Hi, I have set up a simple subsriber like the one in [this tutorial](https://answers.ros.org/question/250143/subscribing-to-a-topic-with-python/) and am trying to use it to get data from the nav_msgs/OccupancyGrid [topic](http://docs.ros.org/api/nav_msgs/html/msg/OccupancyGrid.html). As the doc specifies, this data is in the form of an `Int8 []` so the only difference between the tutorial and my code is I replaced "String" with "Int8MultiArray"
I am running what I think is a fairly standard hector_mapping setup, and then I try and launch my subscriber node. The node launches fine and sits there forever, never receiving any messages. When I try `rostopic list` the topic is not there, despite the fact that the [documentation for hector_mapping](http://wiki.ros.org/hector_mapping#Published_Topics) says that it publishes to this nav_msgs/OccupancyGrid topic.
It seems like I'm missing something to tell hector_mapping to publish, except that using `rosrun map_server map_saver` works perfectly, so clearly the data is there. What am I doing wrong?
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