--- title: "Index contour and indirect contouring" subject: "Surveying II" chapter: 4 key: "semester_4/surveying_ii:index-contour-indirect-method" source: https://www.bctengineer.com/q/semester_4/surveying_ii/index-contour-indirect-method site: BCT Engineering --- # Index contour and indirect contouring Surveying II · Chapter 4: Contouring - Asked in: 2071 Bhadra, 1+5 marks - Video: [#2 General Contour & Index Contour || Unit 1: Contouring || Surveying-II](https://www.youtube.com/watch?v=aKnLL0m7x5A) — Er. Iswor Rawat - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/surveying_ii/index-contour-indirect-method - Whole chapter: https://www.bctengineer.com/md/semester_4/surveying_ii/chapter-4 ## Question Describe index contour and explain indirect method of contouring for field control. ## Short answer Index contour is every fifth contour, drawn thick and labelled. Indirect contouring surveys only spot levels; contours are later interpolated. Common methods: grid method for flat areas, cross-sections along routes, radial lines from a central point, and tacheometry for hilly terrain. Control is established by traverse and benchmarks. ## Answer #### 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**. #### Indirect method of contouring In the **indirect method** only selected points (spot levels) are surveyed and plotted, and the contours are drawn later by interpolation. It is cheaper, quicker and suitable for large areas and small-scale maps. 1. **Method of squares (grid method):** The area is divided into squares of 5 m to 20 m side (smaller for rough ground). The corners are pegged and their levels are taken with a level. The grid is plotted, RLs are written at the corners and the contours are interpolated along the grid lines. It is suitable for small flat areas. 2. **Method of cross-sections:** Cross-sections are taken at right angles to a base line or a main line at fixed intervals (10 m to 30 m). Levels are taken at regular intervals and at the break points along each section. Used for route survey (roads, railways, canals). 3. **Radial line method:** Radial lines are set out from a central station at known angles; points on them are located by distance and their levels are taken (by a level or by tacheometry). It is suitable for a hilly area where a hill can be observed from the summit. 4. **Tacheometric method:** Spot heights and positions are obtained together with a theodolite and staff; used for hilly ground. **Procedure for control:** the area is first controlled by a traverse and levelled BMs. Spot levels of the points (grid corners, cross-section points or radial points) are taken from these. The points are plotted to scale, their RLs are written, and the contours are drawn by interpolation, with every fifth contour as an index contour. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. With CI = 2 m, which values are index contours? A. Multiples of 10 m B. Every 2 m C. Every 5 m D. Only the highest contour Answer: A. Every fifth contour (10, 20, 30 m) is thick and labelled. (Key point: Index contour) 2. What is the principle of the indirect method? A. Survey and plot selected spot levels, then interpolate contours B. Mark every contour on the ground C. Use no control D. Draw contours by eye without levels Answer: A. Spot levels are plotted and contours are drawn by interpolation. (Key point: Indirect principle) 3. The method of cross-sections is mainly used for? A. Route surveys such as roads, railways and canals B. Small flat areas C. Hilltops viewed from summit D. Plane table work Answer: A. Cross-sections at right angles to a base line suit route surveys; grid suits small flat areas and radial lines hilly areas. (Key point: Cross-section method) 4. What is done before taking spot levels? A. The area is controlled by a traverse and levelled BMs B. Contours are drawn C. The map is printed D. Nothing Answer: A. Spot levels are taken from controlled stations and BMs, then plotted and interpolated. (Key point: Control) ## Related in Surveying II - [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 - [Direct contouring method, uses of contour map](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Adirect-contouring-uses-contour-map) — Surveying II, chapter 4 - [Methods of contouring and uses of contour maps](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Acontouring-methods-uses) — Surveying II, chapter 4 - [Contouring methods, direct method field procedure](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Acontouring-methods-direct-field-procedure) — 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 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