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

[David J. Bakeman]

On 02/14/2013 10:10 AM, David J. Bakeman wrote:
> 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.
OK the delaunay was a bad idea I don't know what I was thinking
:-[           Anyway I still think it might be useful to include the
points from the building polygon so you always have enough points.
>> 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
>> --
>> View this message in context: http://osgeo-org.1560.n6.nabble.com/Any-ideas-Splitting-polygons-to-only-have-one-point-in-each-polygon-tp5034081p5034373.html
>> Sent from the Quantum GIS - User mailing list archive at Nabble.com.
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