Thanks! It's from OpenStreetMap, if available, otherwise made up (for now). In the future we'll be more accurate (perhaps lidar), at least a better educated guess.

You can infer building height from OSM through either an explicit height tag (usually in meters), or from a "levels" tag, that you can convert with some kind of estimate of standard floor height. Here's a demo that lets you see where that data exists: http://osmbuildings.org/?lat=52.52111&lon=13.40988&zoom=16

Thanks! That's actually exactly how we do it right now, based on the techniques in OSM Buildings actually. Ideally I want to infer height from area and building type if no height-based tags exist. I have a proof-of-concept working.