how to get turtlebot's yaw in gazebo?

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Hello, I'm trying to do line tracking through turtlebot3 (model burger) at gazebo. I think it need to know the value of Turtlebot's heading angle to control the steering. when running the launch file by adding turblebot urdf file, there are the following topics.

<launch>
  <arg name="model" default="$(env TURTLEBOT3_MODEL)" doc="model type [burger, waffle, waffle_pi]"/>
  <arg name="first_tb3"  default="tb3_0"/>
  <arg name="second_tb3" default="tb3_1"/>

  <arg name="first_tb3_x_pos" default="-3.0"/>
  <arg name="first_tb3_y_pos" default=" 0.0"/>
  <arg name="first_tb3_z_pos" default=" 0.0"/>

  <arg name="first_tb3_yaw"   default=" 0.0"/>




<arg name="second_tb3_x_pos" default=" 0.0"/>
  <arg name="second_tb3_y_pos" default="-3.0"/>
  <arg name="second_tb3_z_pos" default=" 0.0"/>
  <arg name="second_tb3_yaw"   default=" 1.57"/>

  <include file="$(find gazebo_ros)/launch/empty_world.launch">
    <arg name="world_name" value="$(find turtlebot3_gazebo)/worlds/empty.world"/>
    <arg name="paused" value="false"/>
    <arg name="use_sim_time" value="true"/>
    <arg name="gui" value="true"/>
    <arg name="headless" value="false"/>
    <arg name="debug" value="false"/>
  </include>

  <group ns = "$(arg first_tb3)">

    <param name="robot_description" command="$(find xacro)/xacro $(find turtlebot3_description)/urdf/turtlebot3_$(arg model).urdf.xacro" />

    <node pkg="robot_state_publisher" type="robot_state_publisher" name="robot_state_publisher" output="screen">
      <param name="publish_frequency" type="double" value="50.0" />
      <param name="tf_prefix" value="$(arg first_tb3)" />
    </node>
    <node name="spawn_urdf" pkg="gazebo_ros" type="spawn_model" args="-urdf -model $(arg first_tb3) -x $(arg first_tb3_x_pos) -y $(arg first_tb3_y_pos) -z $(arg first_tb3_z_pos) -Y $(arg first_tb3_yaw) -param robot_description" />
  </group>

  <group ns = "$(arg second_tb3)">

    <param name="robot_description" command="$(find xacro)/xacro $(find turtlebot3_description)/urdf/turtlebot3_$(arg model).urdf.xacro" />

    <node pkg="robot_state_publisher" type="robot_state_publisher" name="robot_state_publisher" output="screen">
      <param name="publish_frequency" type="double" value="50.0" />
      <param name="tf_prefix" value="$(arg second_tb3)" />
    </node>
    <node name="spawn_urdf" pkg="gazebo_ros" type="spawn_model" args="-urdf -model $(arg second_tb3) -x $(arg second_tb3_x_pos) -y $(arg second_tb3_y_pos) -z $(arg second_tb3_z_pos) -Y $(arg second_tb3_yaw) -param robot_description" />
  </group>

and roslaunch 'rostopic list' are below

/clock
/gazebo/link_states
/gazebo/model_states
/gazebo/parameter_descriptions
/gazebo/parameter_updates
/gazebo/set_link_state
/gazebo/set_model_state
/rosout
/rosout_agg
/tb3_0/cmd_vel
/tb3_0/imu
/tb3_0/joint_states
/tb3_0/odom
/tb3_0/scan
/tb3_1/cmd_vel
/tb3_1/imu
/tb3_1/joint_states
/tb3_1/odom
/tb3_1/scan
/tf
/tf_static

Here, I take the imu topic and calculate the heading angle as a quaternion

example rostopic echo /tb3_1/imu

header: 
  seq: 1211
  stamp: 
    secs: 49
    nsecs: 470000000
  frame_id: "base_footprint"
orientation: 
  x: -0.00272906196186
  y: 0.00272004812062
  z: 0.706975273161
  w: 0.707227768613
orientation_covariance: [0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0]
angular_velocity: 
  x: -3.8725728505e-05
  y: 0.00018048770886
  z: 8.43839290564e-06
angular_velocity_covariance: [0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0]
linear_acceleration: 
  x: 6.26591426547e-11
  y: -3.62427788769e-10
  z: -1.67375709478e-11
linear_acceleration_covariance: [0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0]

here are quaternion code ... (more)