var ID = navigator.geolocation.watchPosition(function(pos){  ...},function(err){  ...},{  ...});

navigator.geolocation.clearWatch(ID);

function toRadians(degree) {

  return degree * Math.PI / 180;

}function haversine(latitude1, longitude1, latitude2, longitude2) {

  var R = 6371;  var deltaLatitude = toRadians(latitude2-latitude1); 

  var deltaLongitude = toRadians(longitude2-longitude1);

  latitude1 = toRadians(latitude1);

  latitude2 = toRadians(latitude2); 

  var a = Math.sin(deltaLatitude/2) * Math.sin(deltaLatitude/2) +   

  Math.cos(latitude1) * Math.cos(latitude2) *   

  Math.sin(deltaLongitude/2) * Math.sin(deltaLongitude/2); 

  var c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1-a)); 

  var d = R * c;  return d;

}