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Could someone shed some light into a method to link breaking outline borders without much distortion from the original like e.g. convex hull does? Convex hull alternative ideas are of course welcome to be tried even if they have similar results.

Below two screenshot of what I try to remove and achieve a more continuous result: img1 img2

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  • What do you mean by "link breaking borders"? Commented Dec 18, 2018 at 18:14
  • The second image is almost self explanatory. I mean that these breaks in the continuous imaginary border "line" is troubling and I want it removed, if there is a developed method of course. If you know what convex hull is, it could be similar but with "higher accuracy". Commented Dec 18, 2018 at 18:18

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You could do an old-school buffer-debuffer technique (discussed many time on this site e.g. here and elsewhere too) followed by hole removal. The key here will be the amount you buffer-debuffer by. By looking at the second image I'd go for about 2 or 3 times the raster resolution.

Alternatively you could try using a concave hull tool (aka alpha shape). This is like a convex hull but a lot tighter. To find this simply type 'concave' into the Processing Toolbox search and it'll pop up in Qgis 3.x (earlier version need a plugin to be installed). It's under Processing Tools->QGIS Vector Geometries and you'll find a couple of options to try. It'll all depend on your tolerances etc.

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  • The buffer-debuffer technique worked like charm. Thank you very much MappaGnosis, both for the ideas and for reminding how engineering works. Commented Dec 18, 2018 at 18:53
  • One comment,, the concave tool alpha needs points as input, not polygon, but one can convert the boundaries to points easily. Commented Dec 18, 2018 at 18:54
  • I was thinking more like converting the raster to points to get the mass. This way you only need to do one conversion (raster to points then concave hull vs. raster to polygon to points and then concave hull), but if buff-debuff worked... it's a moot point :) Commented Dec 19, 2018 at 7:40

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