How to feed physical dimensions of robot to navigation stack?

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First of all, the URDF file is not only used by simulator(e.g. Gazebo), but also used by RViz, which can display a robot model after reading the URDF file.

As for navigation stack, the URDF file doesn't affect the navigation. The local and global planner are setup by several configuration files(costmap_common_params.yaml, global_costmap_params.yaml, local_costmap_params.yaml and base_local_planner_params.yaml). The robot footprint is setup in the costmap_common_params.yaml.

For more details, you can see navigation/Tutorials/RobotSetup.

EDIT

It is not necessary to spawn the URDF, it doesn't affect the navigation. RViz can display a robot outline according to the parameters(footprint or robot_radius) in costmap_common_params.yaml. But with URDF, RViz can also display a good-looking robot model. It just for good-looking.