I've got an entire city in Microstation DGN format. I'm able to convert it into Google Earth (KML), DXF, SVG and other formats, and then in .osm with ogr2osm.
However, the 3D model is build entirely with wireframes (triangulated polygon strings).
This does not seem suitable for .OSM.
Does anyone have experience with such problems and is it possible, and will it solve it, to change the model from wireframes into solid surfaces or solid blocks.
No, there is currently no way to full import 3D city models into OSM. The reason is, that OSM 3D abilities use a completely different approach for 'semi 3D' micromapping, while existing formats as City GML use pure 3D models.
If you want to mix both aspects, you need to export an OSM 3D model and export your properitary 3D model and combine both in a open format. You can use for example Blender3D that is complex but very versatile :) But please keep in mind, that this can result in a licensing clash!
