--- title: "Contour interval factors and characteristics" subject: "Surveying II" chapter: 4 key: "semester_4/surveying_ii:contour-interval-factors-characteristics" source: https://www.bctengineer.com/q/semester_4/surveying_ii/contour-interval-factors-characteristics site: BCT Engineering --- # Contour interval factors and characteristics Surveying II · Chapter 4: Contouring - Asked in: 2076 Bhadra, 6 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-characteristics - Whole chapter: https://www.bctengineer.com/md/semester_4/surveying_ii/chapter-4 ## Question Explain the factors that affect contour interval. Explain characteristics of contour with neat sketches. ## Short answer Factors affecting contour interval include purpose, scale, nature of ground, time/cost, accuracy required, and area size; CI is kept constant on a map. Contours are closed curves, all points on a contour have same elevation, they never cross (except overhanging cliff), closely spaced contours indicate steep slope, they cross ridges/valleys at right angles, and concentric closed contours indicate hill or depression. ## Answer #### Factors affecting 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 route survey. 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). A rough guide is CI (m) ≈ scale denominator / 2000 (0.25 m at 1:500, 5 m at 1:10 000), so that the lines do not crowd. 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 the cost increases. 5. **Accuracy required:** if the work needs high accuracy (for example, earthwork volumes), a small CI is chosen. 6. **Size of the area:** for a large area the CI is large; for a small area it is small. The contour interval should be kept constant over one map, so that the slope can be judged from the spacing of the contours. #### Characteristics of contours with sketches 1. **Contours are closed curves.** Every contour must close on itself, either inside the map or outside it. It cannot start or end abruptly. 2. **All points on a contour have the same elevation.** 3. **Contours never cross each other**, except at an overhanging cliff or a cave. Two contours of different elevation cannot meet or cross, because one point cannot have two elevations. 4. **Contours of different elevations do not unite** to form one line, except at a vertical cliff, where they coincide and appear as one line. 5. **Closely spaced contours indicate a steep slope; widely spaced contours indicate a gentle slope.** Equally spaced contours indicate a uniform slope, and straight, equally spaced, parallel contours indicate an inclined plane. 6. **Contours cross ridge lines and valley lines at right angles.** The direction of the steepest slope is at right angles to the contour. 7. **A set of concentric closed contours** with the higher values inside represents a hill; with the lower values inside it represents a depression (pond or pit). 8. **Contours bend in V or U shapes across a ridge and a valley.** For a ridge (spur) the bend of the contour points towards the lower ground; for a valley the bend points towards the higher ground. 9. **A contour of the same elevation may reappear** on the other side of a ridge or a valley, so one contour value can appear twice on a map. 10. **A single contour cannot lie alone between two contours of higher (or lower) value.** If the ground dips (or rises) between them, it crosses that level twice, so the value appears as two lines or as one line closing on itself. ```text HILL DEPRESSION .--100--. .--100--. / .-110-. \ / .--90-. \ | / (120) \ | | / (80) \ | \ '-----' / \ '-----' / '-------' '-------' RL rises to the centre RL falls to the centre ``` ```text RIDGE (v points downhill) VALLEY (^ points uphill) 130 \ / 120 /\ \ / / \ 120 \ / 110 / \ \_/ / \ 110 \ / 100 / \ \___/ high ground at top high ground at top ``` ```text Steep slope Gentle slope Uniform slope (close) (wide apart) (equal spacing) ----100 ----100 ----100 ----101 ----101 ----102 ----102 ----103 ----101 ----103 ``` ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How does map scale affect contour interval? A. Large-scale maps use a small CI; small-scale maps use a large CI B. Large-scale maps use a large CI C. Scale has no effect on CI D. Small-scale maps need CI of 0.25 m Answer: A. A large-scale map such as 1:500 has a small CI (0.25-0.5 m); a small-scale map has a large CI so lines do not crowd. (Key point: Scale and CI) 2. For steep hilly ground the contour interval should be: A. Small, 0.25 m, to show relief B. Large (5-10 m or more), otherwise contours crowd together C. Zero D. Changed frequently on the same map Answer: B. Steep ground with a small CI would crowd the contours, so a large CI is used; flat ground needs a small CI. (Key point: Ground nature and CI) 3. Why does a small CI increase cost? A. It needs fewer field points B. It needs more field points and more time C. It requires bigger paper only D. It needs a different datum Answer: B. A smaller CI means more contours and more field points, so time and money increase. (Key point: Cost and CI) 4. Why is CI kept constant on one map? A. So that slope can be judged from contour spacing B. To make all contours index contours C. To avoid interpolation D. So contours can cross Answer: A. With constant CI, close spacing means steep and wide spacing means gentle slope. (Key point: Constant CI on a map) 5. Concentric closed contours with the higher value inside represent: A. A depression B. A hill C. A valley D. A saddle Answer: B. Higher value inside indicates a hill; lower value inside indicates a depression. (Key point: Hill and depression) ## Related in Surveying II - [Factors affecting contour interval](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Acontour-interval-factors) — 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 characteristics, guide points A and B](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Acontour-characteristics-six-guide-points) — 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 ## Related in other subjects - [Bridge site factors and components with sketches](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Abridge-site-factors-components-sketches) — Transportation Engineering II, chapter 5 - [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 - [Factors for residual soil characteristics](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aresidual-soil-characteristics-factors) — Soil Mechanics, chapter 1