Explanation:
That depends on the tension on the tape.
As defined, the tape, due to gravity would form a catenary, a curve, so the measurement would be less than 30 m. The tension on the tape would affect the degree to which the measurement would be less. A tension well within the limits of the steel used to make the tape would be would be well within the accuracy of the tape. Over 30 m, the tape is likely +/- 2 mm, the recorded value could be 30m, +1mm/-3mm. If you need more than that degree of accuracy, you don’t use a tape.