Buildings, aerials, etc. are placed according to obstructions data etc. See the FlightGear Scenery Object Database for some examples.
OpenStreetMap buildings and objects
Buildings, roads, power cables/pylons, railways, railway platforms, piers, and other man-made objects, as mapped in OSM, can be turned into 3D models for use in FlightGear scenery. This is done with a custom written python script. More information at the FlightGear wiki including generated objects for countries and regions available for download.
- http://wiki.flightgear.org/Terragear_Corine - Howto about Building Europe scenery with SRTM v4, Corine landclass and OpenStreetMap data
- http://forum.flightgear.org/viewtopic.php?f=5&t=9737 - Forum topic about building from OSM data