A good part of my professional life has been linked with GIS and Virtual Reality applications, mainly 3d models of cities.
Urban0 is an umbrella name I have for all the technology I've used to generate and view 3d models of cities based on real data.
Most major challenges was the scope of the data (requiring streaming systems), data conversion (hundreds of squared kilometers of data that are not exactly well formed) and content generation, which had to mix procedurally generated data with hand-made models.
One of our first challenges was generating 3d data for the city of Lisbon, which we successfully did and spend the next couple of years refining. Some years later, we did the same for the city of Cascais
In these projects, I did most of the programming of the core technology (rendering and streaming systems), and some of the content exporters.