4

I'm having problems in intersecting Voronoi polygons with another shape. As the image shows, I want to clip the Voronoi layer at the red costal perimeter of the UK so that the additional surface area of those polygons doesn't affect distance calculations. The Voronoi polygons represent telephone exchange areas.

enter image description here

When I attempt to intersect the two layers in QGIS via the Vector-Geoprocessing menu, it produces a few random shapes, as detailed in the image below:

enter image description here

Any ideas?

  • Try converting your boundary line layer to polygon (Vector > Geometry Tools > Lines to Polygons) and intersect this with your voronoi polygon layer. – Joseph May 10 '16 at 11:11
  • Hi Joseph, the boundary is already a Polygon I'm afraid, so this didn't work. – Thirst for Knowledge May 10 '16 at 11:27
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I think you need to use clip not intersect from vector -> geoprocessing to clip your data to the specific boundary.

  • Hi Ahmadhanb, I tried clip and it didn't work. It produced an empty shapefile again. I changed the coordinate system and this did not solve the issue. – Thirst for Knowledge May 10 '16 at 11:29
  • It looks like you have different projections in the two figures above. Are you sure the projection is same for both input data? – ahmadhanb May 10 '16 at 12:18
  • Yes, I've made sure it's in the correct projection. I tried it with each layer saved as EPSG: 27700 (OSGB 1936) and as EPSG: 3395 (WGS 84 / World Mercator). Whether I try intersect or clip, the new shapes do not show up. Any suggestions? – Thirst for Knowledge May 10 '16 at 15:01
  • Yes, clip may well be the right tool to use, but it's not working due to vector topology errors. – Thirst for Knowledge May 10 '16 at 19:58
  • Then, you need to check the topology first by running topology checker, and fix any error detected. – ahmadhanb May 11 '16 at 1:28
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To provide a summary and answer to the issues here:

  • Clip instead of intersect is the correct geoprocessing tool;
  • The QGIS Voronoi tool introduced quite a few imperfections in the polygons produced, and this seems to be the culprit for the later geoprocessing not working as planned;
  • I tried to clean up the Voronoi polygons produced in QGIS, but I found the quickest solution was to produce them in Arcmap and then export them for use back in QGIS.

Upvotes for Ahmadhanb & Tamas Kosa for useful suggestions.

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just tested it with my own data. UK boundary plus active GNSS stations voronoi, and I can confirm the intersection is working in my Qgis, however if I manually create a topology error in my boundaries the intersection query will work just partly. Run the topology checker maybe you can see any errors.

  • Thanks Tamas, you're right, it's a topology error issue. I'm working on this now and managed to remove a large number of errors by increasing the Voronoi buffer zone (although I'm not sure how this will affect the end result yet). I have 5 gaps which I'm trying to rectify, any suggestions? – Thirst for Knowledge May 10 '16 at 19:59
  • gaps can be sorted with creating new polygon and merge with the old one – Tamas Kosa May 10 '16 at 20:05

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