--- title: "Contour interval, index contour, plotting, uses" subject: "Surveying II" chapter: 4 key: "semester_4/surveying_ii:contour-interval-index-plotting-procedure-uses" source: https://www.bctengineer.com/q/semester_4/surveying_ii/contour-interval-index-plotting-procedure-uses site: BCT Engineering --- # Contour interval, index contour, plotting, uses Surveying II · Chapter 4: Contouring - Asked in: 2081 Chaitra, 1+1+2+2 marks - Video: [How to Plot Contours on a Survey Plan Using Reduced Levels (RL) | Manual Interpolation Method](https://www.youtube.com/watch?v=v5Q1xag4_Yg) — Architecture Skills - 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-index-plotting-procedure-uses - Whole chapter: https://www.bctengineer.com/md/semester_4/surveying_ii/chapter-4 ## Question Define the term Contour Interval and Index Contour. Describe the procedure for contour plotting with applying surveying principle. Write uses of contours. ## Short answer Contour interval is the constant vertical distance between consecutive contours; index contour is every fifth contour drawn thicker and labelled. Plotting follows working from whole to part: control survey, point location, plotting, interpolation by proportion, drawing smooth curves, and checking. Contours are used for determining elevations, slopes, sections, alignments, intervisibility, reservoir capacity, site selection, and earthwork quantities. ## Answer #### Contour interval **Contour interval (CI)** is the constant vertical distance between two consecutive contours, for example 1 m or 5 m. It is kept the same over one map. #### Index contour **Index contour** is every fifth contour (for example, when CI = 2 m, every 5th line, 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**. #### Procedure for contour plotting (applying the surveying principle) The surveying principle of **working from the whole to the part** is followed: a strong framework of control is fixed first, and the details and contours are added within it. 1. **Control survey:** A traverse or triangulation framework is run and plotted accurately. Benchmarks are fixed by levelling. 2. **Field survey of contour points:** The ground points are located (and their RLs found) from the control stations by the direct or the indirect method (grid, cross-section or tacheometry). 3. **Plotting the points:** The control stations are plotted first by coordinates; the survey points are then plotted from them to scale. The RL of each point is written beside it. 4. **Interpolation:** Between each pair of neighbouring points, the contours of the required values are located by proportion, assuming a uniform slope between the points. 5. **Drawing the contours:** Points of equal RL are joined by smooth curves. Every fifth contour is drawn thick as an index contour and labelled. Contours must not cross each other. 6. **Checking:** The map is checked against the ground (shape of hills, valleys, ridge lines), and the title, scale, CI and north line are added. #### Uses of contours 1. To find the elevation of any point and the slope between two points from the map (without going to the field). 2. To draw the longitudinal section and cross-sections of a road, railway, canal or pipeline on paper. 3. To fix the best alignment of a road, railway or canal with a given gradient (contour gradient). 4. To find the intervisibility of two points (to decide whether a signal station can be seen from another). 5. To calculate the capacity of a reservoir, the catchment area and the volume of earthwork (from the areas enclosed by contours). 6. To select suitable sites for a dam, building, bridge, airport or water tank, and to mark the drainage lines (watershed). 7. To find the quantity of cutting and filling for levelling a site, and to plan irrigation and drainage layouts. 8. To study the general topography (hills, valleys, ridges) of a large area, and for military purposes and town planning. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Contour interval is: A. The horizontal distance between consecutive contours B. The constant vertical distance between consecutive contours on a map C. The distance between index contours on the ground D. The scale of the map Answer: B. CI is the constant vertical distance between two consecutive contours and is the same over one map. (Key point: Contour interval) 2. An index contour is: A. Every contour that closes on itself B. Every fifth contour, drawn thicker and labelled with RL C. The lowest contour on the map D. A dotted contour of unknown RL Answer: B. Every fifth contour is drawn thick and labelled for quick reading; the others are intermediate contours. (Key point: Index contour) 3. Which surveying principle is followed in contour plotting? A. Working from the part to the whole B. Working from the whole to the part, fixing control first C. Taking the field work before any control D. Reciprocal observations Answer: B. A strong control framework (traverse, benchmarks) is fixed first, then details and contours are added within it. (Key point: Whole to part principle) 4. Which is the correct order of contour plotting? A. Interpolation, control survey, plotting points, drawing contours B. Drawing contours, plotting points, interpolation, control survey C. Control survey, field survey of points, plotting, interpolation, drawing contours D. Plotting points, drawing contours, control survey, interpolation Answer: C. Control comes first, then points are surveyed and plotted, contours are interpolated between them and then drawn by joining equal RLs. (Key point: Order of procedure) ## Related in Surveying II - [Contour index, fixing contours between 1240.12 and 1264.32](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Acontour-index-fixing-uniform-slope-points) — 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 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 characteristics and uses of contour map](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Acontour-characteristics-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 ## Related in other subjects - [Field procedure of chain surveying](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Afield-procedure-chain-survey) — Surveying I, chapter 3, asked 4× - [Principle of chain surveying](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Aprinciple-of-chain-survey) — Surveying I, chapter 3, asked 3× - [Engineering surveying; two-way principle application](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Aengineering-surveying-two-way-angle) — Surveying I, chapter 1 - [Principle of chain surveying; well-conditioned triangle](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Achain-principle-well-conditioned-vs-error) — Surveying I, chapter 3