---
title: "Two-point resection in plane tabling"
subject: "Surveying I"
chapter: 6
key: "semester_3/surveying_i:two-point-resection-plane-table"
source: https://www.bctengineer.com/q/semester_3/surveying_i/two-point-resection-plane-table
site: BCT Engineering
---

# Two-point resection in plane tabling

Surveying I · Chapter 6: Plane Table Survey

- Asked in: 2067 Asar, 8 marks
- Video: [Two Point Problem| Surveying|Devesh Saini](https://www.youtube.com/watch?v=UpUnqyfF3u0) — Civil Beats
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/surveying_i/two-point-resection-plane-table
- Whole chapter: https://www.bctengineer.com/md/semester_3/surveying_i/chapter-6

## Question

Explain the procedure for solving the two point resection problem of plane tabling.

## Short answer

Choose auxiliary Q. At Q: rays to A,B → q; sight P → p1 (estimated QP). At P: orient q‑p1, ray from a → p on line. Sight B → b′. Error = ∠ab–ab′. Alidade on ab′ fix rod; alidade on ab rotate to bisect rod. Then intersect a and b rays for P.

## Answer

#### Two point resection
**Statement:** locate on the sheet the station P occupied by the table when only two well-defined points A and B (already plotted as **a** and **b**) are visible from P. An *auxiliary station* Q is used. It is chosen near P so that A and B are visible from Q and the angles PAQ and PBQ are not too small, so that the rays intersect well.

**Procedure**

1. **At Q:** set up the table at Q and orient it approximately by compass or by eye. Pivot the alidade on **a**, sight A and draw a ray back; pivot on **b**, sight B and draw a ray back. The rays meet at **q**, the approximate position of Q.
2. Pivoting on q, sight P and draw a ray. Mark **p1** on it by estimating the distance QP.
3. **At P:** set up the table with p1 over P and orient it by back-sighting Q (alidade along p1-q). Pivot on a, sight A and draw a ray back. It cuts the line q-p1 at **p**.
4. Pivoting on p, sight B and draw a ray. It cuts the ray q-b (drawn in step 1) at **b′**. The figure a, b′, p, q is similar to the ground figure A, B, P, Q but is turned by the orientation error, so the angle between ab and ab′ is the error of orientation.
5. Place the alidade along **ab′** and fix a ranging rod R far away in that line. Then place the alidade along **ab** and turn the table until R is bisected. The table is now correctly oriented; clamp it.
6. Pivoting on a and b, sight A and B and draw rays back. Their intersection is the true position **p** of P. Check with a third point if one is visible.

```text
   A *                        * B      (ground stations)

        a ---------------- b           (plotted points)
         \               /  \
          \             /    b'  <- ray p->B meets ray q-b
           \           /
            p --------
            |   (p lies on line q-p1)
            q   (auxiliary station Q)

   angle b-a-b' = error of orientation
```

The method gives a good result when Q is chosen so that the angles PAQ and PBQ are not very small. It needs two set-ups, so it is laborious, and it is used only when a third point is not visible.

## Check yourself

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

1. Why is an auxiliary station Q used in the two-point problem?
   A. To measure the base line
   B. To orient the table correctly, since only two points are available
   C. To check the compass
   D. To replace point A
   Answer: B. Q gives a line from which the correct orientation is obtained. (Key point: Auxiliary station)

2. What is obtained at the first set-up at P?
   A. The exact position p
   B. The point q2
   C. An approximate position p1 where the rays from a and b intersect
   D. The magnetic meridian
   Answer: C. The table is oriented approximately, so p1 is only approximate. (Key point: First set-up)

3. How is the table oriented at Q?
   A. By compass only
   B. By sighting A
   C. By eye
   D. By back-sighting P along the line q1-p1
   Answer: D. Q is oriented using the line q1-p1. (Key point: Station Q)

4. How is the table finally oriented at P?
   A. Alidade along q2-p1, turn the board until Q is bisected
   B. Alidade along ab
   C. Compass at zero
   D. Board turned until A touches B
   Answer: A. The line q2-p1 gives the correct direction, so the board is oriented. (Key point: Final orientation)

## Related in Surveying I

- [Intersection versus resection in plane tabling](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Aintersection-vs-resection-plane-table) — Surveying I, chapter 6
- [Resection by three point problem](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Athree-point-resection-sketches) — Surveying I, chapter 6
- [Two point problem; advantages, disadvantages](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Atwo-point-problem-adv-disadv) — Surveying I, chapter 6
- [Advantages and disadvantages of plane tabling](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Aadvantages-disadvantages-plane-table) — Surveying I, chapter 6, asked 2×
- [Orientation in plane tabling; reliable method](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Aorientation-more-reliable-method) — Surveying I, chapter 6, asked 2×

## Related in other subjects

- [Resection, two point problem, intersection](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Aresection-two-point-problem-ab-p) — Surveying II, chapter 5
- [Two point resection, point C from A, B, D](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Atwo-point-resection-acb-dca-adb) — Surveying II, chapter 5
- [Two point resection, point C (A, B at 7928.474 N)](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Atwo-point-resection-c-a-b) — Surveying II, chapter 5
- [Two point resection method](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Atwo-point-resection-method-describe) — Surveying II, chapter 5