@Override
    public void zoomToBounds(final Viewport bounds) {
        zoomToBounds(new BoundingBox(bounds.bottomLeft.getLatitudeE6(), bounds.bottomLeft.getLongitudeE6(), bounds.topRight.getLatitudeE6(), bounds.topRight.getLongitudeE6()));
    }
call this instead of VTM.layers().add so that we can keep track of layers added by the tile provider
main/src/main/java/cgeo/geocaching/unifiedmap/mapsforgevtm/MapsforgeVtmFragment.java