--- title: "Contour interval, index contour, contouring methods" subject: "Surveying II" chapter: 4 key: "semester_4/surveying_ii:contour-interval-direct-indirect-methods" source: https://www.bctengineer.com/q/semester_4/surveying_ii/contour-interval-direct-indirect-methods site: BCT Engineering --- # Contour interval, index contour, contouring methods Surveying II · Chapter 4: Contouring - Asked in: 2078 Chaitra, 2+5 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-interval-direct-indirect-methods - Whole chapter: https://www.bctengineer.com/md/semester_4/surveying_ii/chapter-4 ## Question Describe contour interval and index contour. Explain the direct and indirect method of contouring. ## Short answer Contour interval is the constant vertical separation between consecutive contours; index contour is every fifth, thicker and labeled. In direct method, contours are located on ground using a level: staff reading = HI − desired RL, points pegged and surveyed. Indirect method uses spot levels from a grid, cross-sections, radial lines or tacheometry, and contours are interpolated later. ## Answer #### Contour interval **Contour interval (CI)** is the constant vertical distance between two consecutive contours, for example 1 m or 5 m. It is the same over one map. Small CI is used for flat ground and large-scale maps; large CI for hilly ground and small-scale maps. #### 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**. #### Direct method In the **direct method** the contours are located and marked directly on the ground, and the points are then surveyed and plotted. It is accurate but slow and costly, so it is used for small areas and large-scale maps. **Steps** 1. Establish control (traverse stations) and a benchmark in the area. 2. Set up a level at a convenient point, take a back-sight on the BM and find the height of instrument: $HI = RL_{BM} + BS$. 3. For a contour of RL $h$, the staff reading required is $r = HI - h$. A staff man moves up or down the slope until the staff reading equals $r$; that point lies on the contour. Mark it with a peg. 4. Find several points of the same contour, then move to the next contour (staff reading changed by the CI). 5. When the level is shifted, a new HI is found by sighting a fixed point (change point). 6. The pegs are then located by the **plane table, theodolite (tacheometer), chain or compass** from the control stations and plotted on the sheet. The points of equal RL are joined by a smooth curve. For steep ground a clinometer or a hand level may be used with a ranging rod. #### Indirect method 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How is the contour interval chosen with respect to ground and scale? A. Small CI for flat ground and large-scale maps; large CI for hilly ground and small-scale maps B. Large CI for flat ground C. Small CI for hilly ground D. CI does not depend on ground Answer: A. A small CI shows relief on flat ground, while hilly ground needs a large CI to avoid crowding. (Key point: Choosing CI) 2. In the direct method, what staff reading marks a point on a contour of RL h? A. r = HI - h B. r = HI + h C. r = h - HI D. r = BS - h Answer: A. The staff reading at RL h is HI - h; the staff man moves until that reading is seen. (Key point: Direct method staff reading) 3. What distinguishes the indirect method from the direct method? A. Only spot levels are surveyed and contours are interpolated later; it is cheaper and suits large areas B. Contours are marked on the ground; used for large areas C. It is more accurate but slower D. It uses no levelling Answer: A. The indirect method surveys selected points and interpolates; the direct method fixes contour points on the ground. (Key point: Direct versus indirect) 4. Which of these is an indirect method of contouring? A. Method of squares (grid) B. Staff reading r = HI - h on pegs C. Marking the contour with pegs on the ground D. Chaining the contour Answer: A. Squares, cross-sections, radial lines and tacheometry are indirect methods. (Key point: Indirect methods) ## Related in Surveying II - [Index contour and indirect contouring](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Aindex-contour-indirect-method) — 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 - [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 - [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 - [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 ## Related in other subjects - [Direct and indirect recursion](https://www.bctengineer.com/question.md?key=semester_4%2Fdata_structure_and_algorithm%3Adirect-indirect-recursion) — Data Structure and Algorithm, chapter 5 - [Define recursion; direct vs indirect](https://www.bctengineer.com/question.md?key=semester_4%2Fdata_structure_and_algorithm%3Adefine-recursion-direct-indirect) — Data Structure and Algorithm, chapter 5 - [Degenerate semiconductors; direct vs indirect bandgap](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Adegenerate-direct-indirect) — Electrical Engineering Material, chapter 6 - [Direct versus indirect band gap semiconductors](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Adirect-indirect-bandgap) — Electrical Engineering Material, chapter 6