Hi Michael,Roger is fully right, this function does not preserve the topology, so be aware of that, some problems can occur... If you want to use shapefiles::dp() just for raw plotting and visual simplification, you can try (on the fly):
library(rgdal) library(shapefiles)fr <- readOGR("polygonFRA_WGS84.shp", "polygonFRA_WGS84") # a shapefile of France with complex topology (holes, islands...) in WGS84 coordinates
pp <- slot(fr, "polygons") # take the polygonscf <- coordinates(slot(pp[[39]], "Polygons")[[1]]) # extract the coordinates of the main polygon ("continental France") pf <- list(x=cf[,1], y=cf[,2]) # list of coordinates, as dp() needs a list and not a matrix or dataframe...
cf1 <- dp(pf, 0.1) # simplification, with a bandwith of 0.1 decimal degree plot(fr) # to see the result points(cf1, col="red", t="l") HTH David Michael Friendly a écrit :
I think, for my application, I'd be happy with the D-P polyline simplification algorithm, because that is what is used in SAS (worked well), and I don't think there are unusual topologies in my map of France that would be so severely out of whack as to lead to *significant* visual artifacts. In fact, you might well expect some artifacts from any visual thinning, but it's a matter of the tradeoff in the way the thinned map is used in a visualization. Mark Monmonier's US Visibility Map might be an extremely thinned, but highly useful example.For R spatial analysis, I think this is worth pursuing and integrating into sp methods. In SAS, proc greduce works simply by adding another variable, density, to the (x,y) coordinates of the spatial polygons, density %in% 1:5, where density==5 is the full map. It is then a simple matter to subset the polygon outlines by sayingdata smallmap; set mymap; where density<4; or proc gmap map=mymap(where=(density<4)); ...Meanwhile, I can't see easily how I could use shapefiles::dp() to thin my Guerry::gfrance maps, because the documentation is,shall we say, somewhat thin. -Michael Roger Bivand wrote:On Wed, 18 Nov 2009, Pinaud David wrote:Hi Michael,maybe you should try the function dp() in the package shapefiles that is an implementation of the Douglas-Peucker polyLine simplification algorithm.Note that its help page does warn that it is not topology-preserving, that is that lines are generalised, but that coincident lines (boundaries of neighbouring polygons) may be generalised differently. GEOS offers a topology-preserving line generalisation facility, which ought to take longer but do better than dp(), because it will not lead to visual artefacts (overlapping polygons, interpolygon slivers, etc.).RogerHTH David Michael Friendly a écrit :The Guerry package contains two maps of france (gfrance, gfrance85) which are quite detailed and large in size (~900K). In writing a vignette for the package, there are quite a few figures that use the map multiple times in a layout, and consequently result in huge file sizes for the .PDF files created. For these purposes, the map need not be nearlyso detailed.I'm wondering if there is a facility to "thin" the map by drawing it at a lower density of lines in the polygon regions. When I was working with SAS, there was a GREDUCE procedure that did this nicely.thanks, -Michael
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