docs: function dst(lat1, lon1, lat2, lon2) {
    // generally used geo measurement function
    var dLat = lat2 * Math.PI / 180 - lat1 * Math.PI / 180;
    var dLon = lon2 * Math.PI / 180 - lon1 * Math.PI / 180;
    var a =
      Math.sin(dLat / 2) * Math.sin(dLat / 2) +
      Math.cos(lat1 * Math.PI / 180) *
        Math.cos(lat2 * Math.PI / 180) *
        Math.sin(dLon / 2) *
        Math.sin(dLon / 2);
    var c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1 - a));
    var d = R * c;
    return d * 1000; // meters
  }