http://dug9.users.sourceforge.net/web3d/temp/geospatial_SRF_Convert_WalkSurface_Nav_Tile_Sys_UG_rev3.html This version includes facilities for viewing underground 1. explicit mention of vertical panning in GeoViewpoint 2. translucencySurface list, and range and translucency value 3. translucency color mask and threshold