WALLS: remove walls below groundplane

worlds/walls.sdf

@@ -166,18 +166,6 @@
     <model name="ground_plane">
       <static>true</static>
       <link name="link">
-        <inertial>
-          <inertia>
-            <ixx>0.16666</ixx>
-            <ixy>0</ixy>
-            <ixz>0</ixz>
-            <iyy>0.16666</iyy>
-            <iyz>0</iyz>
-            <izz>0.16666</izz>
-          </inertia>
-          <mass>10</mass>
-          <pose>0 0 0 0 0 0</pose>
-        </inertial>
         <collision name="collision">
           <geometry>
             <plane>
@@ -225,20 +213,20 @@
       <self_collide>false</self_collide>
     </model>
     <model name="box1">
-      <pose>5 0 0 0 0 0</pose>
+      <pose>5 0 7.5 0 0 0</pose>
       <static>true</static>
       <link name="link1">
       <collision name="collision">
         <geometry>
           <box>
-            <size>1 20 30</size>
+            <size>1 20 15</size>
           </box>
         </geometry>
       </collision>
       <visual name="visual">
         <geometry>
           <box>
-            <size>1 20 30</size>
+            <size>1 20 15</size>
           </box>
         </geometry>
         <material>
@@ -251,19 +239,19 @@
     </model>
     <model name="box2">
       <static>true</static>
-      <pose>-3 5 0 0 0 0</pose>
+      <pose>-3 5 7.5 0 0 0</pose>
       <link name="link2">
       <collision name="collision">
         <geometry>
           <box>
-            <size>10 1 30</size>
+            <size>10 1 15</size>
           </box>
         </geometry>
       </collision>
       <visual name="visual">
         <geometry>
           <box>
-            <size>10 1 30</size>
+            <size>10 1 15</size>
           </box>
         </geometry>
         <material>
@@ -276,19 +264,19 @@
     </model>
     <model name="box3">
       <static>true</static>
-      <pose>13 -10 0 0 0 0</pose>
+      <pose>13 -10 7.5 0 0 0</pose>
       <link name="link2">
       <collision name="collision">
         <geometry>
           <box>
-            <size>17 1 30</size>
+            <size>17 1 15</size>
           </box>
         </geometry>
       </collision>
       <visual name="visual">
         <geometry>
           <box>
-            <size>17 1 30</size>
+            <size>17 1 15</size>
           </box>
         </geometry>
         <material>
@@ -300,20 +288,20 @@
       </link>
     </model>
     <model name="box4">
-      <pose>12 0 0 0 0 0</pose>
+      <pose>12 0 7.5 0 0 0</pose>
       <static>true</static>
       <link name="link1">
       <collision name="collision">
         <geometry>
           <box>
-            <size>1 20 30</size>
+            <size>1 20 15</size>
           </box>
         </geometry>
       </collision>
       <visual name="visual">
         <geometry>
           <box>
-            <size>1 20 30</size>
+            <size>1 20 15</size>
           </box>
         </geometry>
         <material>