How to localize the turtlebot3 after changing the position in a known map

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Hello there! i am pretty new to ROS. After gmapping and navigating the turtlebot3 in a map(simulation), i can see that the turtlebot3 can localize itself when i take the robot and place it in another known place in the map with the help of the laser data.Does anybody help me specifically on how it does....?

I am trying to implement the turtlebot3 navigation in my own robot and it cannot detect the position if i take it and place on a new position in a known map...

thanks...