This is using the Digital Surface Model gathered by the GIS Centre of Zurich.
Take a look here. Loads 70 MB of JSON, so be patient, please :)
You'll get a pretty, colourful model that you can zoom into, turn around and bask in its gorgeousness. Looks like this:
The code within this Repository is licensed under the MIT License but the actual GeoTIFF images are licensed under the GIS-ZH License! Basically, if you use the data, put up a source note, saying
Grundlage: Geodaten © GIS-ZH
and you should be good. But I'm not a lawyer and not liable for anything you do with this data or that results from your use of that data.