--- title: "Contour index, fixing contours between 1240.12 and 1264.32" subject: "Surveying II" chapter: 4 key: "semester_4/surveying_ii:contour-index-fixing-uniform-slope-points" source: https://www.bctengineer.com/q/semester_4/surveying_ii/contour-index-fixing-uniform-slope-points site: BCT Engineering --- # Contour index, fixing contours between 1240.12 and 1264.32 Surveying II · Chapter 4: Contouring - Asked in: 2079 Chaitra, 1+3+2 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/surveying_ii/contour-index-fixing-uniform-slope-points - Whole chapter: https://www.bctengineer.com/md/semester_4/surveying_ii/chapter-4 ## Question What is contour index? Explain the method of fixing the index and minor contour from two uniform slope ground points such as 1240.12 m and 1264.32 m and CI = 3.0 m. Why do contour line never touch or cross? ## Short answer An index contour is every fifth contour drawn thicker and labelled. For points 1240.12 m and 1264.32 m with CI 3 m, index contours are at 1245 m and 1260 m; minor contours at 1242, 1248, 1251, 1254, 1257, 1263 m. Positions are found by proportion: distance from lower point = (contour RL - 1240.12)/24.20 × L. Contours never touch or cross because a point cannot have two elevations, except at vertical or overhanging cliffs. ## Answer #### Contour index An **index contour** is every fifth contour, drawn as a thicker line and labelled with its RL so that contours can be identified easily. With CI = 3 m, the index contours are at every 15 m: 1245 m, 1260 m, ... The contours between them are the **intermediate (minor) contours**. #### Fixing index and minor contours between 1240.12 m and 1264.32 m The ground between the two points has a uniform slope, so contour positions are found by proportion. 1. Contour values are multiples of CI = 3 m. Between 1240.12 and 1264.32 these are 1242, 1245, 1248, 1251, 1254, 1257, 1260, 1263 m. 2. Of these, 1245 m and 1260 m are multiples of 15 m, so they are **index contours**; the other six are minor contours. 3. The difference in level between the points is $1264.32 - 1240.12 = 24.20$ m. If the points are $L$ cm apart on the map, the distance of a contour of value $h_c$ from the lower point is $$ x = \frac{h_c - 1240.12}{24.20}\times L $$ Taking the plotted distance as $L = 12.1$ cm (so that $x = (h_c - 1240.12)/2$ cm): | Contour (m) | Type | Distance from 1240.12 point (cm) | |---|---|---| | 1242 | Minor | 0.94 | | 1245 | Index | 2.44 | | 1248 | Minor | 3.94 | | 1251 | Minor | 5.44 | | 1254 | Minor | 6.94 | | 1257 | Minor | 8.44 | | 1260 | Index | 9.94 | | 1263 | Minor | 11.44 | 4. Mark these positions on the line joining the points, repeat for the neighbouring points, join equal values with smooth curves, and draw the 1245 m and 1260 m contours thick and label them. ```text 1240.12 1242 1245 1248 1251 1254 1257 1260 1263 1264.32 o-------|----||---|----|----|----|----||---|-------o idx idx ``` The spacing between neighbouring contours is equal ($3/24.2\times 12.1 = 1.50$ cm) because the slope is uniform. #### Why contour lines never touch or cross All points of a contour have the same RL. If two contours of different RL touched or crossed, the common point would have two different elevations, which is impossible. The only exceptions are a **vertical cliff**, where contours of different value fall on the same plan line and appear to merge, and an **overhanging cliff or cave**, where the upper contour is drawn over the lower one and appears to cross it (the hidden part is dotted). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. With CI = 3 m and every fifth contour as index contour, index contours lie at: A. Every 3 m B. Every 5 m C. Every 15 m (e.g. 1245 m, 1260 m) D. Every 30 m Answer: C. 5 x 3 m = 15 m, so index contours are multiples of 15 m such as 1245 and 1260. (Key point: Index contours for CI = 3 m) 2. Which contours (multiples of 3 m) lie between 1240.12 m and 1264.32 m? A. 1240 to 1264 m B. 1242 m to 1263 m, eight contours C. 1245 m to 1260 m only D. 1243 m to 1264 m Answer: B. Multiples of 3 m within the range are 1242, 1245, 1248, 1251, 1254, 1257, 1260, 1263 m. (Key point: Contours between points) 3. Why are the contour spacings equal between the two points (1.50 cm for L = 12.1 cm)? A. Because the ground slope is assumed uniform B. Because the contours are all index contours C. Because the points are at the same RL D. Because CI is always 1.5 cm on the map Answer: A. Under uniform slope, spacing = CI/(level difference) x L = 3/24.2 x 12.1 = 1.50 cm. (Key point: Equal spacing) 4. Why do contour lines never touch or cross? A. They would have different intervals B. A common point would have two different RLs, except at a vertical cliff or overhang C. They are drawn with different pens D. The surveyor avoids it for neatness Answer: B. Different-value contours sharing a point would give it two elevations; only vertical cliffs (merge) and overhangs/caves (appear to cross) are exceptions. 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