buildings

Does anyone know of an app or website that shows the height of surrounding buildings? This would be helpful for RTH and Intelligent Modes. It would be awesome to see an overlay on a map that highlighted buildings that were within a range of 100ft-400ft.

So ive been flying over cities quite abit, I am currently flying over a National Stadium, I was wondering how high I have to fly to get the entire stadium in the frame? is there any calculations or equations i can do to figure this out? or are there any available software or ideas available...