Sheep, >Its a bit like being a linguist studying the syntax of a language by only >researching poetry What a wondeful statement! You have just given me a very nice phrase to put on the website. I am indeed doing that. Let me assure you that my aim is not to derive any analytical references to the usability of these buildings but merely to make a collage of some nice buildings (mostly unusual shaped) and their SS properties. I mean some famous buildings may not even have interesting structural chracteristics (other than the facade) because they were designed merely to serve the functional needs e.g. boxes on top of each other. Many of the "accessibility indicators" have been researched in other disciplines e.g. influence of colour, shapes, sounds, patterns but more from a product design and scientific visualisation perspective. There is some literature in the human-computer interaction research. The idea of multi-layered graph makes sense in a way that it tries to be a practical solution. There is a lot of work being done by evacuation researchers (some based at CASA, http://www.casa.ucl.ac.uk/cjec/phd/personalhomepage/research.htm#sdss_for_emergency_services) on the topic of "physical accessibility" graphs. Actually, 3D visibility graph are not a lot difficult either other than they are computationally intensive and don't have the nice completely-connected nature of 2D isovist polygons. Sanjay.