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I've created some travel time isochrones using the 'OSM Tools' plugin in QGIS. I now need to remove overlapping sections of the isochrone layer so that I'm left with discrete polygons for each isochrone (in this case travel time to a central point). This is so I can calculate the proportion of a 'district' that is within x and y and z, mins of the city centre. So for example here are my isochrones

this is my starting point

If I now remove the 'shortest travel time' (yellow polygon) it's clear that all other polygons depicting travel times in excess of the yellow polygon are beneath:

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I would like to cookie cut out the longer travel time polygons so that none of the individual isochrones overlap i.e. my layer looks like this (with the yellow isochrone hidden):

enter image description here

I had thought that the difference or symmetrical differnece tool would do the trick but no joy. Is there an easy way to cut/clip polygons on the same layer from within QGIS?

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2 Answers 2

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To modify topologically incorrect polygons (overlapping polygons) as in the following example:

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you need to use GRASS -> v.clean tool from Processing toolbox -> GRASS GIS 7 commands -> Vector -> v.clean, and in cleaning tool use Break, as you can see below:

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The output result is now topologically correct:

enter image description here

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  • thanks this is really helpful. Next stage: can you think of an easy way of automating deletion of those overlapping polygons i.e. the polygons are now split where they intersect, but I now need to delete the overlapping polygons - preferably not manually...
    – marty_c
    Apr 18, 2018 at 8:36
  • Try to use Delete duplicate geometries from QGIS Toolbox -> QGIS geoalgorithms -> Vector general tools -> Delete duplicate geometries. It may solve the problem.
    – ahmadhanb
    Apr 18, 2018 at 8:42
  • that nearly does what I want, although that tool does not consider attributes of my data, so it just deletes everything except the lowest layer:( Are there any more complex operations I can perform to specify what data to delete?
    – marty_c
    Apr 19, 2018 at 13:52
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Use 'Clipper' plugin. Install it from "Plugins > Manage and Install Plugins". (I use QGIS 2.18)

After installing, select center polygon and click on the Clipper button on the Plugin Toolbar

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Then select other outer polygon and click button and so on.

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Result:

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