The allothetic source corresponds the sensors of the robot, like a camera, a microphone, laser, lidar or sonar.[citation needed] The problem here is "perceptual aliasing". This means that two different places can be perceived as the same. For example, in a building, it is nearly impossible to determine a location solely with the visual information, because all the corridors may look the same.[2] 3-dimensional models of a robot's environment can be generated using range imaging sensors[3] or 3D scanners.[4][5]