[philbarr]

Wow - how does it do that?! Image recognition techniques alone (as far as I know) couldn't do that, right? So how does it know how far away something is?

[rsp1984]

My guess is they estimate one or several dominant scene planes from the sparse triangulated feature points and get scale through incorporating accelerometer measurements.

[doublerebel]

You only need two camera angles and information about camera lens distortion to make a very good estimate. Look up "3d reconstruction".