[Qgis-user] Re: Clipping overlapping polygons with QGIS?

[Alexandre Leroux]

Hi list -

I quick update. What I've been trying to do may possibly be done by the Intersection function of my layer with itself. I tried applying the intersection geoprocessing function via both QGIS and gvSIG and both failed to generate a full cookie cutting of all the polygons in my layer by themselves. The larger polygons are simply not cut by the smaller ones (the "empty space" polygon in my example below is never created).

If I look at gvSIG's documentation (QGIS's pdf documentation is not as detailed): http://www.gvsig.org/web/projects/gvsig-desktop/docs/user/gvsig-desktop-1-11-manual-de-usuario/gvsig-desktop-1-11-manual-de-usuario-en/gvsig_freemind_toc_view?doc=gvSIG%201.11%20User%20manual/Analysis%20and%20data%20processing/Vector/Geoprocessing%20tools/Intersection/Introduction it seems like it's exactly what I want to do. But running the command just doesn't create the expected result (as I said, the larger polygons are not cut by the smaller ones). I'm wondering if the problem is not with my shapefile (but I don't see why that would be the case).

If anyone has a clue, that would be great -

Thanks -
Alex


Le 2011-09-18 à 3:05, Alexandre Leroux a écrit :
> 
> 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