modifying a known map gmapping

I was wondering if it's possible to create a partial map using the navigation stack and then reloading the map later to complete it. Correct me if I am wrong but it seems you can load previously known maps to the amcl demo but that program does not build on the map, trying the same command to the gmapping demo doesnt seem to work: $ roslaunch turtlebot_navigation gmapping_demo.launch map_file:=/tmp/my_map.yaml How does one complete a partial map? Also is it possible to edit the map.pgm file in something like gpaint to correct small errors?