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I have old land-use maps in a scanned format. I am trying to obtain proper land-use maps from it, similar to what is described here: https://gis.stackexchange.com/questions/71650/how-to-compare-areas-in-scanned-historical-land-use-maps . The problem I'm facing is that some of the land-use classes are depicted as pattern (e.g. forests below are green with sprinkled black dots).

My fairly general question is for tips on how to do this in GiMP? I.e. creating an image file, where the patterns are replaced by plain areas in high-contrast colors.

Example from the map: enter image description here

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One thing you could try is using the Select-by-Colour tool in Hue select-by mode (rather than the usual Composite select). [See the Tool Options, "Select by:" parameter.] To eliminate holes, grow the resultant selection by 1 or 2 pixels and then shrink back by the same amount.

A quick test on the sample image you posted above suggests that, used in that way, the tool will pick the clay-coloured regions en-masse quite accurately, likewise for the water. But, of course the real problem is going to be distinguishing the 4 other "textures" shown, which re-use the same basic colours (originally probably just 2 shades of green, 2 shades of yellow, plus black). Again, the Select-by-Colour tool in Hue select-by mode may go some way towards separating these regions - it will for example allow you to select all shades of green simultaneously quite accurately, but you will then have to combine/subtract/intersect selections to refine the regions.

  • Thanks a bunch for this. Playing with the select-by mode helps. However, the forest (black dots on green) is the most important part for my, so I'm looking for something that works a little better there. Also, this is just a tiny excerpt, the whole map is a /lot/ larger :/ – sheß Nov 8 '15 at 22:29
  • Maybe, the way to go is just to get rid of all the black lines (lashed ones, outlines etc) and then just treat all black that remains as forest.... – sheß Nov 8 '15 at 22:30
  • Hmm. yes I guessed this was just small sample, that's all the more reason to find a method that works right across the whole map. – Fiddlesticks Nov 8 '15 at 22:55
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    Getting rid of the black lines may not be all that easy because of the dithering/pixelation created by scanning. You could try Select-by-Colour (in composite select-by mode) absolute black, change Mode to "add to selection" click on a few shades of grey, grow selection by say 10px, then intersect with a selection of all greens (as above) to get all green pixels with 10px radius of black/grey. – Fiddlesticks Nov 8 '15 at 23:08

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