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It seems to me that a good place to start is to look at the occupancy grid map message from the nav_msgs package http://docs.ros.org/api/nav_msgs/html/msg/OccupancyGrid.html

Also look at the laser scan message from sensor_msgs http://docs.ros.org/api/sensor_msgs/html/msg/LaserScan.html

To visualize it you will probably want to use rviz with the map display type http://wiki.ros.org/rviz/DisplayTypes/Map

Now assuming you have a stream of LaserScan messages you just need to figure out the math to map each ray to a grid cell. To do this you need to know the pose of the laser scanner. You will have to translate the ray direction from the laser scanners coordinate frame to the world coordinate frame. The tf package might help with that http://wiki.ros.org/tf

The hard part will be to handle noise and to properly merge together multiple laser scans over time to get a good probabilistic map.