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I am trying make a layer of buildings, divided into apartments, by making polygons from crossing & intersecting lines in QGIS. When I use the vector/geometry tools/lines to polygons tool, I get just one polygon with no divisions at the inner lines, or otherwise odd polygons. The attached screenshot shows a random example of three polygons which were created, with the associated lines I hoped to use for the divisions. I also then tried the single part to multipart, but could not make that divide the polygon by the separate line file. I don't know if I am simply using the tools incorrectly, or if there is some setting I am missing.

enter image description here

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    In Arc the tool to do this would be looking at enclosed areas and you'd get the result you're expecting. In the example on the left if might not work if the dividing lines weren't actually snapped to or crossing the boundary line. But from your right example, it appears the QGIS tool is ignoring crossing lines for some reason. Do the lines have z values, such that maybe they aren't actually crossing? Someone more familiar with QGIS might immediately recognize another issue or possibility. – Chris W Jul 21 '15 at 18:14
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There is a tool in the Processing toolbox called 'Polygonize' which will do what you want. It needs shapely to run.

The tool used to be called the 'Polygonizer', there is a note on it here:

http://confound.me.uk/maps/ppv4.pdf

N.

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