geoDistance.js

// Converts from degrees to radians.
Math.radians = function(degrees) {
  return degrees * Math.PI / 180;
};

// Converts from radians to degrees.
Math.degrees = function(radians) {
  return radians * 180 / Math.PI;
};

/**
 * Returns the destination point from point having travelled the given distance on the
 * given initial bearing (bearing normally varies around path followed).
 *
 * @param   {Array} coord - coordinates (lat, lon) in degrees.
 * @param   {number} distance - Distance travelled, in same units as earth radius (default: metres).
 * @param   {number} bearing - Initial bearing in degrees from north.
 * @returns {Array} Destination point. Coordinates (lat, lon) in degrees
 *
 * @example
 *     var p1 = [51.4778, -0.0015];
 *     var p2 = destinationPoint(p1, 7794, 300.7);
 *
 * From: http://www.movable-type.co.uk/scripts/latlong.html
 */
function destinationPoint(coord, distance, bearing) {
    var radius = 6371e3;

    var φ1 = Math.radians(coord[0]);
    var λ1 = Math.radians(coord[1]);

    // see http://williams.best.vwh.net/avform.htm#LL

    var δ = distance / radius; // angular distance in radians
    var θ = Math.radians(bearing);

    var φ2 = Math.asin( Math.sin(φ1)*Math.cos(δ) +
                        Math.cos(φ1)*Math.sin(δ)*Math.cos(θ) );
    var λ2 = λ1 + Math.atan2(Math.sin(θ)*Math.sin(δ)*Math.cos(φ1),
                             Math.cos(δ)-Math.sin(φ1)*Math.sin(φ2));
    λ2 = (λ2+3*Math.PI) % (2*Math.PI) - Math.PI; // normalise to -180..+180°

    return [Math.degrees(φ2), Math.degrees(λ2)];
}