How does this function sound (convexhull.xy {spatstat})?

http://bm2.genes.nig.ac.jp/RGM2/R_current/library/spatstat/man/convexhull.xy.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@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@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
>
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-- 
In God we trust, all others bring data.

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