Detect whether there is a spatial polygon in a spatial extent

Question

Given a spatial extent (for example cbind(c(0,0),c(10,10)),I want to see whether there is a part of a SpatialPolygons falls in this extent? How can I do this? Thank you very much!

Answer 1

bbox(spatialpolygons) will give you the bounding box of a spatialpolygons object. You just need to test if any of the bounds are outside your extent with four greater-than or less-than tests. There's no need to do any clipping.

Never use gpclib - it has a restrictive license. When you do want to do polygon clipping and buffering and all those groovy GIS things, use rgeos.

Answer 2

Expanding on spacedman's suggestion, here is how you can get the intersection of two extents (NULL if not intersecting). 'sp' is a Spatial* object

library(raster)
e <- extent(c(0,10,0,10))
intersect(e, extent(sp))

But it may indeed be, as whuber points out, that the polygon is in fact not inside extent e. If you need to know that, do

library(rgeos)
gIntersects(as(e, 'SpatialPolygons'), sp)

For large problems, it may be efficient to do

in <- intersect(e, extent(sp))
if (isTRUE(in)) { in <- gIntersects(as(e, 'SpatialPolygons'), sp) }