void bgWorker_DoWork(object sender, DoWorkEventArgs e)
        {
            BackgroundWorker worker = (BackgroundWorker)sender;

            int[] range   = (int[])e.Argument;
            int   minzoom = range[0];
            int   maxzoom = range[1];

            _emptyQuadKeys = _rasterFS.LoadEmptyTilesFile();

            for (int z = minzoom; z <= maxzoom; z++)
            {
                if (worker.CancellationPending)
                {
                    return;
                }
                GenerateTileSet4Zoom(worker, z);
            }
        }