....you don't need autonomous charging stations for city-wide surveillance. You can just fly the drone back home.
Don't forget about quick deployment either.
A well placed charging station with a drone already inside ready to deploy at any time could shave off a number of minutes for getting first eyes on the scene. And sometimes, that very short initial lead time could be crucial in locating a drowning victim, or identifying a fleeing bank robber, or seeing what's going on just a few milliseconds after some gunshots are heard.