public Nullable<double> Altitude {get;}

private void SubscribeToSnapshotEvents()
{
  SnapshotChangedEvent.Subscribe(OnSnapshotChanged);
}
private void OnSnapshotChanged(SnapshotChangedEventArgs args)
{
  if (args == null)
    return;

  var snapshot = args.Snapshot as NMEASnapshot;
  if (snapshot == null)
    return;

  QueuedTask.Run(() =>
  {
    var pt = snapshot.GetPositionAsMapPoint();
    if (pt?.IsEmpty ?? true)
      return;

    // access properties
    var alt = snapshot.Altitude;
    var dt = snapshot.DateTime;
    var vdop = snapshot.VDOP;
    var hdop = snapshot.HDOP;
    // etc

    //TODO: use the snapshot
  });
}