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You are working with three frames: the fixed frame (let's say, odom), the base frame(base_link) and the laser frame (laser)

  1. The pose2D message is the same as the odom->base_link tf being published. It is provided only for convenience (in case you want to subscribe to a topic and not a tf). The value of the message is the pose of the base frame in relation to the fixed frame.

  2. Make sure you note the distinction between the theta input and theta output. The theta input is optional. It can be from an IMU or odometry source. If you have these sensors, it improves the speed and accuracy of the scan matching, because it provides an initial guess. The theta output is the best guess for the angle, given the laser scans. So yes, the theta output is valid even if you don't have any theta input.

  3. That tf best implemented with a static tf publisher. The launch file you linked works.

I hope this helps.