What is tf_static?

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In my launch file I use:

<!-- tf transform -->
  <node pkg="tf" type="static_transform_publisher" name="map_nav_broadcaster" args="-0.45 0 0 3.1765 0 3.1415 /base_link /laser 100"/>

Has this anything to do with /tf_static or is it just the /map_nav_broadcaster node?

No. The map_nav_broadcaster node is only publishing /tf. The only node subscribing to /tf_static is slam_gmapping.

The slam_gmapping node is listening for transforms on both the /tf and /tf_static topics.

Is there an easy way to prevent tf::TransformListeer from subscribing to the /tf_static topic? (only for the sake of clarity of the rqt_graph...)

it is possible to remap in launcher /tf_static to /tf. But I don't know if it is good idea:

<remap from="tf_static" to="tf" />

In Melodic rqt_graph has an option to group the two tf topics into a single group for visualization: https://github.com/ros-visualization/...