Identifying which points in a regular lattice are within a polygon's boundaries

Question

I would like to work out which points that define a regular lattice are within a polygon. The code below does this but VERY VERY slowly:

#the polygon that I want to check each point against
glasgow_single <- readShapePoly(
    fn="data/clipped/glasgow_single"
)

#interpolated contains the coordinates of the regular grid
points_to_check <- expand.grid(
    x=interpolated$x,
    y=interpolated$y
    )

#function to be called by plyr    
fn <- function(X){
    this_coord <- data.frame(lon=X["x"], lat=X["y"])
    this_point <- SpatialPoints(this_coord)
    out <- gContains(glasgow_single, this_point)
    out <- data.frame(x=X["x"], y=X["y"], val=out)
    return(out)
}

#plyr call
vals <- adply(points_to_check, 1, fn, .progress="text")
vals$val <- as.numeric(vals$val) 

Taking into account both thinking time and computing time, is there a much faster way of doing this?

Answer 1

Yes, there's a much better approach. For this and many other topological operations, the rgeos package has you well covered. Here, you're wanting rgeos::gWithin():

## Required packages 
library(rgdal)
library(raster)  ## For example polygon & functions used to make example points
library(rgeos)   

## Reproducible example
poly <- readOGR(system.file("external", package="raster"), "lux")[1,]
points <- as(raster(extent(poly)), "SpatialPoints")
proj4string(points) <- proj4string(poly)

## Test which points fall within polygon 
win <- gWithin(points, poly, byid=TRUE)

## Check that it works
plot(poly)
points(points, col=1+win)