[R-sig-Geo] [R] [ESRI-L] outline polygons of point clumps

[Xiaohua Dai]

Hi Roger,

Thank you for your kindly help. It is exactly what I want. I will try
to understand your programming.  I think the program should be useful
in many fields as you indicated.

I did not expect the problem can be solved so easily. Again, R is very
powerful with so many good functions. I am quite interested in lapply
and tapply. I will find some references/websites on their
applications.

Also thanks to remind me of the post guide.

Best wishes
Xiaohua

On 5/12/06, Roger Bivand <Roger.Bivand at nhh.no> wrote:
> On Fri, 12 May 2006, Xiaohua Dai wrote:
>
> > Sorry, I did not make my question clear. Since I have a point theme
> > with many points, some of them may clump together. the problems here
> > are:
>
> A more specific follow-up after my reply directly to R-help.
>
> I'm assuming you have a running R, and that convex hulls are what you
> want.
>
> Take a random data set:
>
> set.seed(1)
> xy <- matrix(runif(500, 0, 10), ncol=2)
> xy_clusts <- hclust(dist(xy), method="complete")
> # complete linkage hierarchical clustering
> plot(xy_clusts)
> # shows the clustering tree
> cl_10 <- cutree(xy_clusts, 10)
> cl_20 <- cutree(xy_clusts, 20)
> cl_30 <- cutree(xy_clusts, 30)
> # cut the tree - the objects contain the memberships
> which_cl_10 <- tapply(1:nrow(xy), cl_10, function(i) xy[i,])
> chulls_cl_10 <- lapply(which_cl_10, function(x) x[chull(x),])
> # construct convex hull polygons for each cluster
> plot(xy)
> res <- lapply(chulls_cl_10, polygon)
> # and repeat for cl_20 and cl_30
> which_cl_20 <- tapply(1:nrow(xy), cl_20, function(i) xy[i,])
> chulls_cl_20 <- lapply(which_cl_20, function(x) x[chull(x),])
> plot(xy)
> res <- lapply(chulls_cl_20, polygon)
> which_cl_30 <- tapply(1:nrow(xy), cl_30, function(i) xy[i,])
> chulls_cl_30 <- lapply(which_cl_30, function(x) x[chull(x),])
> plot(xy)
> res <- lapply(chulls_cl_30, polygon)
>
> If you need the list of convex hulls out as a shapefile, we can do that,
> if you need a raster, I'd suggest using kernel density with different
> bandwidths for a start, and NA out the densities below a chosen threshold.
>
> Hope this helps,
>
> Roger Bivand
>
> PS. Use package maptools, function readShapePoints() to read your
> shapefile, and coordinates() of the input object to extract the
> coordinates. If need be, project from geographical coordinates using
> transform methods in package rgdal.
>
>
> > 1.  how to find clumps in a point theme?
> > 2.  the convex-hull extension I found only deal with all the points in
> > a theme at each time?  how to make each convex hull around each point
> > clump automatically?
> >
> > Thanks.
> >
> > Xiaohua
> >
> >
> >
> > On 5/12/06, Bob Booth <bbooth at esri.com> wrote:
> > > Xiaohua,
> > >
> > > That would be one way to do it. There are others.
> > > Try searching ArcScripts for "convex hull"
> > > http://arcscripts.esri.com/
> > >
> > > For example:
> > > http://arcscripts.esri.com/details.asp?dbid=14535
> > > or
> > > http://arcscripts.esri.com/details.asp?dbid=12084
> > >
> > > Bob
> > >
> > >
> > > -----Original Message-----
> > > From: ESRI-L [mailto:ESRI-L at esri.com] On Behalf Of Xiaohua Dai
> > > Sent: Friday, May 12, 2006 2:33 AM
> > > To: ESRI-L at esri.com
> > > Subject: [ESRI-L] outline polygons of point clumps
> > >
> > > Dear all,
> > >
> > > How to generate one outline polygon for each point clump? Are there
> > > any present functions in ArcView, ArcGIS, R or some freewares? I just
> > > had a quick look at the package adehabitat and did not find the
> > > function.
> > >
> > > To my knowledge, I could do it as follows: 1) make a grid map of my
> > > study area with cell values = 0; 2) assign 1 to the cells containing
> > > at least one point; 3) convert the 1-value cells into polygons.
> > >
> > > Am I right?
> > >
> > > Thanks
> > > Xiaohua
> > >
> > >
> >
> >
> >
>
> --
> Roger Bivand
> Economic Geography Section, Department of Economics, Norwegian School of
> Economics and Business Administration, Helleveien 30, N-5045 Bergen,
> Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43
> e-mail: Roger.Bivand at nhh.no
>
>