Mesh Editing: Flatten a surface

TODO: Rewrite this

Due to sensor inaccuracies flat floors sometimes are mapped roughly...

straightened out the ground plane using Blender:

1. enable ortographic view (numpad 5),
2. look from the side (numpad 3),
3. enable X-Ray (Alt-z),
4. box select vertices,
5. manually fix the selection,
6. then scale z to 0 (s z 0)

Link: https://www.youtube.com/watch?v=TmKt9RFphf0