How to publish map continously?

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Ad 1) I didn't find this type of function in the code. You can call map_server every time you need it in another subscribed node, save it in that node, and then publish it at a frequency of your choice. You should not call the map service in the publisher callback(!).

Ad 2) I don't think it's a good practice, you should call a map service only when you need it because it puts unnecessary strain on the network.

By default the map_server node does not allow that to publish the map continuously, it only publishes the loaded map once on a latched topic. This is due to the assumption that the loaded map by this node is not going to be modified and, usually, if the map is static there is not any good reason to publish it more than once.

To summarize and to provide a bit more information on the topic, unless you have an occupancy grid that is ever changing, for example if you are mapping an unknown region, in which case you would typically not use the map_server anyway, but probably a SLAM approach based node instead (that would handle the update and publishing of the map), it is most likely not an optimal solution to keep publishing the same static map at a given frequency.

However, I do not know your use-case and some assumptions went into this answer... Why would you like to publish the same map with a given frequency? Do you have any specific use-case that you can share?

The map_server node has a built in service called /change_map (I am just a beginner). Maybe it would help you if you could call this service from within your code at a certain frequency?