TF tree : incoherent info between tf_monitor and rqt_tf_tree leading to impossible SLAM in gazebo

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Hi all,

I'm facing some weird issues trying to map a simulated environment in Gazebo with hector mapping. First of all, as you can see in the screenshot besides, tf_monitor and rqt_tf_tree are outputting incoherent data on tf trees. Who should I trust more? And why?

Secondly, here is my launch file

<launch>

<!-- these are the arguments you can pass this launch file, for example paused:=true -->
<arg name="paused" default="false"/>
<arg name="use_sim_time" default="true"/>
<arg name="extra_gazebo_args" default=""/>
<arg name="gui" default="true"/>
<arg name="recording" default="false"/>
<!-- Note that 'headless' is currently non-functional.  See gazebo_ros_pkgs issue #491 (-r arg does not disable
 rendering, but instead enables recording). The arg definition has been left here to prevent breaking downstream
 launch files, but it does nothing. -->
<arg name="headless" default="false"/>
<arg name="debug" default="false"/>
<arg name="physics" default="ode"/>
<arg name="verbose" default="false"/>
<arg name="world_name" default="Parking_resized_processed/model.sdf"/> <!-- Note: the world_name is with respect to GAZEBO_RESOURCE_PATH environmental variable -->
<arg name="respawn_gazebo" default="false"/>
<arg name="use_clock_frequency" default="false"/>
<arg name="pub_clock_frequency" default="100"/>

<!-- set use_sim_time flag -->
<group if="$(arg use_sim_time)">
    <param name="/use_sim_time" value="true" />
</group>

<!-- set command arguments -->
<arg unless="$(arg paused)" name="command_arg1" value=""/>
<arg     if="$(arg paused)" name="command_arg1" value="-u"/>
<arg unless="$(arg recording)" name="command_arg2" value=""/>
<arg     if="$(arg recording)" name="command_arg2" value="-r"/>
<arg unless="$(arg verbose)" name="command_arg3" value=""/>
<arg     if="$(arg verbose)" name="command_arg3" value="--verbose"/>
<arg unless="$(arg debug)" name="script_type" value="gzserver"/>
<arg     if="$(arg debug)" name="script_type" value="debug"/>

<!-- start gazebo server-->
<group if="$(arg use_clock_frequency)">
    <param name="gazebo/pub_clock_frequency" value="$(arg pub_clock_frequency)" />
</group>
<node name="gazebo" pkg="gazebo_ros" type="$(arg script_type)" respawn="$(arg respawn_gazebo)" output="screen"
args="$(arg command_arg1) $(arg command_arg2) $(arg command_arg3) -e $(arg physics) $(arg extra_gazebo_args) $(arg world_name)">
    <!-- Added to prevent TF collision with Hector mapping-->
    <!--<remap from="tf" to="gazebo_tf"/>-->
</node>

<!-- start gazebo client -->
<group if="$(arg gui)">
    <node name="gazebo_gui" pkg="gazebo_ros" type="gzclient" respawn="false" output="screen"/>
</group>

<!-- fetch the robot model -->
<arg name="model" default="$(find rover)/urdf/rover_fixed.urdf"/>

<!-- Parse urdf file -->
<param name="robot_description" command="$(find xacro)/xacro --inorder $(arg model)" />

<!-- Spawn the robot into Gazebo -->
<node name="spawn_urdf" pkg="gazebo_ros" type="spawn_model" args="-param robot_description -urdf -model rover -x 9 -y 0 -z 0" />

<!-- Start rviz display -->
<arg name="rvizconfig" default="$(find rover)/rviz/urdf.rviz" />
<node name="joint_state_publisher" pkg="joint_state_publisher" type="joint_state_publisher" />
<node name="robot_state_publisher" pkg="robot_state_publisher" type="state_publisher" />
<node name="rviz" pkg="rviz" type="rviz" args="-d $(arg rvizconfig)" required="true" />

<!-- Start turtlebot teleop key -->
<node name="teleop_key" pkg="turtlebot_teleop" type="turtlebot_teleop_key">
    <remap from="teleop_key/cmd_vel" to="cmd_vel"/>
</node>

<!-- start hector_mapping node -->
<arg name="scan_topic" default="scan" />
<node pkg="hector_mapping" type="hector_mapping" name="hector_mapping" output="screen">
    <param name="map_frame" value="map" /><!-- map -->
    <param name="base_frame" value="base_link" /><!-- base_link -->
    <param name="odom_frame" value="odom" /><!--odom-->
    <param name="scan_topic" value="$(arg scan_topic)" />
    <param name="use_tf_scan_transformation" value="true" />
    <param name="use_tf_pose_start_estimate" value="true" /> <!-- false -->
    <param name="map_resolution" value="0.05" />
    <param name ...