choose the image with the best lighting/colors as the source for blending. You would have to cut off the borders first though.

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