Gazebo 2 and SDF V1.5 - Using Populations

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Hi there,

I currently have ROS Indigo running Gazebo 2.2.6 on my Ubuntu 14.04 LTE system without any issues.

My aim is to create a farm world in Gazebo. I've successfully loaded a single .dae grassland mesh in the simulator through a .world file using SDF V1.4. Now, I'm trying to follow this tutorial to add a randomly distributed population of .dae bush models to the world. Looking at this link, I noted that the 'population' tag was introduced in SDF V1.5. I've therefore changed my code to use this format:

<?xml version="1.0"?>
<sdf version="1.5">
  <world name="default">

    <... farmland model, world physics ...>

    <population name="bush_population">
      <model name="bush">
        <include>
          <static>true</static>
          <uri>model://meshes/bush.dae</uri>
        </include>
      </model>
      <pose>0 0 0 0 0 0</pose>
      <box>
        <size>10 10 0</size>
      </box>
      <model_count>10</model_count>
      <distribution>
        <type>random</type>
      </distribution>
    </population>
  </world>
</sdf>

However, this version doesn't seem to work with Gazebo anymore. When I try simulating the world, I get this error:

Error [Converter.cc:127] Unable to convert from SDF version 1.5 to 1.4
Error [parser.cc:697] XML Element[population], child of element[world] not defined in SDF. Ignoring.[world]
Error [parser.cc:688] Error reading element <world>
Error [parser.cc:348] Unable to read element <sdf>
Error:   Could not find the 'robot' element in the xml file
     at line 81 in /build/buildd/urdfdom-0.2.10+dfsg/urdf_parser/src/model.cpp
Error [parser_urdf.cc:2608] Unable to call parseURDF on robot model
Error [parser.cc:273] parse as old deprecated model file failed.

followed by a crash. I suspect the problem has something to do with the libsdformat-dev package installed, but I'm not sure how to go about fixing this without causing dependency issues. I've tried both re-installing Gazebo 2 and migrating to Gazebo 4, but I'd prefer to stay with 2 as 4 seems to cause some other less obvious problems in my programs.

I would really appreciate if anyone has any clues on how to fix this! I'm quite new to Linux/Gazebo/ROS so input from someone with more experience would be very appreciated.

Many thanks in advance! :-)