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

[Michael Werner Maur]

Hi Alex,

I have the same problem ! I can't find a way to detect selfintersections with 
QGis. There is a vector analysis tool, but it don't find and show the invalid 
geometry. Up to date I use the easy OpenJump   to find this situations in 
vector data ! ...Is there anybody, who knows how it works with QGis ?

Thanks

michael.


Am Dienstag 20 September 2011 22:43:50 schrieb 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%20processi
> ng/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
> 
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-- 
Michael Werner Maur

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