Sunday, May 29, 2011

R functions for Earth geographic coordinate calculations

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)
}
Created by Pretty R at inside-R.org


#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)
}
Created by Pretty R at inside-R.org


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)
}
Created by Pretty R at inside-R.org


#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)
} 
Created by Pretty R at inside-R.org

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