Imagine you want to evaluate e.g. a SLAM-approach using a ground truth. Both of components provide a transformation via TF2 from a map-frame to a base_link-frame. This results in two distinct TF-trees. You can not simply give the same name to the base links because this would break the tree. In some cases it might be possible to flip one of these transformations, but this might not always feasable.
This problem can be solved using tf_tree_connector. tf_tree_connector adds a
static transformation between the two map frames. This is done in a way, that
during the start of tf_tree_connector bot base_link-frames concided, so that
the transformations reference_map -> reference_base_link and
reference_map -> base_link are identical.
