[R-sig-Geo] voronoi (thiessen) polygons inside irregular area

[Rolf Turner]

The problem is that the tile corresponding to point 8 consists of
two discontiguous polygons one of which is empty of points of the 
pattern.  The empty polygon is juxtaposed with one of the polygons
comprising the tile for point 10, which makes points 10 and 8 neighbours.

I think it suffices to eliminate from each tile any polygons that do not 
contain any points of the pattern.  The following function does
that:

bar <- function(X) {
require(spatstat)
ttt <- dirichlet(X)
sss <- ttt$tiles
f <- function(x,pat){
     www <- lapply(x$bdry,function(y){owin(poly=y)})
     gp  <- sapply(www,function(w){
                any(inside.owin(pat$x,pat$y,w))})
     www[[seq(along=gp)[gp]]]
}
rrr <- lapply(sss,f,pat=X)
ttt$tiles <- rrr
ttt
}

Try:

junk <- bar(poly.w.points)
junk.sp <- as(junk,"SpatialPolygons")
nays <- poly2nb(junk.sp)
nays[10] # Gives 3  7 11 14.  OMMMMMMM!!!!

Rolf Turner

On 21/01/14 20:10, buzzfuzz006 wrote:

> Hi,
> Is there a simple way to draw voronoi/thiessen polygons inside an irregular
> polygon similar to this, but where the polygons created do not extend over
> the boundaries of the overlaying polygon. E.g
> Using the packages
>
> library(spatstat);library(spdep)
>
> If we had set points:
>
> x<-c(0.9,1.7,2.4,2.9,4.83, 0.73, 2.31, 3.69, 4.23, 2.86, 1.91, 4.32, 4.60,
> 1.82)
> y<-c(1.9,0.9,2.8,1.9,1.81, 1.66, 4.54, 5.66, 1.99, 4.03, 4.32, 5.98, 5.56,
> 3.41)
>
> within the set irregular polygon:
>
> x.p<-c(0.1, 6.0, 6.0, 1.0, 5.0, 3.0, 3.5, 0.1)
> y.p<-c(0.1, 1.0, 6.5, 5.5, 5.0, 1.0, 4.8, 5.0)
>
> now I set the polygon to needed dirichlet format
>
> poly.l<-list(x.p,y.p)
> names(poly.l)<-c("x","y")
>
> and create the spatial point pattern
>
> poly.w.points<-ppp(x=x,y=y,poly=poly.l)
>
> then carry out the voronoi tessellation
>
> spatstat.options("gpclib"=T)
> tess<-dirichlet(poly.w.points)
>
> unfortunately if we are to plot this it looks like this
>
> plot(tess)
> text(x,y,1:length(x))
>
> and if we check the neighbours we see that the voronoi polygons extend
> outside the polygon area
> tess.sp<-as(tess,"SpatialPolygons")
> neighs<-poly2nb(tess.sp)
>
> e.g.
> neighs[10]
> shows that point 10 is neighbours (shares boundary) with 8,9 and 13 – whilst
> I want a result whereby 10 would only be neighbours with 7,11,14 and 3.