Use existing map and create a new map at the same time using gmapping

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Hi,

I already have an unclear map which might be suitable for some of the locations but cannot suffice for each location. Therefore I want to initialize the robot using the old map (using amcl and nav_stack) but at the same time create a new map as it moves automatically on the old map (using gmapping). There is a similar question about using gmapping and amcl together. In the comments to one of the answers, someone has mentioned that since gmapping already provides localisation and mapping at the same time so we don't need amcl. But in my case, I am using the old map to get the initial and destination points from the user and then creating a new map once initialized and then saving the map for the next time. How can I start gmapping if I am already navigating on another map?