[Gvsig_english] QUESTION gvSIG 1.9 (BN 1253) -- Corrupted Polygon / Create new polygon using nodes

[Antonio Falciano]

Il 01/07/2010 12.30, Jorge Gaspar Sanz Salinas ha scritto:
> El 01/07/10 12:08, Antonio Falciano escribió:
>> Il 01/07/2010 10.56, Simon Cropper (Botanicus Australia Pty Ltd) ha scritto:
>>> Hi,
>>>
>>> I have inherited a polygon -- nodes in the polygon do not run
>>> sequentially from clockwise or anti-clockwise. This is throwing an error.
>>>
>>> I was able to extract the nodes from the polygon but have had problems
>>> finding the routine to create a new polygon using these points (there is
>>> no holes in the polygon).
>>>
>>> Can anyone please point me to a suitable routine?
>>
>> Hi Simon,
>> you can try with the SEXTANTE library: in detail, applying "Minimum
>> enclosing shapes" (Convex hull) on vertices and then subtract the more
>> external triangles (selected manually) computed with "Delauney
>> triangulation". This is a quite huge task, also known as "Concave hull".
>> I don't know if this problem has been solved in GFOSS world...
>> http://code.flickr.com/blog/2008/10/30/the-shape-of-alpha/
>> Cheers,
>> Antonio
>>
>
> Schuyler Erle developed for flickr a free software to create alpha
> shapes[1], but the trac/svn[2] is closed now...
>
> More info about the flickr project at [3]
>
> It would be great to have a SEXTANTE geoprocess for that, indeed.
>
> Best
>
> [1] http://biogeometry.duke.edu/software/alphashapes/index.html
> [2] http://code.flickr.com/trac/browser/trunk/clustr
> [3] http://code.flickr.com/blog/2008/10/30/the-shape-of-alpha/

Hi Jorge,
I'm agree with you about the need of a such tool. Its algorithm is quite
complex, however there are a lot of interesting resources on the web [4]
or in the literature [5]. Furthermore, there are many possibilities to
obtain good approximations. For instance, see [6]. Maybe, with a good
SEXTANTE model or script it's possible to obtain something of useful.

Cheers,
Antonio

[4] 
http://stackoverflow.com/questions/83593/is-there-an-efficient-algorithm-to-generate-a-2d-concave-hull
[5] 
http://www.wipo.int/pctdb/en/wo.jsp?WO=2008107859&IA=IB2008050849&DISPLAY=DESC
[6] http://grass.osgeo.org/wiki/Create_concave_hull

-- 
Antonio Falciano
http://www.linkedin.com/in/antoniofalciano