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I'm not sure what you and Alex are asking, exactly. But if you
import a vector layer into GRASS, cleaning is done and topology
automatically created. The GRASS topological vector model does not
allow overlapping polygons. So new boundaries are created everywhere
that polygons overlap - what you called "cookie-cutting". If the
process is successful (not always: sometimes impossible situations
like self-intersecting boundaries cause it to fail), the new GRASS
vector will be a kind of "merge" of all the polygons in the
original. That is, every point covered by the vector will be in at
most 1 polygon. And all intersecting boundaries will have a node at
every intersection. Maybe that will help?<br>
-- <br>
Micha<br>
<br>
On 21/09/2011 09:11, Michael Werner Maur wrote:
<blockquote
cite="mid:201109210811.01000.michael.maur@lvermgeo.rlp.de"
type="cite">
<pre wrap="">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:
</pre>
<blockquote type="cite">
<pre wrap="">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):
<a class="moz-txt-link-freetext" href="http://www.gvsig.org/web/projects/gvsig-desktop/docs/user/gvsig-desktop-1">http://www.gvsig.org/web/projects/gvsig-desktop/docs/user/gvsig-desktop-1</a>-
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 :
</pre>
<blockquote type="cite">
<pre wrap="">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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<pre wrap="">
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