js: distance to (geolocation)

distanceTo_geolocation.js

distanceTo : function(point) {
    var self = this;
    var R = 6371; // mean radius of earth
    var location = self.origin.getPosition();
    var lat1 = self.toRad_(location.lat());
    var lon1 = self.toRad_(location.lng());
    var lat2 = self.toRad_(point.lat);
    var lon2 = self.toRad_(point.lng);
    var dLat = lat2 - lat1;
    var dLon = lon2 - lon1;

    var a = Math.sin(dLat / 2) * Math.sin(dLat / 2) +
            Math.cos(lat1) * Math.cos(lat2) *
            Math.sin(dLon / 2) * Math.sin(dLon / 2);
    var c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1 - a));
    return R * c;
}