[R-sig-Geo] creating polygons of clusters of points?

[Dylan Beaudette]

Hi,

Here is another approach, using something like a 'concave hull' -- i.e. alpha 
shapes:

http://casoilresource.lawr.ucdavis.edu/drupal/node/919

Cheers,
Dylan

On Friday 23 April 2010, Roman Luštrik wrote:
> How does this function sound (convexhull.xy {spatstat})?
>
> http://bm2.genes.nig.ac.jp/RGM2/R_current/library/spatstat/man/convexhull.x
>y.html
>
> Convex Hull of Points Description
>
> Computes the convex hull of a set of points in two dimensions.
> Usage
>
> convexhull.xy(x, y=NULL)
>
> Arguments x vector of x coordinates of observed points, or a 2-column
> matrix giving x,y coordinates, or a list with components x,y giving
> coordinates (such as a point pattern object of class "ppp".) y (optional)
> vector of ycoordinates of observed points, if
> x is a vector.
>
> Cheers,
> Roman
>
> On Fri, Apr 23, 2010 at 4:37 PM, Hans Ekbrand <hans at sociologi.cjb.net>wrote:
> > How to create nice polygons from a number of clusters?
> >
> > I have sorted list of clusters representing hot-spots
> >
> > > sorted.list.of.clusters
> >
> >  [1]  7  1 11  9 26  2  5 10 33 40 18 28 37 20 30 47 53
> >
> > whereas my.cluster holds the classification of events (criminal
> > fire-setting) to clusters:
> >
> > head(my.cluster, 100)
> >  [1]  1  2  3  3  4  5  6  7  8  5  9 10 11 12  1 13  2 14 15 10 16 17  5
> > 13  7  7 18 19 18 20 20  2 21 16 22 18 10  1
> >  [39]  9 23  4  7 24  9  9  7  7  5  2 11 25 12  7  7 26 27 13  7  7 28
> > 18 28 29  9  2  5 30 11 31 14 18  1 32 26  2 31
> >  [77] 29 33  2 34  2 26 35 36  7  2 35 37 26 38 11  2 11 26 30  7  4 11 
> > 8 11
> >
> > > number.of.fires.at.hotspots <- sapply(sorted.list.of.clusters,
> >
> > function(x) {length(which(my.cluster == x))})
> >
> > > number.of.fires.at.hotspots
> >
> >  [1] 44 39 38 32 22 21 21 21 15 15 14 14 14 12 12 12 12
> >
> > The "SpatialPointsDataFrame" object clust.geo.test holds the events.
> >
> > A list coordinates of the events of each cluster is generated with:
> > > sapply(sorted.list.of.clusters, function(x)
> > > {clust.geo.test at coords[which(my.cluster
> >
> > == x),]})
> > [[1]]
> >           Ost    Nord
> >  [1,] 322737.4 6406939
> >  [2,] 323104.9 6406968
> >  [3,] 323104.9 6406968
> > ...
> >
> > [[2]]
> >           Ost    Nord
> >  [1,] 315359.9 6393110
> >  [2,] 315177.0 6394031
> >  [3,] 314697.8 6394565
> > ...
> >
> > And I can plot the points in the hot-spots with
> >
> > > plot(sdn)
> > > sapply(sorted.list.of.clusters, function(x)
> >
> > {plot(clust.geo.test[which(my.cluster == x),], add = T)})
> >
> > But would like to plot each cluster as a polygon, ideally filled with
> > darker colors for clusters with higher number of fires.
> >
> > I guess I need to create polygons for each cluster and then plot each
> > polygon. How do you do that?
> >
> > For your convenience I have created a RData object containing the
> > objects refered to above and uploaded to this URI
> >
> > http://sociologi.cjb.net/temp/to-polygons-help.RData, so
> >
> > print(load(url("http://sociologi.cjb.net/temp/to-polygons-help.RData")))
> > plot(sdn)
> > sapply(sorted.list.of.clusters, function(x)
> > {plot(clust.geo.test[which(my.cluster == x),], add = T)})
> >
> > would get you to the point I am now.
> >
> > Hans Ekbrand
> >
> > -----BEGIN PGP SIGNATURE-----
> > Version: GnuPG v1.4.9 (GNU/Linux)
> >
> > iEYEARECAAYFAkvRsLEACgkQfCyHKnBQYU4JrQCfWMlOgECamin5ZugGoHcUpqx3
> > fuQAoNDoG6iMVlLJCOYfoP5MRbNYHGlI
> > =jKTU
> > -----END PGP SIGNATURE-----
> >
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> > R-sig-Geo at stat.math.ethz.ch
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-- 
Dylan Beaudette
Soil Resource Laboratory
http://casoilresource.lawr.ucdavis.edu/
University of California at Davis
530.754.7341