While building tf map for gmapping, how to set tf tree?

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While building tf map for gmapping, how to set tf tree?

Following document, http://wiki.ros.org/gmapping

Subscribed Topics tf (tf/tfMessage)Transforms necessary to relate frames for laser, base, and odometry (see below) scan (sensor_msgs/LaserScan)Laser scans to create the map from

I understood like as follow.

subscribe topie as tf made it from laser , base, odom topic,  and publish map topic. to transform tf using [ laser , base, odom ]

And in the document say like below to for making tf tree

Required tf Transforms <the frame="" attached="" to="" incoming="" scans=""> → base_linkusually a fixed value, broadcast periodically by a robot_state_publisher, or a tf static_transform_publisher. base_link → odomusually provided by the odometry system (e.g., the driver for the mobile base)

Provided tf Transforms map → odomthe current estimate of the robot's pose within the map frame

Document say, [ map → odom ] [ base_link → odom ]

but, the project I run gmapping & built map show the tf tree like below [ map -> odom -> base_footprint -> base_link -> base_scan ]

I can not understand what does document saying. As I guess,

[ <the frame="" attached="" to="" incoming="" scans=""> → base_link ] this could be changed as

[  base link -> base scan  ] [  odom -> base link ] like that.

please understand me.

Thankyou.