ROS Occupancy Grid

An occupancy grid is how a mobile robot sees the world: the territory is split into square cells, each marked as free, occupied or unknown. In ROS and ROS 2 such a map is stored as a pair of files — a raster .pgm and a .yaml description — served through map_server, and the Nav2 planners build a route on it.

Usually the robot builds such a map itself, driving around the territory with a scanner. A map from OpenStreetMap is needed where the robot has not driven yet: to model deliveries across a block, to test a planner on a real street and courtyard network, to prepare a scene for simulation, to give the robot an initial idea of the district.

What is inside: a ZIP with map.pgm (a black-and-white raster) and map.yaml with the resolution, the origin and the occupancy thresholds. Buildings, water and permanent barriers such as walls and fences become occupied cells; roads, sidewalks and parks become free ones; everything else stays unknown. The default resolution is 0.5 meters per pixel and can be set down to 0.1 meters; the raster side is capped at 10,000 pixels, so a large district has to be taken at a coarser resolution.