dem - Retrieving average building height in urban areas?

I am looking for a way to collect data on the height profile of some cities over time. I don't need to produce a detailed 3D reconstruction of cities, nor do I need to detect and measure individual buildings. I would simply need a coarse proxy of how "vertical" a city is - for instance, average building height, or maximum height.

I have in mind a few possible options but I'm not sure how involved they are and what kind of software can do this:

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  Look at the difference between a DSM and a DTM - I am still looking into possible sources (ASTER?)

  
  


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  LiDAR data

  
  


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  Stereo imagery (e.g. IKONOS)

  
  



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  Retrieving building height from shadows – I think even an ArcGis extension does this, but this is not automated and would require a building by building process.

  
  

I was told that software such as Socet Gxp, Leica LPS/XPro, and Pixel Factory can generate height data from satellite imagery automatically. Has anyone heard of those or know of any open source alternatives?