---
title: "Purposes of triangulation; tertiary triangulation"
subject: "Surveying I"
chapter: 8
key: "semester_3/surveying_i:triangulation-purpose-tertiary"
source: https://www.bctengineer.com/q/semester_3/surveying_i/triangulation-purpose-tertiary
site: BCT Engineering
---

# Purposes of triangulation; tertiary triangulation

Surveying I · Chapter 8: Triangulation and Trilateration

- Asked in: 2079 Bhadra, 1+3 marks
- Video: [TRIANGULATION || Fundamental Techniques used for Surveying](https://www.youtube.com/watch?v=lw5q_gqaEgM) — Civil Engineering Guru
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/surveying_i/triangulation-purpose-tertiary
- Whole chapter: https://www.bctengineer.com/md/semester_3/surveying_i/chapter-8

## Question

What are the purposes of the triangulation and trilateration survey? Describe briefly. Write down the general specification of tertiary triangulation.

## Short answer

Triangulation and trilateration provide accurate horizontal control for mapping, engineering works, geodetic surveys, photogrammetry, deformation monitoring, and boundary demarcation. Tertiary triangulation (third order) has sides 1.5–10 km, baseline 0.5–3 km, average triangular error 6″, maximum 12″, base error 1/250 000.

## Answer

**Triangulation** is a method of horizontal control survey in which control stations are fixed at the vertices of a chain or net of triangles. One side (the **base line**) is measured very accurately, all the angles of the triangles are measured, and the lengths of the other sides are computed with the sine rule. **Trilateration** is the method in which the lengths of **all the sides** of the triangles are measured directly with an EDM (electromagnetic distance measuring) instrument, and the angles are computed from the sides using the cosine rule.

#### Purposes of triangulation and trilateration
- To establish accurate horizontal control (position of stations) for topographic and engineering surveys of large areas, with plane tabling, tacheometry or traverses based on the control stations.
- To determine the size and shape of the earth (geodetic surveys) and to fix positions on the earth by latitude and longitude.
- To provide control for photogrammetric maps and aerial surveys.
- To set out the exact alignments and centre lines of large engineering works, such as long bridges, tunnels, dams and canals.
- To monitor movement or deformation (of dams, landslides, the crust) by repeating the survey.
- To fix the boundaries of states or countries and provide a reference framework for cadastral surveys.

#### General specification of tertiary (third order) triangulation
| Item | Specification |
|---|---|
| Length of base line | 0.5 to 3 km |
| Length of sides of triangles | 1.5 to 10 km |
| Average triangular error (closure) | 6" |
| Maximum triangular error | 12" |
| Probable error of base line | 1 in 250,000 |
| Discrepancy between two measures of the base | 20 mm $\sqrt{K}$ ($K$ in km) |
| Probable error of computed distance | 1 in 5,000 to 1 in 20,000 |

Tertiary triangulation is based on the secondary stations and is used for closely spaced engineering control.

## Check yourself

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

1. What distinguishes trilateration from triangulation?
   A. Only angles are measured in trilateration
   B. All sides are measured by EDM and angles are computed by the cosine rule
   C. A base line is measured and angles by sine rule
   D. It uses a plane table only
   Answer: B. Trilateration measures all sides directly with EDM and computes angles with the cosine rule. (Key point: Triangulation vs trilateration)

2. Which is a main purpose of triangulation and trilateration?
   A. Measuring vertical heights only
   B. Finding soil bearing capacity
   C. Establishing accurate horizontal control for topographic and engineering surveys
   D. Estimating earthwork cost
   Answer: C. They give accurate positions of control stations on which detailed surveys and setting out are based. (Key point: Purpose of control surveys)

3. How can triangulation help monitor movement of a dam or landslide?
   A. By measuring it once only
   B. By using the contour interval
   C. By measuring rainfall
   D. By repeating the survey and comparing station positions
   Answer: D. Repeated surveys reveal changes in station positions, showing deformation. (Key point: Deformation monitoring)

4. Which is the correct specification for tertiary triangulation?
   A. Base line 0.5-3 km, sides 1.5-10 km, average error 6"
   B. Base line 5-15 km, average error below 1"
   C. Base line 1.5-5 km, average error 3"
   D. Base line 30-150 km, average error 12"
   Answer: A. Tertiary: base 0.5-3 km, sides 1.5-10 km, average triangular error 6", maximum 12". (Key point: Tertiary specification)

## Related in Surveying I

- [Triangulation, trilateration; specifications](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Atriangulation-trilateration-specifications) — Surveying I, chapter 8, asked 5×
- [Triangulation principle; 1st order trilateration](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Atriangulation-principle-first-order-trilateration) — Surveying I, chapter 8
- [Trilateration over triangulation; primary specs](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Atrilateration-advantages-primary-triangulation) — Surveying I, chapter 8
- [Triangulation versus trilateration](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Atriangulation-vs-trilateration) — Surveying I, chapter 8, asked 7×
- [Triangulation; EDM principle for trilateration sides](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Atriangulation-and-edm-principle-trilateration) — Surveying I, chapter 8
- [Principle of triangulation and trilateration](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Aprinciple-triangulation-trilateration-note) — Surveying I, chapter 8

## Related in other subjects

- [Types of specification: general and detailed](https://www.bctengineer.com/question.md?key=semester_8%2Fconstruction_management%3Aspec-types-general-detailed) — Construction Management, chapter 12, asked 7×
- [Horizontal control specification for major traverse](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Atraverse-horizontal-control-specification) — Surveying II, chapter 1
- [Purposes of emergency lighting installation](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Aemergency-lighting-purposes) — Industrial Power Distribution and Illumination, chapter 9
- [Define RT works, objectives, purposes of spur](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Ariver-training-objectives-spur-purposes) — Irrigation and Drainage Engineering, chapter 6