--- title: "Methods of contouring and uses of contour maps" subject: "Surveying II" chapter: 4 key: "semester_4/surveying_ii:contouring-methods-uses" source: https://www.bctengineer.com/q/semester_4/surveying_ii/contouring-methods-uses site: BCT Engineering --- # Methods of contouring and uses of contour maps Surveying II · Chapter 4: Contouring - Asked in: 2069 Bhadra, 8 marks - Video: [Methods of contouring](https://www.youtube.com/watch?v=NYr_Ic6o_xw) — WIT Solapur - Professional Learning Community - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/surveying_ii/contouring-methods-uses - Whole chapter: https://www.bctengineer.com/md/semester_4/surveying_ii/chapter-4 ## Question What are the methods of contouring? Explain briefly. Write the uses of contour maps. ## Short answer Contouring methods are direct (contours marked in the field) and indirect (spot levels surveyed, contours interpolated) – including grid, cross-section, radial line, and tacheometric methods. Contour maps help determine elevations, slopes, alignments, intervisibility, reservoir capacity, earthwork volumes, and are used for site selection, drainage planning, and town planning. ## Answer #### Methods of contouring Contouring is done by two methods: the **direct method** and the **indirect method**. #### 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. #### Uses of contour maps 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 pipe line 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 if a signal station can be seen from another). 5. To calculate the capacity of a reservoir, the catchment area and the volume of earthwork (area between contours). 6. To select suitable sites for a dam, building, bridge, airport or a 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 layout. 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. Contouring is carried out by which two methods? A. Grid method and radial method B. Direct method and indirect method C. Level method and theodolite method D. Estimation method and calculation method Answer: B. The two main methods are the direct method (contours traced on the ground) and the indirect method (spot levels plus interpolation). (Key point: Two methods of contouring) 2. In the direct method, for a contour of RL h the staff reading required is: A. r = HI + h B. r = h - HI C. r = HI - h D. r = RL of BM + h Answer: C. HI = RL_BM + BS, and the staff reading at a point of RL h is r = HI - h; the staff man moves until this reading is obtained. (Key point: Staff reading in direct method) 3. Why is the indirect method preferred for large areas and small-scale maps? A. It is cheaper and quicker, as only selected spot levels are surveyed and contours are interpolated B. It marks every contour on the ground with pegs C. It is more accurate than the direct method D. It needs no levelling at all Answer: A. The direct method is accurate but slow and costly, so the indirect method with spot levels and interpolation is used for large areas. (Key point: Direct vs indirect suitability) 4. For which work is the method of cross-sections mainly used? A. Small flat building sites B. Hills observed from a summit C. Route surveys such as roads, railways and canals D. Underwater mapping only Answer: C. Cross-sections at right angles to a main line suit long narrow strips such as roads, railways and canals. (Key point: Method of cross-sections) 5. Which is a use of a contour map? A. Measuring the magnetic declination B. Finding reservoir capacity, catchment area and earthwork volume C. Correcting the closing error of a traverse D. Setting out a curve by deflection angles Answer: B. Areas between contours give capacity of reservoirs, catchment area and volumes of earthwork. (Key point: Uses of contour maps) ## Related in Surveying II - [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 interpolation and uses of contour maps](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Acontour-interpolation-uses) — 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 - [Index contour and indirect contouring](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Aindex-contour-indirect-method) — 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 - [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