--- title: "Contour gradient, index contour, interval factors" subject: "Surveying II" chapter: 4 key: "semester_4/surveying_ii:contour-gradient-index-interval-factors" source: https://www.bctengineer.com/q/semester_4/surveying_ii/contour-gradient-index-interval-factors site: BCT Engineering --- # Contour gradient, index contour, interval factors Surveying II · Chapter 4: Contouring - Asked in: 2078 Baisakh, 6 marks - Video: [Contour Intervals, Index Contours and Spot Heights explained](https://www.youtube.com/watch?v=lOj41sG3q54) — Fell Running Guide - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/surveying_ii/contour-gradient-index-interval-factors - Whole chapter: https://www.bctengineer.com/md/semester_4/surveying_ii/chapter-4 ## Question Define contour gradient and index contour and explain the factors determining contour interval. ## Short answer Contour gradient is a line of constant slope where horizontal distance between successive contours is uniform, used for route location. Index contour is every fifth contour, thicker and labeled. Contour interval depends on map purpose, scale, ground nature, required accuracy, time/cost, and area size; smaller intervals for flat ground, large-scale, detailed design. ## Answer #### Contour gradient **Contour gradient** is the line of constant slope (uniform gradient) that cuts the contours at the same horizontal equivalent. It is laid on the ground to fix the route of a road, railway, canal or pipe line. The gradient = CI / HE = 1 in (HE/CI). Example: for a road of 1 in 20 gradient and CI = 1 m, the horizontal equivalent is 20 m, so a divider opened to 20 m (at map scale) is stepped from contour to contour to trace the route. #### Index contour **Index contour** is every fifth contour (for example every 5th line when CI = 2 m, giving 10 m, 20 m, 30 m ...) drawn as a thicker line and labelled with its RL, so that the map can be read quickly. The other contours are called **intermediate (minor) contours**. #### Factors determining contour interval 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. For a road of 1 in 20 gradient and CI = 1 m, what is the horizontal equivalent? A. 1 m B. 20 m C. 0.05 m D. 21 m Answer: B. Gradient = CI/HE, so HE = CI × 20 = 20 m; a divider is opened to 20 m at map scale. (Key point: Horizontal equivalent for a gradient) 2. What is a contour gradient used for? A. To mark index contours B. To fix the route of a road, railway, canal or pipeline at constant slope C. To find the area of a hill D. To measure bearings Answer: B. The divider stepped from contour to contour traces a route of uniform gradient. (Key point: Use of gradient) 3. How does the nature of the ground affect the CI? A. Flat ground needs a small CI; steep ground a large CI B. Flat ground needs a large CI C. Steep ground needs a small CI D. Ground nature has no effect Answer: A. A small CI is needed to show relief on flat ground; steep ground with small CI would crowd contours. (Key point: Nature of ground) 4. Why does a small CI raise cost? A. It needs more field points and time B. It needs fewer points C. It needs a thicker pen D. It reduces accuracy Answer: A. A small CI needs more spot levels, so time and cost increase. (Key point: Cost and time) ## Related in Surveying II - [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 - [Factors affecting contour interval](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Acontour-interval-factors) — Surveying II, chapter 4 - [Contour interval factors and characteristics](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Acontour-interval-factors-characteristics) — Surveying II, chapter 4 - [Index contour and indirect contouring](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Aindex-contour-indirect-method) — Surveying II, chapter 4 ## Related in other subjects - [Practical factors in sampling; sampling interval for X1(t)X2(t)](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Asampling-practical-x1x2) — Communication Systems, chapter 4 - [Refractive index of liquid film by interference](https://www.bctengineer.com/question.md?key=semester_1%2Fengineering_physics%3Acoherent-sources-film-refractive-index-method) — Engineering Physics, chapter 4