code: function (pt1, pt2) {
      var lon1 = pt1.coordinates[0],
        lat1 = pt1.coordinates[1],
        lon2 = pt2.coordinates[0],
        lat2 = pt2.coordinates[1],
        dLat = this.numberToRadius(lat2 - lat1),
        dLon = this.numberToRadius(lon2 - lon1),
        a = Math.pow(Math.sin(dLat / 2), 2) + Math.cos(this.numberToRadius(lat1))
          * Math.cos(this.numberToRadius(lat2)) * Math.pow(Math.sin(dLon / 2), 2),
        c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1 - a));
      return (6371 * c) * 1000; // returns meters
    }
docs: from http://www.movable-type.co.uk/scripts/latlong.html