---
title: "Factors affecting contour interval"
subject: "Surveying II"
chapter: 4
key: "semester_4/surveying_ii:contour-interval-factors"
source: https://www.bctengineer.com/q/semester_4/surveying_ii/contour-interval-factors
site: BCT Engineering
---

# Factors affecting contour interval

Surveying II · Chapter 4: Contouring

- Asked in: 2072 Magh, 4 marks
- Video: [Factors Affecting to Contour Interval](https://www.youtube.com/watch?v=Ztjc4clKBHg) — Learn Civil Tech & Tricks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/surveying_ii/contour-interval-factors
- Whole chapter: https://www.bctengineer.com/md/semester_4/surveying_ii/chapter-4

## Question

Explain the factors that affect the contour interval.

## Short answer

Contour interval is chosen based on map purpose (small for design, large for reconnaissance), scale (small CI for large scale), nature of ground (small for flat, large for steep), accuracy needed, time/cost, and size of area. It is kept constant for one map.

## Answer

The **contour interval** is the constant vertical distance between two successive contours. It is chosen from the following factors.

1. **Purpose of the map:** a small CI is used for detailed engineering design (dam, building site); a large CI for a general reconnaissance map or a preliminary route location.
2. **Scale of the map:** a large-scale map (1:500) has a small CI (0.25 m to 0.5 m); a small-scale map has a large CI (5 m to 20 m). In general, the CI is increased as the scale becomes smaller, so that the contours do not crowd together.
3. **Nature of the ground:** flat ground needs a small CI (0.25 to 1 m) to show the relief; steep or hilly ground needs a large CI (5 to 10 m or more) or the contours crowd together.
4. **Time and money available:** a small CI needs more field points and more time, so cost increases.
5. **Accuracy required:** if the work needs high accuracy (earthwork volume) a small CI is chosen.
6. **Size of the area:** for a large area the CI is large; for a small area the CI is small.

The contour interval should be kept constant for one map so that the slope can be seen from the spacing of the contours.

## Check yourself

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

1. Which map needs a small contour interval?
   A. A reconnaissance map
   B. A detailed engineering design map such as a dam or building site
   C. A route survey
   D. A map of a large area
   Answer: B. Detailed design needs fine relief, so a small CI is used. (Key point: Purpose)

2. How does map scale affect CI?
   A. Large scale uses a small CI; small scale uses a large CI
   B. Large scale uses a large CI
   C. Scale has no effect
   D. Small scale uses a small CI
   Answer: A. At 1:500, CI is 0.25 to 0.5 m; at small scale, CI is 5 to 20 m so lines do not crowd. (Key point: Scale)

3. Which statement is correct?
   A. High accuracy work such as earthwork volume needs a small CI; a large area uses a larger CI
   B. High accuracy needs a large CI
   C. Large area uses a small CI
   D. Small area uses a large CI
   Answer: A. A small CI gives greater accuracy but costs more, so large areas use a larger CI. (Key point: Accuracy and area)

## Related in Surveying II

- [Contour interval factors and characteristics](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Acontour-interval-factors-characteristics) — Surveying II, chapter 4
- [Contour gradient, index contour, interval factors](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Acontour-gradient-index-interval-factors) — Surveying II, chapter 4
- [Contour interval, index contour, why never cross](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Acontour-interval-index-plotting-never-cross) — Surveying II, chapter 4
- [Contour interval, index contour, contouring methods](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Acontour-interval-direct-indirect-methods) — Surveying II, chapter 4
- [Contour interval, index contour, plotting, uses](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Acontour-interval-index-plotting-procedure-uses) — Surveying II, chapter 4
- [Contour terms and characteristics with sketch](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Acontour-terms-characteristics) — Surveying II, chapter 4, asked 3×

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