How not to scan the ground as a wall when mapping

asked 2022-03-03 07:03:16 -0500

H.Horimoto
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Hi, Thank you for your help. When I was gmapping a robot with velodyne-16 velodyne_points using pointcloud_to_laserscan, I encountered a phenomenon that the ground in front of the robot was recognized as a wall. if I changed min_height or min_height to extremely narrow, it would be recognized as a wall. The ground is not recognized as a wall, but that does not make the gmapping work, and obstacles below velodyne cannot be detected. I would like to get some advice on what to do. As far as I can think, I would like to remove the laser points if they are widely spaced, but I don't know how to do that. Thank you very much. Thank you in advance.

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Is the lidar always detecting the ground, or does it happen just sometimes while the robot is moving? Could the body of the robot be bouncing? How far away is the real wall?

Mike Scheutzow ( 2022-03-05 08:03:30 -0500 )edit

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answered 2022-03-04 15:55:53 -0500

Mike Scheutzow
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The Velodyne VLP-16 simultaneously scans at different vertical angles from the horizontal plane, producing 16 scans per revolution. If you are merging all 16 scans together, and treating the result as if it was a horizontal scan, that's not going to work well.

When gmapping with this device, most people select just 1 of the 16 scans (one that is approximately horizontal.)

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Thank you for your help. I am new using Velodyne VLP-16. If you don’t mind, please let me know when you have idea to select 1 of the 16 scans(one that is approximately horizontal.) I am sorry if I am wrong, I though that I should use laserscan from velodyne vlp-16 launcher and set ring param to 7? I must not use pointcloud to laserscan? I don't know what should I do. thank you in advance.

H.Horimoto ( 2022-03-04 22:29:35 -0500 )edit

If you are using standard ros packages with a ring value, you are selecting just one of the scans. I think these rings are numbered 0 to 15, with the most-downward-angle scan being 0, however my web searches are turning up no documentation to confirm that. Does the problem happen less if you increase the ring number?

Mike Scheutzow ( 2022-03-05 08:00:45 -0500 )edit

Thank you for your reply. These are websites about it. (https://index.ros.org/p/velodyne_lase...) (https://github.com/ros-drivers/velody...) From what I gather from these two documents, there is a good chance that the default settings will be fine. I'll experiment outdoors next time. what do you think about it?

H.Horimoto ( 2022-03-05 08:17:07 -0500 )edit

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How not to scan the ground as a wall when mapping

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