I have a city that I would like to divide up by its streets with the end result being polygons that are the blocks divided by its streets. In the original data that I have, the city boundary should be the outside boundary. It is a polygon. The city should be divided up by its streets. I have those in two files, I could use either - one is the road right of way which leaves space for the streets, the other is the road centerlines. Those are polylines.

Again, what I would like to do is subdivide the city into blocks. I have tried Union and then polygonize, which yielded an error message. When I checked the geometry the entire boundary came up invalid. When I try to convert lines to polygons, almost the entire city disappears leaving only a couple of blocks.

It's become rather circular at this point. I try one way, run into some problems, and someone tells me to try the other. I try the other way, someone tells me to try the first way. and on and on.

Here is an image of the boundary with the road centerlines. enter image description here

And here is a conceptual image (not from GIS, only for illustration). In the concept image, the thick blue line would be the polygon corporate boundary and the thin orange lines the road centerlines/rights of way.

enter image description here

What I would like to do is to merge the geometries so that each subdivided segment is its own polygon. In other words, in the end there would be five polygons from the conceptual image.

How can I do this?

Just to be clear I want the result to be polygons not polylines.

1 Answer 1


you can do this with the SAGA algorithm called Polygon-line intersection, available via Processing (tested in QGIS 2.18)

Here's an example where I've digitized a polygon and a set of lines...

enter image description here

Using the processing toolbox, search for "polygon-line"

enter image description here

end result is the polygon layer, split into chunks by the lines:-

enter image description here

  • Thanks for the tip. So, I ran it, and I got... something. :) I'd post a screenshot or two but I think I'm up against the limit. Maybe I should create a new post. I anticipated that I might run into some issues because some lines aren't closed but I'm not sure how to interpret what I've got here/how to fix.
    – GDog
    Apr 30, 2017 at 17:30

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