[regularfry]

Motor encoders drift. If the wheels ever slip, your encoders are now telling you the wrong position. In fact, that's how this system works: they read encoders to get a dead reckoning position, then correct it with the visual system to give full accuracy.

The advantage of a photo survey is that you can do it with the bot hardware (as in fact they do here; they dragged it around but I don't see why you couldn't do random-walk to cover an area), so setup cost should be relatively constrained. Computationally you're not exactly breaking the bank either - it's running at 4fps on an Nvidia Jetson TX1, which I suspect is overkill.