On Sun, 11 Jan 2015, David Depew wrote:
Dear list,
given a set of polygons created from points with a specified buffer
distance,
library(sp)
library(rgeos)
box <- readWKT("POLYGON((0 0, 0 1000, 1000 1000, 1000 0, 0 0))")
plot(box)
set.seed(1)
pts <- spsample(box, n=40, type="random")
pols <- gBuffer(pts, byid=TRUE, width=50)
plot(pols, add=TRUE)
I'd like to merge polygons where the buffers overlap
i.e.
merg.poly=gUnaryUnion(pols)
plot(merg.poly)
but I'm having difficulty figuring out how to specify that the merged
polygons retain some unique Id. Clearly in the package help, one would
specify this by use of the "id" argument, however since the polygons
started as points, overlapping points do not have the same ID. Suppose one
approach is to define a common ID for overlapping polys before using
gUnaryUnion, but I'm at a loss where to begin with that approach. Any
advice would be greatly appreciated,
Thanks for a self-contained example!
Right, this isn't obvious, as you need to go to a list/graph
representation of the intersects predicate, which takes us outside rgeos.
We'll compare this with the equivalent for distance neighbours in spdep:
library(spdep)
dnb <- dnearneigh(pts, 0, 100) # 2*50 buffer
nc <- n.comp.nb(dnb) # find graph components
merg.poly <- gUnaryUnion(pols, nc$comp.id)
plot(merg.poly)
which jumps over the buffered objects. To use the buffered objects, do:
gI <- gIntersects(pols, pols, byid=TRUE, returnDense=FALSE) # list
for (i in seq(along=gI)) {gI[[i]] <- gI[[i]][-(which(gI[[i]] == i))];
if (length(gI[[i]]) == 0) gI[[i]] <- 0L} # remove i<->i edges
class(gI) <- "nb"
attr(gI, "region.id") <- names(gI)
nc1 <- n.comp.nb(gI)
all.equal(nc, nc1) # same result
So what this is doing is having gIntersects return a list. Since i
intersects i, we have to drop the i<->i edges, pretend that this is an nb
object, and use the n.comp.nb() function to find the graph components.
These two approaches give the same results. There is a new vignette in
spdep starting to look at representations of neighbours as graph objects:
http://cran.r-project.org/web/packages/spdep/vignettes/nb_igraph.html
Hope this helps,
Roger
Thanks,
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Roger Bivand
Department of Economics, Norwegian School of Economics,
Helleveien 30, N-5045 Bergen, Norway.
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e-mail: roger.biv...@nhh.no
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