R functions for Earth geographic coordinate calculations

May 29, 2011
By

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Here are some functions that I regularly use for geographic data (e.g. binning, filtering, calculation of new positions etc.).

#distance in kilometers between two long/lat positions (from "fossil" package)
earth.dist <- function (long1, lat1, long2, lat2)
{
rad <- pi/180
a1 <- lat1 * rad
a2 <- long1 * rad
b1 <- lat2 * rad
b2 <- long2 * rad
dlon <- b2 - a2
dlat <- b1 - a1
a <- (sin(dlat/2))^2 + cos(a1) * cos(b1) * (sin(dlon/2))^2
c <- 2 * atan2(sqrt(a), sqrt(1 - a))
R <- 6378.145
d <- R * c
return(d)
}

#degree bearing between two long/lat positions (from "fossil" package)
earth.bear <- function (long1, lat1, long2, lat2)
{
rad <- pi/180
a1 <- lat1 * rad
a2 <- long1 * rad
b1 <- lat2 * rad
b2 <- long2 * rad
dlon <- b2 - a2
bear <- atan2(sin(dlon) * cos(b1), cos(a1) * sin(b1) - sin(a1) *
cos(b1) * cos(dlon))
deg <- (bear%%(2 * pi)) * (180/pi)
return(deg)
}

new.lon.lat <-
function (lon, lat, bearing, distance)
{
rad <- pi/180
a1 <- lat * rad
a2 <- lon * rad
tc <- bearing * rad
d <- distance/6378.145
nlat <- asin(sin(a1) * cos(d) + cos(a1) * sin(d) * cos(tc))
dlon <- atan2(sin(tc) * sin(d) * cos(a1), cos(d) - sin(a1) *
sin(nlat))
nlon <- ((a2 + dlon + pi)%%(2 * pi)) - pi
npts <- cbind(nlon/rad, nlat/rad)
return(npts)
}

#tells which lon lat positions are within the defined limits to the west, east, north, and south
lon.lat.filter <-
function (lon_vector, lat_vector, west, east, north, south)
{
if(west>east) {
lon_vector_new=replace(lon_vector, which(lon_vector<0), lon_vector[which(lon_vector<0)]+360)
east_new=east+360
} else {
lon_vector_new=lon_vector
east_new=east
}
hits=which(lon_vector_new < east_new & lon_vector_new > west & lat_vector < north & lat_vector > south)
return(hits)
}

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