Google Cartographer 3D map data interpretation

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Hi,

currently I'm working on custom robot platform and I would like to map an outdoor bumpy environment. I have tested different kinds of sensor configuration (horizontal and/or vertical mounted laser scanner, 3D laser scanner like the Velodyne VLP-16) via the usage of GAZEBO. Additionally an IMU and odometry topic is available.

My problem is, that the generated map only visualizes the surrounding objects but not the ground (there is a lot of unknown space). If I try the 2D version the ground is visualized as free space.

If also tried the provided examples from the cartographer ros page, and I'm getting the same result.

Have anyone in the community some hints, how I can map the whole 3D environment? or an example how I can process further?

Thanks, Clemens

Update:

If I run e.g. this example (Link to example), I get a map which contains some obstacles in a light red color (values inside the map topic are round about 50 ). But I get no information about the ground/floor. All other spaces are represented inside the map topic as unknown space. Is it possible to change e.g. a parameter to recognize the floor?