Figure from ROBOT, Moravec, Oxford, 1998, Chapter 2: Caution! Robot Vehicle!, page 35
2D sonar occupancy grid

1990-2D evidence grid of a difficult hallway
A robot with a belt of 24 Polaroid sonar units obtained 624 range measurements along an 8 meter hallway. More than half the ranges were too long or missing because of deflections by the mirrorlike walls. A good reconstruction, in a 64 by 32 cell grid, was nevertheless obtained because the evidence patterns representing sonar ranges had been well adjusted for this kind of environment by a learning process. The traversed hallway runs left-right. The start of an adjoining hallway, running upwards, can be seen on the right.