---
title: "Errors eliminated by face observation"
subject: "Surveying I"
chapter: 7
key: "semester_3/surveying_i:face-observation-errors-and-temporary-adjustment"
source: https://www.bctengineer.com/q/semester_3/surveying_i/face-observation-errors-and-temporary-adjustment
site: BCT Engineering
---

# Errors eliminated by face observation

Surveying I · Chapter 7: Transit and Theodolite

- Asked in: 2076 Asoj, 2+2 marks
- Video: [Sources of Errors in Theodolite - Theodolite Traversing - Surveying 1](https://www.youtube.com/watch?v=PlNdmbSgJ30) — Ekeeda
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/surveying_i/face-observation-errors-and-temporary-adjustment
- Whole chapter: https://www.bctengineer.com/md/semester_3/surveying_i/chapter-7

## Question

List out the errors which are eliminated by taking face observations of the theodolite; also explain the mechanism of elimination with neat sketches. Explain in brief about the temporary adjustment of theodolite.

## Short answer

Face observations eliminate collimation error, trunnion axis error, vertical index error, eccentricity, and non-parallelism of line of collimation and altitude bubble. Changing face reverses the error sign; the mean of FL and FR readings cancels them. Teporal adjustments are centering, levelling, and parallax elimination.

## Answer

#### Errors eliminated by face observations
**Face observation** means observing a point with the telescope in the face-left position (vertical circle on the left, FL), then again after transiting the telescope and turning the instrument through 180 degrees (face right, FR), and taking the mean.

| Error | Cause |
|---|---|
| Line of collimation error | Line of sight not perpendicular to the horizontal axis |
| Horizontal (trunnion) axis error | Horizontal axis not perpendicular to the vertical axis |
| Vertical circle index error | Vernier zero not exactly at the zero of the vertical circle when the telescope is horizontal |
| Eccentricity of the vertical circle / vernier (partly) | Centre of the circle not on the axis |
| Error due to the line of collimation and altitude bubble not being parallel | Altitude bubble axis not parallel to the line of sight |

#### Mechanism of elimination
**1. Collimation error.** On face left the line of sight deviates from the correct direction by a small angle $e$ to one side. After changing face, the deviation is on the *opposite* side, so one reading is too large by $e$ and the other is too small by $e$. The mean is free of the error.

```text
  FL:   X-------- true line ---------> P
          \_ line of sight (+e)
  FR:   X-------- true line ---------> P
          /_ line of sight (-e)
        mean of FL and FR = true direction
```

**2. Trunnion axis error.** If one end of the horizontal axis is higher, the telescope sweeps an inclined plane. After changing face, the high end is on the opposite side, so the angular error changes sign and cancels in the mean. (The error is large only for steep sights.)

**3. Index error of the vertical circle.** If the vertical circle reads $+i$ on face left when the line of sight is horizontal, the same error has the opposite effect on face right. The mean of the two vertical angles is free of $i$.

For horizontal angles, both verniers (A and B, 180 degrees apart) are also read and averaged, which removes the error due to **eccentricity of the centre of the circle**.

#### Temporary adjustments
1. **Setting up and centering** over the station mark using the plumb bob or optical plummet.
2. **Levelling up** with the three foot screws and the plate level: centre the bubble with the level parallel to two foot screws, turn the instrument through 90 degrees and centre it with the third screw, and repeat until the bubble stays central in all positions.
3. **Elimination of parallax:** focus the eyepiece until the cross hairs are sharp, then focus the objective on the target so that the image does not move relative to the cross hairs when the eye is moved.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Why does the mean of face left and face right remove collimation error?
   A. The error is the same sign on both faces
   B. The deviation is on opposite sides on the two faces, so one reading is too large and the other too small by e
   C. The vertical circle is read on both faces
   D. The bubble is re-centred between faces
   Answer: B. After changing face the deviation e switches side, so the readings err by +e and -e and the mean is free of it. (Key point: Collimation error cancellation)

2. Which error is NOT eliminated by taking both faces?
   A. Line of collimation error
   B. Trunnion axis error
   C. Vertical circle index error
   D. Error of centring over the station
   Answer: D. Face observation removes instrumental errors like collimation, trunnion and index errors, not centring errors. (Key point: Errors eliminated by face observation)

3. Which practice removes error due to eccentricity of the centre of the horizontal circle?
   A. Reading both verniers A and B (180° apart) and averaging
   B. Observing on face left only
   C. Repeated levelling
   D. Focusing the eyepiece
   Answer: A. Verniers 180° apart show equal and opposite eccentricity errors, so their mean is free of it. (Key point: Eccentricity)

4. When is trunnion axis error large?
   A. For horizontal sights
   B. For steep sights
   C. Only on face right
   D. Only when the bubble is off-centre
   Answer: B. The inclined plane swept by the telescope has more effect on the angle when sights are steep. (Key point: Trunnion axis error)

## Related in Surveying I

- [Face observation errors; optical theodolite](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Aface-observation-errors-optical-theodolite) — Surveying I, chapter 7, asked 2×
- [Plotting theodolite traverse; repetition errors](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Aplotting-theodolite-traverse-repetition-errors) — Surveying I, chapter 7
- [Levelling methods; errors eliminated by balancing](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Aprinciple-methods-leveling-balancing-sight) — Surveying I, chapter 5, asked 2×