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I have a line file of airport runways across my state. I'm looking to symbolize them as part of a thematic town parcel layer, so I'd like them all to be in 2-D. My problem is, some bigger airports (that is, an airport with more than one landing strip) have lines bounding the landing strips, while smaller ones are just straight lines. Further, when I go to polygonize my line layer, the tool leaves behind everything it sees as 'unpolygonizable".

before: see before

&after: and after...

Is there a way to polygonize the entire layer WITHOUT having to manually edit every single feature? There are more than 200 of them...

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I see you are using Q-GIS. Unfortunately I am not 100% familiar with the Tools available. I can maybe point you in a direction.

ESRI has a tool called "Collapse Dual Line to Centreline"It takes 2 parallel lines and gives you the centreline between these lines. Using this centreline you can buffer a distance and get the correct polygon shape.

Doing a quick Google search, I see there are some QGIS tools out there. Here are some of the links I found:

  1. https://github.com/migurski/Skeletron - free tool for generalising lines
  2. http://osgeo-org.1560.x6.nabble.com/Centre-of-the-roads-from-polygons-to-lines-td4978702.html
    • describes a method of rasterising the polygon to re-create a vector / centreline

I once had a similar issue trying to create a centreline from a drainage network. I had some success in creating Thiessen Polygons and extracting out the centrelines from that. Unfortunately I cannot find the links.

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You may be interested in creating either bounding box(bounds) and/or convex hull.

enter image description here


Before start, please make sure you have one geometry per each airport. I mean, if your airport has three runways, you need to combine these three into single multi-part geometry.

Before: Airport_3 and Airport_4 has three runways. enter image description here

After Vector | Geometry Tools | Singleparts to multipart

enter image description here

Then Processing Toolbox | QGIS geoalgorithms | Vector geometry tools | Convex hull (and Boundary) will give you something like the topmost picture.

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