[Qgis-user] Any ideas? Splitting polygons to only have one point in each polygon

[David J. Bakeman]

On 02/14/2013 08:58 AM, magerlin wrote:
> Hi - I've tried the one building a time approach but have not been able to
> get through.
> It is very kind of you Victor to offer to have a look at it and perhaps
> include it in your blog.
Perhaps you could use the extract nodes routine to extract the points
from each building and include those along with the points inside the
building polygons?  That way even if there are no points inside a
building there would still be at least 3 points?  Ofcourse the ftools
extract nodes does not appear to handle selected polygons either but you
could do that in grass with v.extract type=point.  Another idea might be
to use a Delaunay triangulation of the points in a building with the
addresses in a building using v.delaunay.
>
> You can get my data and model files here:  Get data
> <https://dl.dropbox.com/u/8189384/Break%20Polygons%20by%20Points.zip>  
> The zip file includes both some test data (looking almost like the picture
> in my first post) but also some nasty real data with about 14.000 buildings!
> Also two model files are included: The one used to create the picture in my
> first post and my attempt to the iterative procedure.
>
> Executing the "Break Polygons by Points iteratively.model" give me an error
> message:
>
> <http://osgeo-org.1560.n6.nabble.com/file/n5034373/Error_msg.png> . 
> There are no messages in the log.
>
> My general experiences so far has been:
> * I need to use the Grass v.voronoi since the Qgis "Voronoi polygons" uses
> the whole layer even if selections exist
> * v.voronoi stops with an error message if less than 3 points in the
> building but it has problems handling situations also with bigger (but
> small) numbers. In the example below with 5 points it has only made 3
> voronoi polygons:
> <http://osgeo-org.1560.n6.nabble.com/file/n5034373/5_points_not_ok.png>  
> Some other situations with 5 points are handled correctly!
> * Due to the above mentioned limitations and since I need to be able to also
> split buildings with only 2 points I have experienced that the addition of
> some temporary additional points at the (far) edge of the Analysis Extent
> will help when they are included in the creation of Veronoi polygons for
> every building
> * With the large number of polygons in my real data I think the iterative
> process at each step should append the new Voronoi polygons inside the
> building to a "collecting" layer of split buildings (at the end beeing the
> result layer) - otherwise the algorithm must at the end be able to merge an
> unknown big number of shape files.
>
>
> My experiences with Sextante: It is marvellous to now be able to make your
> own routines and it is quite simple with the modeler! I have no knowledge of
> python so I have not come to the scripts yet.
>
> I have an undocumented feeling that Sextante in the background "saves" some
> setting and sometimes unintentionally (at least from my point of view ;-)
> keeps them for later runs of other geoalgorithms? 
> Sometimes it could seem to be due to (not reported) problems with
> overwriting existing files. I have experienced several times that my
> "Break_Polygons_by_Points.model" stops to function (nothing happens when
> running it) but closing down Qgis and restarting makes it OK again. It could
> perhaps also be some saved analysis extents?
>
>
>
>
>
>
>
> -----
> Regards Morten
>
> Qgis 1.8.0 Stand alone installer in Win7 64 bit
> --
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