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I am drawing administrative regions that have boundaries follow streets in another layer. As these streets are pretty long with many vertices, I don't want to re-trace them. I am looking for a way to "split" the polygon like "split features" tool (the one with the scissors icon).

Before: one whole polygon with a line running through it

enter image description here

After: the line works as a pair of scissors and split the polygon into 2 parts (in the picture below, the polygons were moved apart for illustrating purpose only)

enter image description here

Thanks a lot.

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Identical:… – Simbamangu Oct 17 '12 at 15:20
This does not appear to be identical to the question in the link. Here, if I understand correctly, Cao Minh Tu wants to extract the "street" part of a polygon. The resulting extracted object would be a (set of ) linestrings. – katahdin Oct 17 '12 at 16:19
@katahdin - I understand that OP wants to "split the polygon" using "streets in another layer" - the output will be split polygons, not lines. – Simbamangu Oct 18 '12 at 3:20
@Simbamangu: I tried Vector|Geoprocessing Tools|Intersect as you suggested with 2 test layers but the result layer have nothing on it. Test data: – Cao Minh Tu Oct 18 '12 at 9:23
@Cao Minh Tu, the pictures help a lot. Good clarification. – katahdin Oct 18 '12 at 12:30
up vote 7 down vote accepted

First you have to union both shapes

enter image description here

Then activate the Polygonizerplugin to reshape the lines to polygones:

enter image description here

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My Ubuntu Qgis complained about missing shapely.ops when I tried to install Polygonizer plugin. Right now I don't know how to fix it. I'll have to resolve this problem before trying your solution. Thanks. – Cao Minh Tu Oct 19 '12 at 2:40
It works. What took me so long to accept your answer is that I didn't know how to install shapely.ops. It turns out very easy: go to Ubuntu software center, search for "shapely" and install the only one python-shapely package. After that I was able to install polygonizer plugin and tried out your solution. Thanks a lot! – Cao Minh Tu Oct 31 '12 at 10:28
I can't install the Polygonizer plugin. – gregmacfarlane Feb 13 '13 at 21:24

This similar question is for ArcGIS, but may provide you with some ideas from the discussion that you can implement in QGIS.

I also created an ArcGIS model that you may be able to replicate using similar concepts in QGIS. I tried to describe the concepts that I used here.

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Thanks for your answer, but right now ArcGIS or programming skill is out of my ability. – Cao Minh Tu Oct 19 '12 at 2:47

If I understand your question correct, the the GRASS command can be used to convert polygons to lines. see the following post: How do I split polygons into line segments?

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I added pictures to clarify the question - thanks a lot. – Cao Minh Tu Oct 18 '12 at 9:25
@Cao Minh Tu: thanks, the pictures make it clear, I did misunderstand you – Kurt Oct 19 '12 at 5:35

If you don't have the ArcInfo license, you can use the command "geom.splitpolysbylines" in the Geospatial Modelling Environment. It work perfectly. Make sure, however, that you have ArcGIS 10.1 or ArcGIS 10.0 SP4 and installed "R" 2.12 or higher.

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My question was for Qgis (showed in the tag) - I updated the header to make it more visible. Thanks anyway! – Cao Minh Tu Nov 16 '13 at 2:27
Sorry about that. Anyhow, if "someone else" has the same type of question, he/she use the option that I suggested above. – Doum Nov 18 '13 at 21:05

I solved this problem with the Processing Toolbox, using the SAGA Polygon-line intersection algorithm.

The process might fail (i.e. no split) if the line end is snapped to a vertex point of the polygon. Extending the line a bit into the neighbouring polygon or outside area sanitizes those pitfalls.

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