How to merge multiple map.pgm files?

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I'm currently trying to generate a map in a large scale environment. I want to divide the environment into several smaller ones, generate a map for each of them, and then merge them offline to generate one large map. But I don't know how to do that. Please let me know of any packages or open source on Github that can be integrated.

Also, if you know another way to create a large scale map, please let me know.

Thank you very much.