var TowerLayer = {};
 TowerLayer.new = func(group, name) {
        var m=Layer.new(group, name, AirportModel ); #FIXME: AirportModel can be shared by Taxiways, Runways etc!!
        m.setDraw( func draw_layer(layer: m, callback: draw_tower, lod:0 ) );
        return m;
}
register_layer("towers", TowerLayer);