After loading the urdf model in gazebo, there is no `map` frame on the tf tree.

edit

I defined a robot model with the urdf file, but after gazebo reads the urdf file, I check out the tf tree and find that there is no map frame on the tf tree. I wanna figure out how I can generate a map frame when I spawn my robot model in gazebo. Any advice will be appreciated.

Screenshot of tf tree:

My launch file:

<?xml version="1.0" encoding="utf-8"?>
<!-- Launch this file to spawn cambot model in gazebo. -->
<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="gui" default="true" />
    <arg name="headless" default="false" />
    <arg name="debug" default="false" />
    <!-- We resume the logic in empty_world.launch, changing only the name of the world to be launched -->
    <include file="$(find gazebo_ros)/launch/empty_world.launch">
        <arg name="debug" value="$(arg debug)" />
        <arg name="gui" value="$(arg gui)" />
        <arg name="paused" value="$(arg paused)" />
        <arg name="use_sim_time" value="$(arg use_sim_time)" />
        <arg name="headless" value="$(arg headless)" />
    </include>
    <param name="robot_description" command="$(find xacro)/xacro --inorder $(find cambot_description)/urdf/cambot.urdf.xacro" />
    <node name="urdf_spawner" pkg="gazebo_ros" type="spawn_model" respawn="false" output="screen" args="-urdf -model cambot -param robot_description"></node>
</launch>