---
title: "Trilateration over triangulation; primary specs"
subject: "Surveying I"
chapter: 8
key: "semester_3/surveying_i:trilateration-advantages-primary-triangulation"
source: https://www.bctengineer.com/q/semester_3/surveying_i/trilateration-advantages-primary-triangulation
site: BCT Engineering
---

# Trilateration over triangulation; primary specs

Surveying I · Chapter 8: Triangulation and Trilateration

- Asked in: 2076 Chaitra, 4 marks
- Video: [Classification of triangulation || Types of triangulation || Triangulation Survey || advance survey](https://www.youtube.com/watch?v=UpuSu41MhO0) — Mukesh Kumar
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/surveying_i/trilateration-advantages-primary-triangulation
- Whole chapter: https://www.bctengineer.com/md/semester_3/surveying_i/chapter-8

## Question

Explain the advantages of trilateration over triangulation. List out the general specifications of primary triangulation.

## Short answer

Trilateration advantages: speed (EDM measures distances quickly), no baseline is needed, accuracy is 1 part in 100,000 to 1,000,000 without accumulation of angular error, fewer observers and cost, possible in poor visibility, simple computation. Primary triangulation specs: baseline 5–15 km, triangle sides 30–150 km, average triangular closure <1″, maximum 3″, probable baseline error 1 in 1,000,000, discrepancy 5 mm√K.

## Answer

#### Advantages of trilateration over triangulation
1. **Speed:** only distances are measured, and EDM takes minutes per side, while observing angles in several sets is slow.
2. **No base line:** no separate base line, base extension or check base is needed; every side is measured with high accuracy.
3. **Accuracy:** modern EDM gives 1 part in 100,000 to 1,000,000, and there is no accumulation of angular error.
4. **Fewer observers and less cost:** the work can be done by a small party, and in many cases no tall towers are needed for signals.
5. **Weather:** work is possible in poor visibility or at night, particularly with microwave instruments.
6. **Simple computation:** angles are computed from the sides by the cosine rule, and the figure is adjusted by the least squares method.
7. **Strong networks:** very long sides and a strong network are possible with the same field effort.

#### General specifications of primary (first order) triangulation
| Item | First order (primary) | Second order (secondary) | Third order (tertiary) |
|---|---|---|---|
| Length of base line | 5 to 15 km | 1.5 - 5 km | 0.5 - 3 km |
| Length of sides of triangles | 30 - 150 km | 8 - 30 km | 1.5 - 10 km |
| Average triangular error (closure) | less than 1" | 3" | 6" |
| Maximum triangular error | 3" | 8" | 12" |
| Probable error of base line | 1 in 1,000,000 | 1 in 500,000 | 1 in 250,000 |
| Discrepancy between two measures of the base | 5 mm $\sqrt{K}$ | 10 mm $\sqrt{K}$ | 20 mm $\sqrt{K}$ |
| Probable error of computed distances | 1 in 60,000 to 1 in 250,000 | 1 in 20,000 to 1 in 50,000 | 1 in 5,000 to 1 in 20,000 |

($K$ is the distance in km. The figures are the common textbook values; Survey Department practice may differ slightly.)

## Check yourself

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

1. Why does trilateration not need a separate base line or check base?
   A. Because angles are measured carefully
   B. Because every side is measured directly with high accuracy by EDM
   C. Because the figure is never closed
   D. Because only one side is used
   Answer: B. All sides are measured by EDM, so no base extension or check base is required. (Key point: No base line)

2. Which is an accuracy advantage of trilateration?
   A. It uses larger angles
   B. It needs no instrument
   C. There is no accumulation of angular error
   D. It ignores meteorological effects
   Answer: C. Since angles are not observed, angular error does not build up. (Key point: Error accumulation)

3. Which working condition favours trilateration, especially with microwave EDM?
   A. Only clear daytime
   B. Only during rain
   C. Only in flat country
   D. Poor visibility or night work
   Answer: D. EDM can be used in poor visibility or at night, unlike angle observation. (Key point: Weather advantage)

4. What base line length and side length apply to primary triangulation?
   A. Base 5-15 km, sides 30-150 km
   B. Base 1.5-5 km, sides 8-30 km
   C. Base 0.5-3 km, sides 1.5-10 km
   D. Base 15-30 km, sides 150-300 km
   Answer: A. First order has base 5 to 15 km and sides 30 to 150 km. (Key point: Primary triangulation specs)

## 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
- [Triangulation versus trilateration](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Atriangulation-vs-trilateration) — Surveying I, chapter 8, asked 7×
- [Triangulation principle; third-order specifications](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Atriangulation-principle-third-order) — Surveying I, chapter 8
- [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

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