Can I extract circular buffers from polygon layers?

Question

I have been working in QGIS with a large and detailed shapefile where remaining forest fragments are represented as polygons. I would like to extract circlular buffers, of various radii, from this vector layer using R. The forest fragment file and a point I need to buffer can be found here, and a radius I am interested in is 1.128km.

There seem to be ways to extract circular buffers from rasters, but not from polygon layers. Is the only way for me to perform this operation to first convert my polygon layer into raster form in GRASS, saga-gis, or GDAL?

Answer 1

Here is an approach. Some example data:

library(dismo)
# points
n <- 10
set.seed(123)
xy <- cbind(runif(n, -150, 150), runif(n, -50,  50))
# polygons
p1 <- rbind(c(-180,-20), c(-140,55), c(10, 0), c(-140,-60), c(-180,-20))
p2 <- rbind(c(-10,0), c(140,60), c(160,0), c(140,-55), c(-10,0))
p3 <- rbind(c(-125,0), c(0,60), c(40,5), c(15,-45), c(-125,0))
p <- spPolygons(p1, p2, p3, attr=data.frame(pID = 1:3), crs="+proj=longlat +datum=WGS84")

Create circles:

cc <- geometry(circles(xy, 1000000, lonlat=TRUE, dissolve=FALSE))
cc <- SpatialPolygonsDataFrame(cc, data.frame(cid=1:n))

Intersect polygons with circles:

pc <- intersect(p, cc)

Inspect results:

plot(p, border='blue', lwd=2)
plot(cc, add=TRUE, border='red', lwd=2)
plot(pc, col='gray', add=TRUE)

P.S. with a raster you can do this:

r <- raster()
values(r) <- 1:ncell(r)
buf <- extract(r, xy, buf=100000)

Answer 2

The buffer tool from the raster package lets you work with both rasters and Spatial* objects. To to create a buffer for a single point:

library(raster)
pt <- SpatialPointsDataFrame(data.frame(525000,9250000), data = data.frame('Pt1'), proj4string = CRS("+init=epsg:32736"))
pt.buf <- buffer(pt, width = 500, )

It returns a spatial polygon:

> summary(pt.buf)
Object of class SpatialPolygons
Coordinates:
      min     max
x  524500  525500
y 9249500 9250500
Is projected: TRUE 
proj4string :
[+init=epsg:32736 +proj=utm +zone=36 +south +datum=WGS84 +units=m
+no_defs +ellps=WGS84 +towgs84=0,0,0]

Plot it:

plot(pt.buf)
points(pt, pch = 19, col = 'red')

For more control over the output, the rgeos package gbuffer has more options. To get a buffer with more segments (e.g. 10) and a smoother curve:

library(rgeos)
pt.buf2 <- gBuffer(pt, width = 500, quadsegs = 10)