Crop for SpatialPolygonsDataFrame

Question

I have two SpatialPolygonsDataFrame files: dat1, dat2

extent(dat1)
class       : Extent 
xmin        : -180 
xmax        : 180 
ymin        : -9         


        

Answer 1

Streamlined method added 2014-10-9:

raster::crop() can be used to crop Spatial* (as well as Raster*) objects.

For example, here's how you might use it to crop a SpatialPolygons* object:

## Load raster package and an example SpatialPolygonsDataFrame
library(raster) 
data("wrld_simpl", package="maptools")

## Crop to the desired extent, then plot
out <- crop(wrld_simpl, extent(130, 180, 40, 70))
plot(out, col="khaki", bg="azure2")

---

Original (and still functional) answer:

The rgeos function gIntersection() makes this pretty straightforward.

Using mnel's nifty example as a jumping off point:

library(maptools)
library(raster)   ## To convert an "Extent" object to a "SpatialPolygons" object.
library(rgeos)
data(wrld_simpl)

## Create the clipping polygon
CP <- as(extent(130, 180, 40, 70), "SpatialPolygons")
proj4string(CP) <- CRS(proj4string(wrld_simpl))

## Clip the map
out <- gIntersection(wrld_simpl, CP, byid=TRUE)

## Plot the output
plot(out, col="khaki", bg="azure2")

Answer 2

see ?crop

corp(x, y, filename="", snap='near', datatype=NULL, ...)

x Raster* object

y Extent object, or any object from which an Extent object can be extracted (see Details

You need to rasterize the first SpatialPolygon, using rasterize function from the raster package

I create some data to show how to use rasterize:

n <- 1000
x <- runif(n) * 360 - 180
y <- runif(n) * 180 - 90
xy <- cbind(x, y)
vals <- 1:n
p1 <- data.frame(xy, name=vals)
p2 <- data.frame(xy, name=vals)
coordinates(p1) <- ~x+y
coordinates(p2) <- ~x+y

if I try :

 crop(p1,p2)
 unable to find an inherited method for function ‘crop’ for signature ‘"SpatialPointsDataFrame"’

Now using rasterize

r <- rasterize(p1, r, 'name', fun=min)
crop(r,p2)

class       : RasterLayer 
dimensions  : 18, 36, 648  (nrow, ncol, ncell)
resolution  : 10, 10  (x, y)
extent      : -180, 180, -90, 90  (xmin, xmax, ymin, ymax)
coord. ref. : +proj=longlat +datum=WGS84 
data source : in memory
names       : layer 
values      : 1, 997  (min, max)

Answer 3

You cannot use crop on sp polygon objects. You will need to create a polygon representing the bbox coordinates of dat2 and then can use gIntersects in the rgeos library.

Edit: This comment was in relation to the version available in 2012 and this is no longer the case.

Answer 4

Here is an example of how to do this with rgeos using the world map as an example

This comes from Roger Bivand on R-sig-Geo mailing list. Roger is one of the authors of the sp package.

Using the world map as an example

library(maptools)

data(wrld_simpl)

# interested in the arealy bounded by the following rectangle
# rect(130, 40, 180, 70)

library(rgeos)
# create  a polygon that defines the boundary
bnds <- cbind(x=c(130, 130, 180, 180, 130), y=c(40, 70, 70, 40, 40))
# convert to a spatial polygons object with the same CRS
SP <- SpatialPolygons(list(Polygons(list(Polygon(bnds)), "1")),
proj4string=CRS(proj4string(wrld_simpl)))
# find the intersection with the original SPDF
gI <- gIntersects(wrld_simpl, SP, byid=TRUE)
# create the new spatial polygons object.
out <- vector(mode="list", length=length(which(gI)))
ii <- 1
for (i in seq(along=gI)) if (gI[i]) {
  out[[ii]] <- gIntersection(wrld_simpl[i,], SP)
  row.names(out[[ii]]) <- row.names(wrld_simpl)[i]; ii <- ii+1
}
# use rbind.SpatialPolygons method to combine into a new object.
out1 <- do.call("rbind", out)
# look here is Eastern Russia and a bit of Japan and China.
plot(out1, col = "khaki", bg = "azure2")