Douglas-Peuker line generalization for Spatial Polygons
The function applies the implementation of the Douglas-Peuker algorithm for line generalization or simplification (originally from shapefiles) to objects inheriting from Spatial Polygons. It does not preserve topology, so is suitable for visualisation, but not for the subsequent analysis of the polygon boundaries, as artefacts may be created, and boundaries of neighbouring entities may be generalized differently. If the rgeos package is available,
thinnedSpatialPolyGEOS will be used with partial topology preservation instead of the R implementation here by passing arguments through.
thinnedSpatialPoly(SP, tolerance, minarea=0)
- an object inheriting from the SpatialPolygons class
- the tolerance value in the metric of the input object
- the smallest area of Polygon objects to be retained
- An object of the same class as the input object
Warnings reporting: Non-finite label point detected and replaced, reflect the changes in the geometries of the polygons induced by line generalization.
Douglas, D. and Peucker, T. (1973). Algorithms for the reduction of the number of points required to represent a digitized line or its caricature. The Canadian Cartographer 10(2). 112-122.
xx <- readShapeSpatial(system.file("shapes/sids.shp", package="maptools"), IDvar="FIPSNO", proj4string=CRS("+proj=longlat +ellps=clrk66")) object.size(as(xx, "SpatialPolygons")) xxx <- thinnedSpatialPoly(xx, tolerance=0.05, minarea=0.001) object.size(as(xxx, "SpatialPolygons")) par(mfrow=c(2,1)) plot(xx) plot(xxx) par(mfrow=c(1,1))