[R-sig-Geo] Merge polygons

[Robert J. Hijmans]

Hi Tyler,

Your example would have been more easily reproducible if it had
started with something like:

temp = list(x=-300358, y=748992.6,  X2=-300450.8,  Y2=748764.8)
library(spatstat)
...

library(maptools)
union = unionSpatialPolygons(polybuffs, ID=rep(1,
times=length(polybuffs at polygons)) )
plot(union)

# then perhaps:
library(raster)
polygonValues(union, raster)

Robert


On Wed, Feb 17, 2010 at 10:17 AM, Tyler Dean Rudolph
<tylerdeanrudolph at gmail.com> wrote:
> I've been attempting for many moons to develop a function which will allow
> me to place a buffer of adjustable radius around an animal's movement
> trajectory with which I may extract underlying pixel values from a raster of
> environmental data.  I would like to be able to extract the habitat values
> using read functions in the raster package as I have tens of thousands of
> observations (trajectories) and importing or cropping images is much too
> demanding on time and processing.
>
> One fairly intuitive solution is to create circular buffers around points
> along each trajectory, thus creating the buffered trajectory with which I
> can interpolate the raster values.  This is fairly simple and looks good:
>
>> temp
>        x        y        X2       Y2
> 1 -300358 748992.6 -300450.8 748764.8
>
> xran<-range(c(temp$x, temp$X)-500, c(temp$x, temp$X)+500)
>  yran<-range(c(temp$y, temp$Y)-500, c(temp$y, temp$Y)+500)
>  subwin<-owin(xrange=xran, yrange=yran)
>  traj<-pointsOnLines(psp(temp$x, temp$y, temp$X, temp$Y, subwin), eps=12)
>  polys<-list()
>    for(i in 1:length(traj$x)) {
>     discbuff<-disc(radius=25, centre=c(traj$x[i], traj$y[i]))
>     discpoly<-Polygon(rbind(cbind(discbuff$bdry[[1]]$x,
> y=discbuff$bdry[[1]]$y), c(discbuff$bdry[[1]]$x[1],
> y=discbuff$bdry[[1]]$y[1])))
>     polys<-c(polys, discpoly)
>    }
>  spolys<-list()
>    for(i in 1:length(polys)) {
>      polybuff<-Polygons(list(polys[[i]]), ID=i)
>      spolys<-c(spolys, polybuff)
>  }
> polybuffs<-SpatialPolygons(spolys)
>
> plot(polybuffs)
>
> ...but is there a way now to merge all these circles into one polygon?  I'm
> afraid I haven't found it yet yet.
>
> Likewise if there are other methods folks can suggest that would accomplish
> the same goal I am all ears.
>
> Tyler
>
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