---
title: "Two peg test and permanent adjustment"
subject: "Surveying I"
chapter: 5
key: "semester_3/surveying_i:two-peg-test-procedure"
source: https://www.bctengineer.com/q/semester_3/surveying_i/two-peg-test-procedure
site: BCT Engineering
---

# Two peg test and permanent adjustment

Surveying I · Chapter 5: Leveling

- Asked in: 2078 Kartik, 4 marks; 2068 Chaitra, 4 marks; 2064 Jestha, 6 marks; 2057 Chaitra, 6 marks (asked 4 times)
- Video: [How to calculate the correct elevations of stations using Two Peg Test of Level|Two Peg Test Method](https://www.youtube.com/watch?v=J9P7l23-BD4) — Master Civil Engineering
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/surveying_i/two-peg-test-procedure
- Whole chapter: https://www.bctengineer.com/md/semester_3/surveying_i/chapter-5

## Question

What is the two peg test (collimation error of a level)? When is permanent adjustment of a level required? Describe the field procedure of testing and adjusting with a neat sketch.

## Short answer

Two peg test checks collimation error of a level. Set up midway between two pegs, get true height difference; then near one peg, compute error from far reading difference. If error exceeds ~10 mm/100 m, permanent adjustment is needed: move cross-hair to correct far staff reading using diaphragm screws.

## Answer

**Two peg test** is the field test used to find the collimation error of a level: the error caused by the line of collimation not being exactly parallel to the axis of the bubble tube when the bubble is central. The line of sight is then inclined up or down, and the staff readings contain an error proportional to the sight distance.

**When permanent adjustment is required:** when the test shows a collimation error larger than the permissible limit (about 10 mm in 100 m for ordinary levelling; 5 mm or less for precise work), or when the instrument has been damaged or dropped. If the error is small, it is eliminated in the field by keeping the backsight and foresight distances equal.

#### Field procedure

1. Choose two points A and B about 60–100 m apart on level ground. Drive pegs or mark them.
2. **First setup:** set up the level at the mid-point F of AB. Level it and read the staff on A ($a_1$) and B ($b_1$). Equal sights cancel the collimation error, so the true difference of level is $h = a_1 - b_1$.
3. **Second setup:** move the level to a point O on the line AB produced (or between A and B), about 2–5 m from one peg, say B. Level it and read the staff on B ($b_2$, near) and on A ($a_2$, far).
4. The apparent difference is $a_2 - b_2$. The error in the far staff reading is $e = (a_2 - b_2) - h$.

```text
 Setup 1:       A       F       B      equal sights
                |  <----+---->  |
 Setup 2:       A                O B    near B
                |  <------------>|-|
```

5. If the apparent difference equals the true difference, there is no error. If $e$ is positive, the line of collimation is inclined upward; if negative, downward.

#### Adjustment

- Let $d_A$ and $d_B$ be the distances of the instrument from A (far) and B (near). The error per metre of sight is $k = e/(d_A - d_B)$. The correct readings for a horizontal line of sight are $a_2' = a_2 - k\,d_A$ on A and $b_2' = b_2 - k\,d_B$ on B (check: $a_2' - b_2' = h$).
- Without moving the instrument, bring the horizontal cross-hair to the correct reading $a_2'$ on the far staff A with the vertical capstan screws of the diaphragm (top and bottom screws: loosen one and tighten the other).
- Check that the bubble stays central, and repeat the test until the error is within the limit.

Collimation error $= \dfrac{e}{\text{difference of distances}}$ (for example 0.004 m in 60 m).

## Check yourself

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

1. What does the two peg test find?
   A. The curvature of the earth
   B. The error because the line of collimation is not parallel to the bubble tube axis
   C. The horizontal distance between pegs
   D. The staff's zero error
   Answer: B. The test measures the collimation error, in which the line of sight is inclined when the bubble is central. (Key point: Meaning of collimation error)

2. Why is the level first set at the mid-point of AB?
   A. Equal sight distances cancel the collimation error so h = a1 − b1 is the true difference
   B. To measure the error directly
   C. To keep the level off the pegs
   D. To avoid refraction only
   Answer: A. With equal sights the error is the same on both readings and cancels, so a1 − b1 is the true level difference. (Key point: First setup at mid-point)

3. From the second setup (near B), how is the error e found?
   A. e = (a2 − b2) − h
   B. e = a1 + b1
   C. e = h − a1
   D. e = a2 + b2
   Answer: A. The apparent difference (a2 − b2) minus the true difference h is the error in the far staff reading. (Key point: Error computation)

4. When is permanent adjustment required?
   A. Always after each use
   B. When the error exceeds the permissible limit (about 10 mm in 100 m) or the instrument was damaged
   C. Only for a perfectly level site
   D. Never; balancing sights is enough in all cases
   Answer: B. Small errors can be handled by balancing sights; larger errors or damage call for adjusting the diaphragm screws. (Key point: When to adjust)

## Related in Surveying I

- [Two peg test: collimation inclined up or down](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Atwo-peg-dumpy-cd-df-120m) — Surveying I, chapter 5
- [Two peg: closing error and precision of level](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Atwo-peg-cd-50m-e-mid-f-5m) — Surveying I, chapter 5
- [Two peg test, AB = 40 m, near B](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Atwo-peg-ab-40m-inner-side-b) — Surveying I, chapter 5
- [Two peg test, AB 100 m, Q 10 m](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Atwo-peg-ab-100m-q-10m-inside) — Surveying I, chapter 5
- [Collimation error with level mid-way and near B](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Atwo-peg-60m-staff-1-855-1-625) — Surveying I, chapter 5
- [Collimation error from 5 m and 45 m setup](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Atwo-peg-collimation-ab-50m-true-diff-0-250) — Surveying I, chapter 5

## Related in other subjects

- [Type I error; difference of two means procedure](https://www.bctengineer.com/question.md?key=semester_5%2Fprobability_and_statistics%3Atype-1-error-difference-two-means) — Probability and Statistics, chapter 6
- [Traverse field procedure between two known stations](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Atraverse-field-procedure-between-known-stations) — Surveying II, chapter 1
- [Chi-square test of independence: 2×2 procedure](https://www.bctengineer.com/question.md?key=semester_5%2Fprobability_and_statistics%3Achi-square-independence-2x2-procedure) — Probability and Statistics, chapter 7
- [Soundness test of cement and procedure](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Acement-soundness-test-procedure) — Civil Engineering Materials, chapter 5