Robot moves through ground plane

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Hi,

I have two ground planes in my simulation, one with 0 rpy and the other pitching by 30 degrees. The aim is to have a robot move along the flat plain and then move along the pitched plane.

The problem is that my robot simply moves though the second plane instead of moving along it.

The world file definition is:

<sdf version='1.6'>
  <world name='default'>
    <light name='sun' type='directional'>
      <cast_shadows>1</cast_shadows>
      <pose frame=''>0 0 10 0 -0 0</pose>
      <diffuse>0.8 0.8 0.8 1</diffuse>
      <specular>0.1 0.1 0.1 1</specular>
      <attenuation>
        <range>1000</range>
        <constant>0.9</constant>
        <linear>0.01</linear>
        <quadratic>0.001</quadratic>
      </attenuation>
      <direction>-0.5 0.5 -1</direction>
    </light>
    <model name='ground_plane'>
      <static>1</static>
      <link name='link'>
        <collision name='collision'>
          <geometry>
            <plane>
              <normal>0 0 1</normal>
              <size>100 100</size>
            </plane>
          </geometry>
          <surface>
            <friction>
              <ode>
                <mu>100</mu>
                <mu2>50</mu2>
              </ode>
              <torsional>
                <ode/>
              </torsional>
            </friction>
            <contact>
              <ode/>
            </contact>
            <bounce/>
          </surface>
          <max_contacts>10</max_contacts>
        </collision>
        <visual name='visual'>
          <cast_shadows>0</cast_shadows>
          <geometry>
            <plane>
              <normal>0 0 1</normal>
              <size>100 100</size>
            </plane>
          </geometry>
          <material>
            <script>
              <uri>file://media/materials/scripts/gazebo.material</uri>
              <name>Gazebo/Grey</name>
            </script>
          </material>
        </visual>
        <self_collide>0</self_collide>
        <kinematic>0</kinematic>
        <gravity>1</gravity>
      </link>
    </model>
    <gravity>0 0 -9.8</gravity>
    <magnetic_field>6e-06 2.3e-05 -4.2e-05</magnetic_field>
    <atmosphere type='adiabatic'/>
    <physics name='default_physics' default='0' type='ode'>
      <max_step_size>0.001</max_step_size>
      <real_time_factor>1</real_time_factor>
      <real_time_update_rate>1000</real_time_update_rate>
    </physics>
    <scene>
      <ambient>0.4 0.4 0.4 1</ambient>
      <background>0.7 0.7 0.7 1</background>
      <shadows>1</shadows>
    </scene>
    <spherical_coordinates>
      <surface_model>EARTH_WGS84</surface_model>
      <latitude_deg>0</latitude_deg>
      <longitude_deg>0</longitude_deg>
      <elevation>0</elevation>
      <heading_deg>0</heading_deg>
    </spherical_coordinates>
    <model name='ground_plane_0'>
      <static>1</static>
      <link name='link'>
        <collision name='collision'>
          <geometry>
            <plane>
              <normal>0 0 1</normal>
              <size>100 100</size>
            </plane>
          </geometry>
          <surface>
            <friction>
              <ode>
                <mu>100</mu>
                <mu2>50</mu2>
              </ode>
              <torsional>
                <ode/>
              </torsional>
            </friction>
            <contact>
              <ode/>
            </contact>
            <bounce/>
          </surface>
          <max_contacts>10</max_contacts>
        </collision>
        <visual name='visual'>
          <cast_shadows>0</cast_shadows>
          <geometry>
            <plane>
              <normal>0 0 1</normal>
              <size>100 100</size>
            </plane>
          </geometry>
          <material>
            <script>
              <uri>file://media/materials/scripts/gazebo.material</uri>
              <name>Gazebo/Grey</name>
            </script>
          </material>
        </visual>
        <self_collide>0</self_collide>
        <kinematic>0</kinematic>
        <gravity>1</gravity>
      </link>
      <pose frame=''>19.5012 -0.03334 0 0 -0.523599 0</pose>
        <scale>1 1 1</scale>
        <link name='link'>
          <pose frame=''>19.5012 -0.03334 0 0 -0.523599 0</pose>
          <velocity>0 0 0 0 -0 0</velocity>
          <acceleration>0 0 0 0 -0 0</acceleration>
          <wrench>0 0 0 0 -0 0</wrench>
        </link>
    </model>
    <state world_name='default'>
      <sim_time>68 495000000</sim_time>
      <real_time>68 588233894</real_time>
      <wall_time>1494140513 740272417</wall_time>
      <iterations>68495</iterations>
      <model name='ground_plane'>
        <pose frame=''>0 0 0 0 -0 0</pose>
        <scale>1 1 1</scale>
        <link name='link'>
          <pose frame=''>0 0 0 0 -0 0</pose>
          <velocity>0 0 0 ...