I think creative use of the overlay() function would accomplish what you want. Some example code that is a bit like what you propose (uses the sp library):
# I have a polydataframe object: my.poly.data <- SpatialPolygonsDataFrame(my.poly, area.lu.table) # I have a set of point objects (derived, in this case, from spsample): samgrid <- spsample(us.spPolys, 500, 'regular') # I'd like to know which poly in my.poly.data each point in samgrid falls in: sample.area <- overlay(my.poly.data, samgrid)$name For this to work, you would need to figure out how to turn contours into polys, which might be tricky, especially if they aren't closed and/or have holes (pits) within them. I admit that I am not very comfortable working with the sp objects, and I would generally advise someone doing GIS-type work to use a GIS. Hawth's (not "Hawk's") Analysis Tools are very useful in an ESRI environment, as are built in commands and extensions in many GIS. yours, Ashton -- Ashton Shortridge Associate Professor [EMAIL PROTECTED] Dept of Geography http://www.msu.edu/~ashton 235 Geography Building ph (517) 432-3561 Michigan State University fx (517) 432-1671 On Wednesday 16 July 2008, Jeebz wrote: > Hi > > I'm new to the R spatial packages, but I have a spatial-oriented task > I would like to accomplish. > > I have a 2d grid of points and each point has a class (13 classes > total in the dataset). The grid is small-ish, only 256x256. I have > run a 2d kernel density estimation (using kde2d) and can create a > contour plot easily (per class). > > What I would like to do is use a contour line to extract all the > points from the original grid that are bounded by that contour. > Ultimately, I wish to calculate a sort of "purity" measure, so given a > particular contour (e.g. a contour level that encloses 95% of the > classes points), how many points from other classes fall in with the > classes that were used to define the contour. > > I know there has to be a way to do this with R... right? A colleague > of mine showed my how it could be done using ArcGIS and "Hawkes > tools", by calculating percent volume contours, but I don't know where > to begin in R. > > Anyone have any thoughts or examples on this? > > Much thanks in advance, > > bct > > _______________________________________________ > R-sig-Geo mailing list > R-sig-Geo@stat.math.ethz.ch > https://stat.ethz.ch/mailman/listinfo/r-sig-geo _______________________________________________ R-sig-Geo mailing list R-sig-Geo@stat.math.ethz.ch https://stat.ethz.ch/mailman/listinfo/r-sig-geo
