2. Angle repetition

When measuring a horizontal angle with a repeating theodolite, it was traditional to advance the horizontal circle so different parts of it would be used for the measurements. This would help compensate for any circle irregularities (recall that a theodolite's circle is etched glass). The most common method was to bring the initial FS reading back to the BS to serve as the beginning of the next measurement, Figure D-5. The angular value accumulated with successive measurements.

 (a) First measurement (b) Second measurement (c) Third measurement Figure D-5 Repeating Measurement

An upper and lower horizontal lock and slow motion was needed to accomplish this. Each would be used at different points in the measurement to either hold the circle or allow it to rotate.

TSIs generally have only a single lock and slow motion for the horizontal circle. The circle itself is electronically encoded so zero can be anywhere the operator chooses. Because the display is digital, the operation can also freeze or hold the reading so it doesn't change as the TSI is rotated. Manipulation of the lock and holding the reading allows the angle to accumulate also on a TSI.