The OctoMap library is an open source library for generating volumetric 3D environment models from sensor data. This model data can then be used by a drone for navigation and obstacle avoidance.
:::note The first line changes your default shell editor to gedit. This is recommended for users who have little experience with vim (the default editor), but can otherwise be omitted. :::
Run the following three lines in separate terminal windows. This opens up Gazebo Classic, Rviz and an octomap server.
In Rviz, change the field 'Fixed Frame' from 'map' to 'world' in the top left of the window. Now click the add button in the bottom left and select MarkerArray. Then double click the MarkerArray and change 'Marker Topic' from '/free_cells_vis_array' to '/occupied_cells_vis_array'
Now you should see a part of the floor.
In the Gazebo Classic window, insert a cube in front of the red rotors and you should see it in Rviz.