private void removeBaseMap() {
        if (baseMap != null) {
            mMapLayers.removeGroup(LayerHelper.ZINDEX_BASEMAP);
        }
        baseMap = null;
    }
call this instead of VTM.setBaseMap so that we can keep track of baseMap set by tile provider
main/src/main/java/cgeo/geocaching/unifiedmap/mapsforgevtm/MapsforgeVtmFragment.java