Is there a way to extract the edge with coordinates out of occupancy map in ROS?

edit

You can iterate over the leafs of your map and set the bounding box(which has values around your robot)

for (octomap::OcTree::leaf_bbx_iterator it = ourmap->begin_leafs_bbx(min_point, max_point);
        it != ourmap->end_leafs_bbx(); ++it)
{
  if(ourmap->isNodeOccupied(*it)) //check if node is occupied or note
   {
       it.getCoordinate();
   }
}

You can also iterate over all the map leafs not just in bounding box. I hope I understood your question in the write way. if you still have questions or doubts just ask

Note: min_point, max_point are defined as follows:

const octomap::point3d min_point(-10, -10, 0);
octomap::point3d max_point(10, 10, 10);