---
title: "Selecting main stations; number of offsets"
subject: "Surveying I"
chapter: 3
key: "semester_3/surveying_i:factors-main-station-number-of-offsets"
source: https://www.bctengineer.com/q/semester_3/surveying_i/factors-main-station-number-of-offsets
site: BCT Engineering
---

# Selecting main stations; number of offsets

Surveying I · Chapter 3: Chain Survey

- Asked in: 2076 Chaitra, 2+2 marks
- Video: [Survey Stations # Main Station # Tie Station # Check Line # Base Line.](https://www.youtube.com/watch?v=NmEqJ2jUEUs) — Civil Samajh
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/surveying_i/factors-main-station-number-of-offsets
- Whole chapter: https://www.bctengineer.com/md/semester_3/surveying_i/chapter-3

## Question

What are the major considerations while selecting the main station and lines of chain survey? How would you decide the number of offsets based on the objects?

## Short answer

Main stations should be intervisible, form well-conditioned triangles, and allow short offsets with minimum lines on level ground, using a long base and check lines. The number of offsets depends on object shape: two for straight edges, many at close intervals for curves, and more at direction changes for irregular boundaries.

## Answer

#### Major considerations in selecting main stations and lines
1. Stations must be **intervisible** and permanent, on firm ground.
2. The framework should consist of **well-conditioned triangles** (angles between $30^\circ$ and $120^\circ$).
3. The main (base) line should be **the longest**, on level ground, near the middle of the area.
4. Use the **minimum number** of stations and lines.
5. Lines should be **close to the details** so that offsets are short (preferably up to 15 m).
6. Avoid obstacles and steep slopes; do not let lines pass over ponds or buildings.
7. Provide **check lines and tie lines** for testing and for the details.
8. Some lines may run along fences or roads to reduce the number of offsets.

#### Number of offsets depending on the object
The number of offsets depends on the nature of the object and the scale of the plan.

| Object | Offsets required |
|---|---|
| Straight boundary or wall | Two (one at each end), plus a check offset |
| Building (rectangular) | Offsets to each corner, or two at one side and a tie |
| Curved boundary (road edge, river bank) | Many, at short regular intervals (5 to 10 m), more for sharper curves |
| Irregular boundary | More offsets at every change in direction |
| Isolated objects (tree, pole, well) | One offset and the chainage |
| Open land of low importance | Few offsets, at 20 to 30 m |

The more irregular the shape and the larger the scale, the more offsets are needed, so that the drawn outline resembles the real shape.

## Check yourself

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

1. The main base line should be:
   A. The shortest, on the edge
   B. On the steepest slope
   C. The longest, on level ground near the middle
   D. Across a pond
   Answer: C. This keeps offsets short. (Key point: Base line)

2. How many offsets are taken to a curved boundary such as a river bank?
   A. Only two, at the ends
   B. One in the middle
   C. None; it is estimated
   D. Many, at short regular intervals (5 to 10 m)
   Answer: D. A curve needs closely spaced points to represent its shape. (Key point: Curved boundary)

3. How is an isolated object like a tree fixed?
   A. One offset with its chainage
   B. Four offsets
   C. A traverse around it
   D. By compass bearing only
   Answer: A. Its position needs only the chainage and offset. (Key point: Isolated object)

4. As the shape becomes more irregular and the scale larger, the number of offsets:
   A. Decreases
   B. Increases
   C. Stays the same
   D. Becomes zero
   Answer: B. The drawn outline must resemble the true shape. (Key point: Irregularity and scale)

## Related in Surveying I

- [Chain stations and lines; well-conditioned triangle](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Afactors-stations-well-conditioned-triangle) — Surveying I, chapter 3
- [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
- [Obstacles in chain survey and remedies](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Aobstacles-chain-survey) — Surveying I, chapter 3, asked 4×
- [Field book and obstacles in chain survey](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Afield-book-and-obstacles) — Surveying I, chapter 3
- [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×
- [Factors for choosing stations; offset types](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Afactors-stations-and-offset-types) — Surveying I, chapter 3

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- [Major components of a power station](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Apower-station-major-components) — Power System Analysis I, chapter 1, asked 2×
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- [Double mass curve, station X and 25 stations, 1941-1976](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Admc-x-25stn-36yr) — Engineering Hydrology, chapter 2