FYI: The floor is the invert, the same in this case but not always. When you dig a tunnel the ground pushes up as well as down so you need to use a tube or else construct an inverted arch to pass the forces out to the walls. For road or rail you then insert a floor or trackbed above the invert. Obviously a storage tank (like a canal tunnel) doesn't need a floor per se.
I don't think that this GTP needs location information!