[Gvsig_english] QUESTION gvSIG 1.10 -- Create new polygon using nodes

[Simon Cropper]

Hi Victor or Antonio,

What happened to this algorithm?

Spatial cluster in Sextante does not create polygons from points as 
discussed in below.

Is their another tool for making a polygon by 'shrink-wrapping' aberrant 
random points?

The thread below was particularly concerned with this and I now have the 
same requirement, but none of the links appear to work anymore.

-- 
Cheers Simon

    Simon Cropper
    Principal Consultant
    Botanicus Australia Pty Ltd
    PO Box 160, Sunshine, VIC
    W: www.botanicusaustralia.com.au

******* ORIGINAL THREAD ****************

On 02/07/10 21:48, Antonio Falciano wrote:
> Il 02/07/2010 12.58, Jorge Gaspar Sanz Salinas ha scritto:
> > El 02/07/10 11:19, Antonio Falciano escribió:
> >> Il 02/07/2010 9.45, Jorge Gaspar Sanz Salinas ha scritto:
> >>> El 01/07/10 13:16, Antonio Falciano escribió:
> >>>> 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
> >>>>
> >>>
> >>> Hi, thanks for the links Antonio.
> >>>
> >>> Schuyler finally pointed me to the current location of Clustr code at
> >>> [1]. He told me that well, it's focused on Flickr needs but maybe 
> can be
> >>> interesting to someone else.
> >>>
> >>> BTW, two days ago I have nice developers workshop about SEXTANTE 
> so just
> >>> to do a matter of exercise I adapted (I mean, copy&pasted) some code
> >>> posted at JTS mailing list [2] and created an algorithm.
> >>>
> >>> It's just a dirty hack and you have to do some "try and error" 
> until you
> >>> get something interesting but well, the results are not soy bad after
> >>> all :-) [3].
> >>>
> >>> Cheers
> >>>
> >>> [1] http://github.com/straup/Clustr
> >>> [2]
> >>> 
> http://jts-devel.219725.n2.nabble.com/polygonizer-and-line-strings-tp2572524p2591002.html
> >>> [3] http://yfrog.com/7fe71p
> >>>
> >>
> >> Hi Jorge,
> >> your results are very interesting, congrats! The alpha shapes algorithm
> >> [1] cited on JTS mailing list is relatively easy to understand and it
> >> seem very effective in order to solve the problem. I'm looking forward
> >> to test your new SEXTANTE geoalgorithm! ;)
> >>
> >> Cheers,
> >> Antonio
> >>
> >> [1] http://www.isprs.org/proceedings/XXXVII/congress/3b_pdf/37.pdf
> >>
> >
> > last shot :-)
> >
> > I've compiled clustr and the results seem pretty fine (as I expected).
> >
> > Using it is easy: you just provide it a text file with a tag (to
> > categorize your clusters if you need) and a pair of long/lat values and
> > it returns a shapefile of polygons. I've get that file from a shapefile
> > with the gvSIG field calculator and some openoffice calc and text 
> replace.
>
> BTW, there are the "Spatial cluster" algorithm in SEXTANTE which allows to
> create "clusters from an input points layer, grouping them according to
> their spatial location" and the "Add coordinates to point" one.
>
> > it don't deals with holes at this time
> >
> > and finally you can let it to calculate the optimal alpha value or
> > provide it this parameter. some simple results:
> >
> > http://yfrog.com/5zzdmp
> >
> > If I would need to use that kind of geoprocess I would use Clustr at
> > this time, but if anyone wants to improve my hack, the algorithm, along
> > with the SEXTANTE workshop simple algorithms, are at:
> >
> > 
> https://svn.prodevelop.es/public/labs/users/jsanz/sextante_algorithms/sextante_tests/
>
> I have tested a bit your alg and it work like a charm! ;) It need only a
> bit of code cleaning, nothing of transcendental!
> It's clear that we have to play a bit with alpha value, however I think
> this feature is better than calculate an optimal alpha value in order to
> control the process. Awesome job!
>
> Cheers,
> Antonio
>