--- title: "Contour interval, index contour, why never cross" subject: "Surveying II" chapter: 4 key: "semester_4/surveying_ii:contour-interval-index-plotting-never-cross" source: https://www.bctengineer.com/q/semester_4/surveying_ii/contour-interval-index-plotting-never-cross site: BCT Engineering --- # Contour interval, index contour, why never cross Surveying II · Chapter 4: Contouring - Asked in: 2079 Jestha, 6 marks - Video: [WHAT IS CONTOUR | CHARACTERSTICS AND METHODS | CIVIL ENGINEERING | MINING ENGINEERING | MINING WORLD](https://www.youtube.com/watch?v=PQfr1LABZWg) — Mining World - 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-never-cross - Whole chapter: https://www.bctengineer.com/md/semester_4/surveying_ii/chapter-4 ## Question Define contour interval. What are contour index? You have to plot the index and minor contour, explain the process with neat sketches and examples relating the principles behind it. Why do contour line never touch or cross? ## Short answer Contour interval is the constant vertical distance between successive contours. An index contour is every fifth contour, drawn thicker and labeled. Plotting uses linear interpolation, e.g., distance = (contour elevation − lower RL)/(difference in RL) × plot distance, with index contours drawn thick. Contours never cross because a point cannot have two elevations; only apparent merging in vertical cliffs or crossing in overhanging cliffs. ## 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. It depends on the scale, relief of the ground and the purpose of the map. #### Contour index **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**. #### Plotting index and minor contours The contours are drawn from the spot levels of surveyed points by interpolation. Example: two surveyed points A (RL 52.4 m) and B (RL 61.8 m) plotted 9.4 cm apart; CI = 2 m, so the index contour is every 5th contour (multiples of 10 m). 1. List the contour values between the two RLs: 54, 56, 58, 60 m. Of these, 60 m is an index contour (multiple of 10 m); 54, 56 and 58 are minor contours. 2. Assuming a uniform slope, find the distance of each contour from A: $x = \dfrac{h_c - 52.4}{61.8 - 52.4}\times 9.4$. | Contour (m) | Distance from A (cm) | |---|---| | 54 | 1.60 | | 56 | 3.60 | | 58 | 5.60 | | 60 | 7.60 | 3. Mark these points on AB, repeat for all the pairs of points, and join the points of equal value with smooth curves. 4. Draw the index contour (60 m) thicker, with its value written in a break in the line; the minor ones are thin lines. ```text A(52.4) 54 56 58 60(index) B(61.8) o--------|----|----|----||--------o ``` #### Why contours never touch or cross A contour joins points of one fixed elevation. If two contours of different elevations touched or crossed, the common point would have two different elevations at the same place, which is impossible. So contour lines never meet. The only exceptions are: - a **vertical cliff**, where the contours of different RL fall on the same plan position and appear to merge into one line, and - an **overhanging cliff or cave**, where the upper contour projects over the lower, so the lines appear to cross (the hidden part is drawn dotted). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What does the choice of contour interval depend on? A. Only the colour of the map B. Scale, relief of the ground and purpose of the map C. Only the number of surveyors D. Only the instrument used Answer: B. CI is selected considering map scale, ground relief and the purpose of the map. (Key point: Contour interval depends on) 2. Points A (52.4 m) and B (61.8 m), CI = 2 m. Which contour values lie between them and which is the index contour? A. 54, 56, 58, 60; the 60 m contour is the index B. 52, 54, 56, 58, 60, 62; 52 is the index C. 53, 55, 57, 59, 61; 55 is the index D. 54, 56, 58, 60; 54 is the index Answer: A. Even multiples of 2 between 52.4 and 61.8; with CI = 2 m, every fifth contour is a multiple of 10 m, so only 60 m. (Key point: Contour list) 3. Using x = (h_c - 52.4)/(61.8 - 52.4) × 9.4, what is the distance of the 56 m contour from A? A. 1.6 cm B. 3.6 cm C. 5.6 cm D. 7.6 cm Answer: B. x = (56 - 52.4)/9.4 × 9.4 = 3.6 cm. (Key point: Distance of contour) 4. Why do contours of different elevations never touch or cross? A. The common point would have two elevations at one place B. They are drawn in different colours C. The scale prevents it D. The interval is too small Answer: A. One point cannot have two RLs; exceptions are vertical cliffs and overhanging cliffs or caves. 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