#' Calculate distance based on spherical law of cosines
#'
#' @param long1 starting longitude point
#' @param lat1 starting latitude point
#' @param long2 ending longitude point
#' @param lat2 ending latitude point
#' @return Distance in km
#' @examples
gcd_slc <- function(long1, lat1, long2, lat2) {
R <- 6371 # Earth mean radius [km]
d <- acos(sin(lat1)*sin(lat2) + cos(lat1)*cos(lat2) * cos(long2-long1)) * R
return(d) # Distance in km
}
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