# R functions for Earth geographic coordinate calculations

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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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