At OpenStreetMap you can download whole cities in 3D: From http://www.openstreetmap.org/export#...53.0767/8.8045 I downloaded the inner city of Bremen as osm file (12.7 MB) With OSM2World (http://wiki.openstreetmap.org/wiki/OSM2World) I converted the osm file to an obj file (28 MB). In Blender I imported the obj file. I scaled the model with the factor 0.02 and applied it, because Blender and espacially the camera behaves strange when the models are bigger then 1000 units. The model contains 12,720 objects. Objects of the same class have the same prefix, e.g. Road5514, RoadJunction2939, Tree2281, Rail4556, Building12049, Fence7478, PostBox1531, ... So it's easy to filter objects of the same class (Menu Select/"Select Pattern...") and join them. The meshes have lots of double faces. I could not get rid of them. But with a little trick I could at least get rid of the render artefacts. With Proportional-Editing-Random and a "Proportional size" covering the the whole model, I moved just one vertex a tiny little in x-, y- and in z-direction. There is a current thread http://blenderartists.org/forum/show...Map-%28-osm%29 about importing osm files with the osm addon in Blender. I thought about to reply on that thread instead of starting a new thread, but all that thread is about an addon to import 2D maps from OpenStreetMap and this thread is about using 3D maps from OpenStreetMap.
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