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This page gives a nice overview and mostly answers the questions you asked: http://wiki.ros.org/navigation/Tutorials/RobotSetup

It is one of the navigation tutorials. They are an excellent resource, well worth going through, and are linked to from that page.

Basically, you start by publishing the output of the encoders and the gyro as an odometry message and also publish it as a transform that connects your robot (e.g., "base_link" or "base_footprint") to the odom reference frame. amcl and gmapping incorporate the laser scan. amcl will provide the map reference frame transform and localize the robot while gmapping will build a map from the laser data.