Display urdf file in gazebo

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Hi, I made a program that autommatically generate an .urdf file from a .fig file (generate by Xfig editor). When I display my urdf file with RViz, the objects and colors are like they should be so everything is normal. I use the command line :

roslaunch urdf_tutorial display.launch model:=PlanSalle.urdf

but when I want to display it in gazebo, it doesn't work. I use :

rosrun gazebo spawn_model -file `pwd`/PlanSalle.urdf -urdf -z 1 -model salle

here is my code :

<robot name="origins">
<link name="base_link">
    <visual>
        <geometry>
            <box size ="18.8 0.4 2"/>
        </geometry>
    </visual>
    <collision>
        <geometry>
            <box size ="18.8 0.4 2"/>
        </geometry>
    </collision>
        <inertial>
                <inertia ixx="0.0" ixy="0.0" ixz="0.0" iyy="0.0" iyz="0.0" izz="0.0"/>
        <mass value="0.1"/>
        </inertial>
</link>

<link name="Mur1">
    <visual>
        <geometry>
            <box size ="11.0 0.4 2"/>
        </geometry>
    </visual>
    <collision>
        <geometry>
            <box size ="11.0 0.4 2"/>
        </geometry>
    </collision>
        <inertial>
                <origin xyz="1 1 1" />
                <inertia ixx="0.0" ixy="0.0" ixz="0.0" iyy="0.0" iyz="0.0" izz="0.0"/>
        <mass value="10"/>
        </inertial>
</link>

<joint name="base_link_to_Mur1" type="fixed">
    <origin xyz="0 0 0" rpy="0 0 0" />      
    <parent link="base_link"/>
    <child link="Mur1"/>
</joint> 
</robot>

Does someone know what's wrong ? thanks