gistfile1.js

// Calculate destination point given start point, initial bearing (deg) and distance (km)
LatLon.prototype.destPoint = function(brng, d) {
  var R = 6371; // Earth's mean radius in km
  var lat1 = this.lat.toRad();
  var lon1 = this.lon.toRad();
  var brng = brng.toRad();
  var lat2 = Math.asin(Math.sin(lat1) * Math.cos(d / R) + 
                       Math.cos(lat1) * Math.sin(d / R) * Math.cos(brng));
                         
  var lon2 = lon1 + Math.atan2(Math.sin(brng) * Math.sin(d / R) * Math.cos(lat1), 
                               Math.cos(d / R) - Math.sin(lat1) * Math.sin(lat2));
  
  // Normalise to -180..+180
  lon2 = (lon2 + Math.PI) % (2 * Math.PI) - Math.PI;

  if (isNaN(lat2) || isNaN(lon2)){
    return null
  } else {
    return new LatLon(lat2.toDeg(), lon2.toDeg());
  }
}

// Convert degrees to radians
Number.prototype.toRad = function(){
  return this * Math.PI / 180;
}

// Convert radians to degrees (signed)
Number.prototype.toDeg = function(){
  return this * 180 / Math.PI;
}

// Convert radians to degrees (as bearing: 0..360)
Number.prototype.toBrng = function(){
  return (this.toDeg()+360) % 360;
}

// Construct a LatLon object: arguments in numeric degrees
// NOTE: All LatLong methods expect & return numeric degrees (for lat/long & for bearings)
function LatLon(lat, lon){
  this.lat = lat;
  this.lon = lon;
}