[OSGeo-Discuss] Re: open source polygon cluster aggregation algorithm?

G. Allegri giohappy at gmail.com
Fri May 7 12:31:11 PDT 2010


Hi Stefan. You guessed right, it's buildings generalization. I know
jts and geos quite well but unfortunatly I  can't use them because I'm
working directly on oracle with plsql. I think buffer is the way, but
not the one oracle provides because it rounds the corners...

Bye,
Giovanni

2010/5/7, Stefan Steiniger <sstein at geo.uzh.ch>:
> how about using R it has alpha shapes and a-like?
>
> What you describe sounds like a problem in map generalization. I.e. the
> approach of buffering is something what a colleague of mine once
> implemented to generalize house-blocks for maps (aggregate the single
> buildings). Unfortunately I don't know of any accessible code for that.
>
> If you are proficient in Java or C++ you could make you custom
> implementation with JTS or Geos (sounds easy to me).
>
> on what geographic objects are you working on? and how much does the
> shape to be need to maintained?
>
> stefan
>
> G. Allegri wrote:
>> <Aside>
>> A nice implementation of alpha shapes with jts:
>> http://www.mail-archive.com/jts-devel@lists.jump-project.org/msg01019.html
>> </Aside>
>>
>> 2010/5/6 G. Allegri <giohappy at gmail.com>:
>>> Thanks Andrea for the links. Yes, I think the problem is similar, in
>>> fact I was also looking for concave hull and alpha shapes algoithms,
>>> but the only open solution I've found is from CGAL [1] and... it's too
>>> complex to extract and reimplement in my context (database procedural
>>> programming).
>>>
>>> IWe have implemented something very "rude":
>>>
>>> 1 - logically aggregate polyongs in clusters (given a certain distance)
>>> 2 - buffer each polygon mantaining the shape (not the usual buffer,
>>> which make rounded artifacts)
>>> 3 - geometrical union
>>> 4 - shrink the result (unbuffer)
>>>
>>> But I have the time for a long holiday waiting the end of the process :)
>>>
>>> [1]
>>> http://www.cgal.org/Manual/last/doc_html/cgal_manual/Alpha_shapes_2/Chapter_main.html
>>>
>>> 2010/5/6 Andrea Aime <aaime at opengeo.org>:
>>>> G. Allegri ha scritto:
>>>>> I'm looking for an algorithm to do polygon cluster aggregation,
>>>>> similar to the ArcInfo "Aggregate Polygon" [1].
>>>>> I know about GEOS "Cascaded Union", but I need two more features:
>>>>>
>>>>> 1 - clustering of polygons that fall within a a certain threshold
>>>>> distance from each other
>>>>> 2 - mantain orthogonality, i.e. the original angles/shapes
>>>> I don't know of any such implementation, but it looks somewhat
>>>> similar to the computation of a concave hull:
>>>>
>>>> http://ubicomp.algoritmi.uminho.pt/local/concavehull.html
>>>> http://stackoverflow.com/questions/83593/is-there-an-efficient-algorithm-to-generate-a-2d-concave-hull
>>>>
>>>> Cheers
>>>> Andrea
>>>>
>>>> --
>>>> Andrea Aime
>>>> OpenGeo - http://opengeo.org
>>>> Expert service straight from the developers.
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