An R function I had to write to calculate the true course/bearing between two lat lon positions. Note that the fossil R library has a function called earth.bear to do the same thing, but hey, who’s installing more libraries at this point?

#finds the bearing aka true course (tc) between two positions

getBearing<-function(lon1, lat1, lon2, lat2) {
 #lon1<-deg2rad(lon1)
 lat1<-deg2rad(lat1)
 
 #lon2<-deg2rad(lon2)
 lat2<-deg2rad(lat2)
 lambda<-deg2rad(lon2-lon1)
 
 Y<-sin(lambda)*cos(lat2)
 X<-cos(lat1)*sin(lat2)-sin(lat1)*cos(lat2)*cos(lambda)
 tc<-atan2(Y,X)
 tc<-rad2deg(tc)
 if (tc<0.0) {
 tc<-360+tc
 }
 tc
}

getBearing(-94.581, 39.099, -90.200,38.627)

#Note that you have to convert degrees to rads and back to make this work, here are the helper functions to do that:

deg2rad <- function(deg) return(deg*pi/180)
rad2deg <- function(rad) return((rad*180)/pi)