Reading coordinates from gmapping map

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Hi. I have a following problem.

I want to localize my robot with triangulation method. I have a rosbag with recorded movement and I managed to create a map with gmapping. I created a launch file where I have declared appropriate parameters, thus I have satisfying map with 1184x1280 resolution in yaml format.

I'm a little stuck right now cause I have no idea how can I use this map in my calculations.

I'm quite a newbie in ROS so I will be grateful for any advice.