stratified random sampling
Hi Gerald: I have an example that may help: It demonstrates the sampling of a polygon shape file with a regularly-spaced point grid, and assignment of polygon values to the sampling points: http://nceas.ucsb.edu/scicomp/GISSeminar/UseCases/SampleVectorPolygonsRasterGrid/SampleVectorPolysRastGrid.html You could modify this example to randomly select a point inside each polygon from the point grid (or a similar modification). Hope this helps.. Rick Reeves
Heuvelink, Gerard wrote:
Dear list, I use readShapePoly to import a shape file. The file has multiple polygons (actually, a 'polygon' may consist of multiple polygons that have the same ID). I want to sample one location at random from each of the polygons. I tried to do this with spsample but did not manage (the option "stratified" does not do what I want). I also looked at dotsInPolys but also without success. I think it should be possible with spsample, possibly after first creating a large number of SpatialPolygons objects, one for each polygon, and next sampling one location at random from each of these. Unfortunately I am not very experienced and also failed to achieve what I want in this way. Can you help me? Thanks, Gerard Heuvelink
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Rick Reeves Scientific Programmer/Analyst and Data Manager National Center for Ecological Analysis and Synthesis UC Santa Barbara www.nceas.ucsb.edu 805 892 2533