R/latlong2km.R

In dill/dillhandy: handy utils

latlong2km<-function(lon,lat,lon0=sum(range(lon))/2,lat0=sum(range(lat))/2) {
## Convert lon,lat co-ordinates to km.n and km.e of west-east and south-north 
## axes through (lon0,lat0) using the "spherical law of cosines"... 
  R <- 6371         ## Earth's mean radius 
  deg2rad = pi/180 ## Conversion factor to convert between degrees to radians
  ## Convert lons and lats from degrees to radians:
  rlon <- lon * deg2rad
  rlat <- lat * deg2rad
  rlat0 <- lat0 * deg2rad
  rlon0 <- lon0 * deg2rad

  delrlon <- rlon - rlon0

  km.n <- sign(rlat-rlat0)*acos(sin(rlat0) * sin(rlat) +
                 cos(rlat0) * cos(rlat) ) * R
  km.e <- sign(rlon-rlon0)*acos(sin(rlat) * sin(rlat) + 
               cos(rlat) * cos(rlat) * cos(delrlon)) * R
  list(km.e=km.e,km.n=km.n)
}