[Qgis-user] Clipping overlapping polygons with QGIS?

[Alexandre Leroux]

Hi list,

I tried to find an answer through The Google, QGIS's wiki, QGIS's plugins and elsewhere, but failed, so here I am.

I have a single layer with multiple polygons. Some polygons includes smaller polygons, thus the larger polygons overlap with some smaller polygons which do not entirely fill the larger polygons. What I need is a layer consisting of the cookie cutting of all polygons by all the polygons of that layer.

Consider this example, a single layer includes a Polygon A, a Polygon B1 and a Polygon B2. "Polygon A" consists of "Polygon B1" + "Polygon B2" + "empty space". I want to clip Polygon A and get the resulting "Polygon B1", "Polygon B2" and "Polygon C", where "Polygon C" is equal to "Polygon A" minus ("Polygon B1" plus "Polygon B2"). In other words, the result that I need are the original polygons cookie cutted by all the lines present in the shapefile.

(my real dataset has thousands of polygons, doing this manually is out of question :-)

* The "Vector -> Geoprocessing Tools -> Clip" feature does not create that "Polygon C" that I need.
* I can't seem to be able to use the "Difference" feature either since all my polygons are on a single layer.
* I tried the Clip feature with lines (i.e. polygon outlines), but the result is empty.

Anyone has a clue how to do that? Am I just dumb? :-)


Thanks a lot!

Alex - slashgeo.org