Transit theodolite — when the instrument is not in perfect adjustment, which observing procedure still yields accurate deflection angles?
-
ASet vernier A to zero on the back station and then plunge the telescope before turning
-
BSet vernier A to zero on the back station and directly turn to the forward station
-
CTake two back sights: one with the telescope normal and one with the telescope inverted, then use the mean
-
DNone of these
Answer
Correct Answer: Take two back sights: one with the telescope normal and one with the telescope inverted, then use the mean
Explanation
Introduction / Context:Theodolites may suffer from residual collimation or axis errors if not fully adjusted. However, careful observing procedures can remove or greatly reduce their effects so that deflection angles remain reliable. This is especially important during route surveys and curve setting where many short turns are made.
Given Data / Assumptions:
- The instrument is not perfectly adjusted (small collimation/axis errors).
- Deflection angles must be measured as accurately as possible.
- Standard reversing (face left/face right) procedures are available.
Concept / Approach:Observations in both positions of the telescope (normal and inverted) reverse the sign of collimation error. By taking readings in both faces and averaging, the error cancels, leaving the true angle. This is the essence of the double-sighting (repetition with faces reversed) procedure widely recommended for precise work with imperfectly adjusted instruments.
Step-by-Step Solution:
Back-sight with telescope normal (face left/right as per instrument), set the reference, and note the reading.Reverse the telescope (plunge), re-sight the same back station, and note the reading again.Turn to the forward station for both faces and compute the deflection angles.Average the two face results to obtain the corrected deflection angle.Verification / Alternative check:The face-averaging method is standard in geodetic practice; residuals indicate adjustment quality and random error magnitude.
Why Other Options Are Wrong:
- Setting zero and plunging without dual observations does not inherently cancel collimation error.
- Turning directly in one face carries the full error into the reading.
- “None” is incorrect because the two-face method is known to work.
Common Pitfalls:Failing to record both faces consistently; not re-sighting carefully on the same points can defeat the cancellation.
Final Answer:Take two back sights: one with the telescope normal and one with the telescope inverted, then use the mean