The NAS is the new common interface for 2D cadastral data in germany and it is based on gml 3.1.1 and the ISO norms 19xxx
NAS conforming data could be visualized using this viewer! It will be projected into the x/y-plane with z=0 for any point not given as 3d point or if the positions are 3d like any other 3d geometry!
http://www.adv-online.de/namespaces/adv/gid/3.1
http://www.alkis.info/ project homepage