URI paths to packages in the SDF model file?

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Typically, there are 3 methods to let Gazebo software load a model.

1. Export GAZEBO_MODEL_PATH properly. (If Gazebo doesn't start at all, then setup Gazebo and environment variables and then export GAZEBO_MODEL_PATH)

2. Move the models to "gazebo/models" folder directly and include the URI in launch file.

3. In case this is being used with ROS (which is the typical case), make sure that the models and the code is withinthe workspace. (I was struggling with this and I was attempting multi-robot simulation. I realised that the environment variables are sourced in the first instance and sourcing them again from second workspace or any other folder can be difficult. Reference : http://wiki.ros.org/catkin/Tutorials/... )

[Note: I am posting this to help people; I myself struggled to get a working solution (for about 10 hours) and I haven't found any similar information as in this answer]

For New users In case of using ros2 and colcon set:

<export>
<gazebo_ros gazebo_model_path="${prefix}/../../../../src/pkg_name/models"/>
</export>

for example in my case all models (including meshes) are in models/ folder, And this works as expected. tested in gazebo 11 and ros2 foxy. The reasons is {prefix} gives path to install/pkg/share/blah-blah.... directory in newer versions.

SDF uses file instead of package:

<visual name='base_link_visual'>
    <pose>0 0 0 0 0 0</pose>
    <geometry>
      <mesh>
        <scale>1 1 1</scale>
        <uri>file://path_to_dae/textured.dae</uri>
      </mesh>
    </geometry>
  </visual>

or you can use the model uri:

<visual name='base_link_visual'>
    <pose>0 0 0 0 0 0</pose>
    <geometry>
      <mesh>
        <scale>1 1 1</scale>
        <uri>model://model_name/meshes/textured.dae</uri>
      </mesh>
    </geometry>
  </visual>

SDF will use ERB in the near furture. This will essentially add ruby scripting into SDF. Here is an example.