What purpose does the base_footprint link serve?
I'm reading through the [Gazebo simulation tutorial](http://www.romin.upm.es/wiki/index.php?title=ROS%3ATutorials/Simulate_Your_Robot_in_Gazebo), and it instructs the user to create an infinitesimally small nearly weightless box which is linked to the base_link, but it doesn't explain what this is or what it's used for, nor does it link to any resources that do. I did see this [similar question](http://answers.ros.org/question/12770/base_link-to-base_footprint-transform/), but that answer doesn't really explain *why* such a link is used. Why does any part of ROS need to know how the model projects onto the ground?
Does every Gazebo model require this link?
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