--- title: "Two point problem; advantages, disadvantages" subject: "Surveying I" chapter: 6 key: "semester_3/surveying_i:two-point-problem-adv-disadv" source: https://www.bctengineer.com/q/semester_3/surveying_i/two-point-problem-adv-disadv site: BCT Engineering --- # Two point problem; advantages, disadvantages Surveying I · Chapter 6: Plane Table Survey - Asked in: 2066 Bhadra, 8 marks - Video: [Two Point Problem In Plane Table Surveying](https://www.youtube.com/watch?v=LJXWZrobg-c) — Ganesh Shegar - 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-problem-adv-disadv - Whole chapter: https://www.bctengineer.com/md/semester_3/surveying_i/chapter-6 ## Question What is the two point problem in plane table resection? Write the advantages and disadvantages of plane table survey. ## Short answer The two‑point resection locates the table station using only two plotted points and an auxiliary station Q. Plane table advantages: simultaneous field plotting, error detection, direct contour drawing, no field books, cheap and simple. Disadvantages: unsuitable in bad weather, heavy equipment, less accurate than theodolite, no re‑plotting from notes, paper instability. ## Answer #### Two point problem The **two point problem** is the resection method of plane tabling in which the table station P is located on the sheet using only two known stations A and B (plotted as a and b), with the help of an auxiliary station Q. It is used when only two plotted points are visible from the table position. **Procedure** 1. **At Q:** choose an auxiliary station Q near P such that the angles PAQ and PBQ are not too small. 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**. 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 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 point **p**. Check with a third point if possible. ```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 ``` #### Advantages and disadvantages of plane table survey **Advantages** - Fieldwork and plotting are done together, so omissions and mistakes are detected on the spot. - Contours and details can be drawn directly while looking at the ground. - Little field book work; no angles need to be booked. - Not affected by local attraction when the table is oriented by back-sighting. - Quick and economical for small and medium-scale maps. - Inaccessible points can be plotted by intersection and resection. - Needs less skill than theodolite surveying, and the instruments are simple. **Disadvantages** - Not suitable in rainy, windy or foggy weather, or in thick forest and built-up areas where intervisibility is poor. - The table and its many accessories are heavy and awkward to carry. - Less accurate than a theodolite survey for large areas; the accuracy depends on the quality of the drawing. - Everything depends on the plan itself; since there are no field notes, the plan cannot be re-plotted to a different scale. - The paper may shrink or expand with the weather. - Plotting must be done in the field, so time is lost in bad weather, and an unskilled person may produce a poor drawing. ## 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. Which is a disadvantage of plane tabling? A. It needs a complex angular field book B. Instruments are heavy and difficult to transport C. It cannot plot inaccessible points D. It is unaffected by weather Answer: B. Many accessories must be carried and weather limits work. (Key point: Disadvantage) ## Related in Surveying I - [Two-point resection in plane tabling](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Atwo-point-resection-plane-table) — Surveying I, chapter 6 - [Methods of plane table survey](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Amethods-of-plane-table-survey) — Surveying I, chapter 6, asked 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 - [Working principle of plane table survey](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Aworking-principle-plane-table) — Surveying I, chapter 6 - [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 - [Plane table principles; orientation by back sighting](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Aorientation-back-sighting) — Surveying I, chapter 6, asked 4× ## 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