map.yaml format

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The [map_name].yaml file only contains metadata about the map, which is given as a separate [map_name].pgm image file. You can create maps by using a SLAM algorithm and saving it using the map_saver tool. In principle, you can also edit maps by editing .pgm files with a graphics editing program of your choice.

Most relevant information can be found on the map_server wiki page.

How to generate maps with SLAM algorithms is described here (gmapping) and here (hector_mapping).