know the diameter of your wheel you know how much you have moved well that's approximately true because the wheel might have slipped there might have been bumps in the road where it's if it's not flat you don't move as far forward as you thought you would have because you really have been going down and back up but it gives you some kind of measurement of how much you've moved then often people with a laser rangefinder on a mobile robot why is that it sends out a laser beam and then it measures how long it takes for that