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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:

enter image description here

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:

enter image description here

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:

enter image description here

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

enter image description here

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

enter image description here

Result:

enter image description here

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