Generate an octomap from your world

Within your <world_name>.world file you should have the following line in between your <world>..</world> tags, which loads the octomap plugin.

<plugin name='gazebo_octomap' filename='librotors_gazebo_octomap_plugin.so'/>

Then you can make a service call as follows, to create an octomap.

rosservice call /world/get_octomap '{bounding_box_origin: {x: 5, y: 0, z: 1.75}, bounding_box_lengths: {x: 20.8, y: 20.8, z: 4}, leaf_size: 0.1, filename: output_filename.bt}'