What exactly is an urban farm?

Does it have to be on buildings? This is an area near my hometown, it has good public transport (the next Metro stop is a bit of a walk but you could take the light rail) https://www.google.de/maps/@49.4845803,11.0337896,15z/data=!...

These are all greenhouses and vegetable gardens.

This is Westland “Food Valley” it’s between Den Haag and Rotterdam ... https://www.google.de/maps/place/Westland,+Niederlande/@51.9...

Is this an urban farm? Is there any advantage to growing vegetables on a roof instead of next door?