Stainless steel reference points are permanently installed at measurement stations along the tunnel or structure. To obtain a measurement, the operator stretches the tape between two reference points, hooking the free end of the tape to one point and the instrument body to the other. The operator tensions the tape by turning a knurled collar until two index marks are aligned and then notes the reading from the tape and the digital display. The sum of these readings is the distance between the two reference points. By comparing the current reading to the initial reading, the operator can calculate the change in distance between the two reference points

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