private void ShowMarkerImport_Checked(object sender, RoutedEventArgs e)
 {
     if (markerImport == null)
     {
         markerImport = new MarkerImportWindow(airfield, this);
     }
     markerImport.Closed += MarkerImport_Closed;
     markerImport.Show();
 }
 private void MarkerImport_Closed(object sender, EventArgs e)
 {
     markerImport = null;
 }