[ESRI-L] outline polygons of point clumps
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