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If you are trying to infer linear position by the number of rotations (really number of the encoder ticks, so full or partial rotations) of the encoder - you'd need to have a way to provide a zero position first - pretty much each time you start the robot program. You might use a bump switch or some other type of fixed position sensor for this. Depending if you are using a set of chains, belts, or winch wire, you might have to also worry about some type of measurement hysteresis...

You might also try a linear magnetic encoder too, you'd get direct linear measurements instead of having to transform the rotational ticks into linear position.