Which of the following statements is true regarding the compensator used in an electronic theodolite?
- AIt compensates for the effect of tilt of the axis of the bubble tube.
- BIt compensates for the effect of horizontal axis tilt.
- CIt compensates for the effect of vertical axis tilt.Correct
- DIt compensates for the effect of tilt of the line of sight.
Solution
Concept
The compensator in an electronic theodolite senses and corrects the error due to small tilt of the vertical (standing) axis.
Step-by-step solution
1. An electronic theodolite has a tilt sensor called a compensator. 2. Even after levelling, the vertical axis may stay slightly tilted. 3. The compensator measures this small remaining tilt of the vertical axis. 4. It then corrects the vertical angles (single-axis type), and also the horizontal angles (dual-axis type). 5. So it compensates for vertical axis tilt. Answer is (C).
Explanation
Perfect levelling is not possible in the field, so the vertical axis is always a little tilted. The compensator senses this tilt and corrects the angle readings automatically. This is why it is called a vertical axis tilt compensator.
Why the other options are wrong
A: The bubble tube axis is only a levelling aid, not what the compensator corrects. B: Horizontal axis tilt (trunnion axis error) is an instrument error removed by face left and face right readings, not by the compensator. D: Tilt of the line of sight (collimation error) is also removed by calibration or by face left and face right readings.
Shortcut
Compensator = corrects for remaining tilt of the vertical (standing) axis.
Common mistake
Students confuse the compensator with horizontal axis or collimation error correction.
Correct answer: C) It compensates for the effect of vertical axis tilt.