Points appear not on ground in sub-360° LIDAR scan

[AstroAure]

Environment

• OS Version: Ubuntu 22.04
• Gazebo version: Ign

Description

Using a LIDAR with less than 360° horizontal scan, the left- and right-most points appear completely out of the plane of the other points, either too low or too high, but always aligned. This is not a visual bug as these abnormal points also show in the topic and bag.

• Expected behavior: All points should fall on the ground surface.
• Actual behavior: Left- and right-most points (first and last vertical scans) appear above or below the surface, not always with the same offset.

Steps to reproduce

LIDAR is defined in URDF as following

 <gazebo reference="lidar">
    <sensor type='gpu_lidar' name='lidar'>"
      <pose>0 0 0 0 0 0</pose>
      <topic>/model/robot/lidar</topic>
      <update_rate>1.0</update_rate>
      <always_on>1</always_on>
      <ignition_frame_id>robot/lidar</ignition_frame_id>
      <visualize>true</visualize>
      <ray>
        <scan>
          <horizontal>
            <samples>900</samples>
            <resolution>1}</resolution>
            <min_angle>${radians(-45)}</min_angle>
            <max_angle>${radians(45)}</max_angle>
          </horizontal>
          <vertical>
            <samples>300</samples>
            <resolution>1</resolution>
            <min_angle>${radians(-30)}</min_angle>
            <max_angle>${radians(0)}</max_angle>
          </vertical>
        </scan>
        <range>
          <min>0.01</min>
          <max>30</max>
          <resolution>0.01</resolution>
        </range>
        <noise type="gaussian">
          <stddev>0</stddev>
        </noise>
      </ray>
    </sensor>
  </gazebo> 

Output

Resulting point cloud in rviz (issue is not only visual).

Directly in the reference frame of the LIDAR (no TF applied), the problem is also present.