R - How to find points within specific Contour

Question

I am creating density plots with kde2d (MASS) on lat and lon data. I would like to know which points from the original data are within a specific contour.

I create

Answer 1

You can use point.in.polygon from sp

## Interactively check points
plot(bw)
identify(bw$lon, bw$lat, labels=paste("(", round(bw$lon,2), ",", round(bw$lat,2), ")"))

## Points within polygons
library(sp)
dens <- kde2d(x, y, n=200, lims=c(c(-73, -70), c(-13, -12)))  # don't clip the contour
ls <- contourLines(dens, level=levels)
inner <- point.in.polygon(bw$lon, bw$lat, ls[[2]]$x, ls[[2]]$y)
out <- point.in.polygon(bw$lon, bw$lat, ls[[1]]$x, ls[[1]]$y)

## Plot
bw$region <- factor(inner + out)
plot(lat ~ lon, col=region, data=bw, pch=15)
contour(dens, levels=levels, labels=prob, add=T)