“…Whenever there is a single surface that the robot uses for navigation, an elevation map is sufficient to model the environment, since overhanging obstacles that are higher than the vehicle, such as trees, bridges or underpasses, can be safely ignored. The strict assumption of a single surface can be relaxed by allowing multiple surfaces per cell (Triebel et al, 2006;Pfaff et al, 2007), or by using classes of cells which correspond to different types of structures (Gutmann et al, 2008). A general drawback of most 2.5D maps is that they do not represent the environment in a volumetric way but discretize it in the vertical dimension based on the robot's height.…”