---
title: "Engineering surveying; two-way principle application"
subject: "Surveying I"
chapter: 1
key: "semester_3/surveying_i:engineering-surveying-two-way-angle"
source: https://www.bctengineer.com/q/semester_3/surveying_i/engineering-surveying-two-way-angle
site: BCT Engineering
---

# Engineering surveying; two-way principle application

Surveying I · Chapter 1: Introduction

- Asked in: 2081 Bhadra, 1+3 marks
- Video: [1.3 Principles of Surveying](https://www.youtube.com/watch?v=Rx1P_2gWLg0) — Civil Modules
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/surveying_i/engineering-surveying-two-way-angle
- Whole chapter: https://www.bctengineer.com/md/semester_3/surveying_i/chapter-1

## Question

What is Engineering Surveying? Describe how the surveying principle can be applied during two-way linear distance measurement and single angle measurement.

## Short answer

Engineering surveying covers measurements and setting out for engineering projects. For two-way linear measurement, a line is measured forward and backward; if the difference is within the allowable limit, the mean is adopted. For single angle measurement, the angle is observed on face left and face right, repeated, and the explement checked to sum to 360°.

## Answer

**Engineering surveying** is the branch of surveying done for engineering works. It covers all the measurements and setting out needed for the planning, design, construction and monitoring of projects such as roads, bridges, dams and buildings. The accuracy is chosen to suit the work.

The survey principles used here are "**make a check on every measurement**" and "**fix a quantity by at least two independent measurements**".

#### Two-way linear distance measurement
A line AB is measured **twice**, once from A to B (forward) and again from B to A (backward), with the same tape and tension.

1. Forward measurement: $L_f$; backward measurement: $L_b$.
2. The difference $d = |L_f - L_b|$ is compared with the allowable limit (for example, 1 in 1000 to 1 in 5000, depending on the ground and the class of work).
3. If it is within the limit, the mean is adopted:

$$
L = \frac{L_f + L_b}{2}
$$

4. If the difference is large, a mistake exists (a miscounted tape length or a wrong reading), so the line is measured again.

The second measurement is independent of the first, so a blunder is caught and random errors are reduced by averaging.

#### Single angle measurement
An angle $\angle AOB$ is measured with a theodolite by:

1. Observing on **face left** and then on **face right**, and taking the mean (this removes instrumental errors such as collimation and trunnion-axis errors).
2. Measuring the angle several times (method of repetition), or in sets with different initial circle readings, and taking the mean. This gives a check on the reading and a smaller error.
3. As a further check, the explement ($360^\circ - \theta$) can be measured on the other side and the two values added to see that they total $360^\circ$, or the angle can be compared with a value calculated from measured sides.

Thus every distance and angle is obtained from two independent observations, and the survey is checked.

## Check yourself

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

1. Engineering surveying is
   A. only the fixing of national control points
   B. surveying for planning, design, construction and monitoring of projects, with accuracy suited to the work
   C. only the study of earth's shape
   D. only the preparation of cadastral maps
   Answer: B. It covers measurements and setting out for engineering works. (Key point: Engineering surveying)

2. In two-way linear measurement, the line is measured
   A. twice in the same direction and the larger value used
   B. once, with a repeated tape reading
   C. forward and backward, and the mean is taken if the difference is within the limit
   D. forward only, with the angle as check
   Answer: C. The independent backward measurement checks for blunders and averaging reduces random error. (Key point: Two-way measurement)

3. If forward and backward measurements differ by more than the permissible limit, you should
   A. find the cause of the mistake and measure again
   B. adopt the mean anyway
   C. adopt the larger value
   D. ignore it for ordinary chaining
   Answer: A. A large difference suggests a mistake such as miscounted tape lengths, so remeasure. (Key point: Large difference)

4. Observing on both face left and face right and taking the mean
   A. fixes the tape tension
   B. increases the horizontal distance
   C. converts angle to bearing
   D. removes instrumental errors like collimation and trunnion errors
   Answer: D. Two faces cancel instrumental errors and give an independent check. (Key point: Angle by face left and right)

## Related in Surveying I

- [Principle applied in two-way linear measurement](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Atwo-way-linear-measurement-principle) — Surveying I, chapter 1

## Related in other subjects

- [Principle of optical distance measurement](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Aoptical-distance-measurement-principle) — Surveying II, chapter 2