R/antipodal.R

In rspatial/geosphere: Spherical Trigonometry

# Author: Robert J. Hijmans
# October 2009
# version 1.0
# license GPL3


antipodal <- function(p1, p2, tol=1e-9) {
	p1 <- .pointsToMatrix(p1)
	p2 <- .pointsToMatrix(p2)
	p <- cbind(p1[,1], p1[,2], p2[,1], p2[,2])
	p[,c(1,3)] <- .normalizeLonDeg(p[,c(1,3)])
	diflon <- abs(p[,1] - p[,3])
	diflat <- abs(p[,2] + p[,4])
	## FIX by Gareth Davies
#	(diflat < tol) & (diflon > (180 - tol))
	## FIX by Jonathan Rynd
#	(diflat < tol) & (abs(diflon%%360 - 180) < tol)
	(diflat < tol) & ((cos(p[,2] * pi/180) * abs(diflon%%360 - 180)) < tol) 	
}


antipode <- function(p) {
	p <- .pointsToMatrix(p)
	p[,1] <- .normalizeLonDeg(p[,1] + 180)
	p[,2] <- -p[,2]
	return( p )
}