[R-sig-Geo] compute buffer from point shapefile to have shapefile

[Roger Bivand]

On Wed, 17 Feb 2010, Tyler Dean Rudolph wrote:

> Here is another way to compute buffered polygons around points:
>
> ## One record (point) at a time (buffer of 25m radius)
>> temp
>        x        y
> 1 -300358 748992.6
>
> library(spatstat)
> discbuff<-disc(radius=25, centre=c(temp$x, temp$y))

Note that from here you can use the coerce method in maptools:

polybuff <- as(discbuff, "SpatialPolygons")

There are many coercion methods in maptools for sp and spatstat classes.

Hope this simplifies!

Roger

>
> library(sp)
> polybuff<-SpatialPolygons(list(Polygons(list(Polygon(rbind(cbind(discbuff$bdry[[1]]$x,
> y=discbuff$bdry[[1]]$y), c(discbuff$bdry[[1]]$x[1],
> y=discbuff$bdry[[1]]$y[1])))), ID=row.names(temp))))
> plot(polybuff)
> points(temp, col="red")
>
> ## Numerous records (points) at once (buffer of 25m radius)
>> temp
>           x        y
> 1  -300358.0 748992.6
> 7  -300450.8 748764.8
> 13 -300415.1 748865.8
> 19 -300504.5 748698.4
> 25 -300488.4 748460.1
>
> polys<-list()
> for(i in 1:nrow(temp)) {
>  discbuff<-disc(radius=25, centre=c(temp$x[i], temp$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)) {
>  spolybuff<-Polygons(list(polys[[i]]), ID=row.names(temp)[i])
>  spolys<-c(spolys, spolybuff)
> }
>
> polybuffs<-SpatialPolygons(spolys)
>
> plot(polybuffs)
> points(temp, col="red")
>
> Tyler
>
> 	[[alternative HTML version deleted]]
>
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-- 
Roger Bivand
Economic Geography Section, Department of Economics, Norwegian School of
Economics and Business Administration, Helleveien 30, N-5045 Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43
e-mail: Roger.Bivand at nhh.no