[mkgmap-dev] Multipolygons and bounding box / bounding shape

I am trying to close polygons that are divided by the splitter or that are located on the border of the OSM dumps. The OSM dumps are a problem for me as they don't have a rectangular bounding box. The bbox variable of the Osm5XmlHandler gives a rectangular boundig box only. Is there any possibility to get the more or less exact bounding shape of an OSM dump? Otherwise I don't see any chance to close polygons reasonable. WanMil