In which file or package the transformation from encoders value to position is made in the case of turtlebot

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I work on turtlebot 2 with ROS hydro. I have the encoder tick, so i make the calculation to get the position. When i compare the calculate position with the position published on the topic /odom I find different value. So I need to know in which file or package the tranformation from encoder to position (in the case of turtlebot) is made in order to compare with my calculation. thank you