How to move turtlebot for a certain pre-defined distance

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One approach that is slightly better than timing is to use TF to see how far you've gone. One example I know of is using TF to measure angle in an older piece of code we used to draw a square on the ground using a TurtleBot. Doing a distance comparison should be even easier than angle comparison. In a straight line, the odometry of a Turtlebot shouldn't be too bad so TF should provide a more repeatable distance than timing (especially over longer distances at slower speeds).