---
title: "Working principle of plane table survey"
subject: "Surveying I"
chapter: 6
key: "semester_3/surveying_i:working-principle-plane-table"
source: https://www.bctengineer.com/q/semester_3/surveying_i/working-principle-plane-table
site: BCT Engineering
---

# Working principle of plane table survey

Surveying I · Chapter 6: Plane Table Survey

- Asked in: 2062 Poush, 8 marks
- Video: [Introduction of plane table surveying](https://www.youtube.com/watch?v=cMEhZMbbrss) — Civil boost
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/surveying_i/working-principle-plane-table
- Whole chapter: https://www.bctengineer.com/md/semester_3/surveying_i/chapter-6

## Question

Describe the working principle of plane table survey.

## Short answer

The plane table is a graphical survey method where plotting is done in the field. The principle is that rays drawn on the sheet from the station towards objects are parallel to the corresponding ground lines, making the plotted figure similar to the ground. This requires levelling the board, centring it over the station, and orientation (by compass or back‑sighting) so that all lines remain parallel.

## Answer

Plane tabling is a graphical surveying method in which fieldwork and plotting are done together on the spot, using a drawing sheet fixed on a board (plane table), an alidade for sighting and a tripod.

**Principle:** the table is set up so that the rays drawn on the sheet from a station point towards the objects are *parallel* to the corresponding lines on the ground. The plotted figure is then similar to the ground figure. This needs three things at every station:

1. **Levelling** - the board is kept horizontal (spirit level on the board).
2. **Centring** - the point on the sheet is kept vertically above the ground station (plumbing fork and plumb bob).
3. **Orientation** - the board is turned so that all lines on the sheet stay parallel to their ground lines (by magnetic needle or by back-sighting a previously plotted line).

**Equipment:** plane table with tripod, alidade (plain or telescopic), plumbing fork with plumb bob, spirit level, trough compass, drawing sheet, pins, ranging rods, tape or chain.

#### Working at a station
1. **Fix the paper** on the board and place the tripod; mount the board and make it roughly level by eye.
2. **Level** the board with the spirit level placed in two directions at right angles, adjusting the tripod legs.
3. **Centre** the table over the ground station with the plumbing fork and plumb bob, so the sheet point is vertically above the ground point.
4. **Orient** the board by trough compass (first station) or by back-sighting a plotted line (later stations).
5. **Sighting and plotting:** pivot the alidade at the plotted point, sight the object through the sight vane and draw a ray along its fiducial edge. Measure the distance and plot to scale (radiation) or fix the point by two rays (intersection).
6. Shift to the next station, re-centre, re-orient, and repeat. Close the traverse by checking the plotted position of the first station.

#### Basic geometrical idea
```text
   Ground:  O ----- A          Sheet:   o ----- a
             \                           \
               B                            b
   Triangle OAB is similar to triangle oab: each ray
   on the sheet is parallel to its ground line and
   drawn to a constant scale.
```

## Check yourself

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

1. What is the basic principle of plane table surveying?
   A. Rays drawn on the sheet are parallel to the corresponding ground lines, so the plan is similar to the ground figure
   B. Rays on the sheet are perpendicular to the ground lines
   C. Angles are booked in a field book and plotted later in the office
   D. Distances are measured with a chain and plotted at a larger scale than the ground
   Answer: A. The table is set so that sheet rays are parallel to the ground lines, which makes the plotted figure similar to the ground figure. (Key point: Core principle)

2. Which three operations are needed at every station of plane tabling?
   A. Levelling, focusing and parallax removal
   B. Levelling, centring and orientation
   C. Centring, booking and reduction
   D. Orientation, traversing and closing
   Answer: B. Levelling keeps the board horizontal, centring puts the sheet point over the ground station, and orientation keeps all lines parallel to the ground. (Key point: Three requirements)

3. How is centring done in plane tabling?
   A. The spirit level is used to keep the board horizontal
   B. The board is turned until the needle floats at zero
   C. The plumbing fork and plumb bob keep the sheet point vertically above the ground station
   D. The alidade is placed along the plotted line
   Answer: C. Centring is achieved with the plumbing fork and plumb bob; the spirit level does levelling. (Key point: Centring)

4. How is the board levelled?
   A. With a spirit level placed in two directions at right angles, adjusting the tripod legs
   B. With the plumb bob only
   C. With the trough compass
   D. By eye alone
   Answer: A. The spirit level is used in two perpendicular directions. (Key point: Levelling procedure)

## Related in Surveying I

- [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×
- [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×
- [Plane table principles; advantages, disadvantages](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Aprinciples-adv-disadv-plane-table) — Surveying I, chapter 6, asked 2×
- [Methods of orienting plane table](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Amethods-of-orienting-plane-table) — Surveying I, chapter 6
- [Chain survey principle and operations](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Achain-principle-and-field-procedure) — Surveying I, chapter 3, asked 2×
- [Principle of chain survey; types of offsets](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Achain-principle-and-offsets) — Surveying I, chapter 3

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