Urban Robots Could Use Manholes to Navigate  GPS is generally
the standard to which all other localization technologies are
compared, and in most outdoor environments, it's hard to beat
for accuracy, precision, and reliability. It's funny, then, that
the times you need GPS the most (in places like downtown New
York or San Francisco) usually end up being the times that GPS
utterly fails due to tall buildings blocking out the sky.
Robots have this same problem, so researchers have been trying
to find other ways that a robot can localize itself where GPS is
intermittent. A common strategy is to use wheel odometers or
inertial measurement units to "guess" where the robot has gotten
to since its last external position fix, but accuracy still
relies on landmarks to help correct for errors. So let's see,
what are some things that are easy to find in urban environments
and don't go anywhere? You probably weren't thinking "manhole
covers," but yeah, it's manhole covers.  The reason manhole
covers might be a good idea