1. A method of operating a mobile robot, the method comprising: receiving three-dimensional depth image data; producing a local perceptual space corresponding to an environment around the robot; classifying portions of the local perceptual space corresponding to sensed objects located above a ground plane and below a height of the robot as obstacles; classifying portions of the local perceptual space corresponding to sensed objects below the ground plane as obstacles; classifying portions of the local perceptual space corresponding to unobstructed area on the ground plane as free space; classifying all remaining unclassified local perceptual space as unknown; and executing a drive command to move to a location in the environment corresponding to local perceptual space classified as at least of free space and unknown.