Toposheet Route and Site Selection on a 1:50,000 Sheet
A student is given a toposheet extract on 1:50,000. She must plan a fair-weather road from village Rampur (grid square 4520) to the rail station at Bela (grid square 4818). On the sheet: contour interval 20 m; a north–south spur separates the two settlements; a direct line between them is 6.4 cm on the map and crosses the 200 m, 220 m and 240 m contours at spacings of 0.8 cm between successive contours; a longer route following the spur is 10.0 cm and crosses only two contours, at 4.0 cm spacing; a river flows east–west with a narrow gorge at grid square 4621 and a broad basin upstream; and the village in 4818 now has a market and a bus stand. The sheet was surveyed in 1972–73. Her practical file already establishes that 2 cm on this sheet equals 1 km on the ground.
1. Convert the distances and compute the gradient of the direct route. Is it acceptable?
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Scale check first: 2 cm = 1 km is consistent with 1:50,000, since 2 cm × 50,000 = 1,00,000 cm = 1 km. So one centimetre on the sheet is 0.5 km.
Direct route: 6.4 cm × 0.5 = 3.2 km. Contour spacing: 0.8 cm × 0.5 = 0.4 km (400 m) between successive contours, i.e. a 20 m rise in 400 m. Gradient: 20 ÷ 400 = 0.05 = 5 per cent, or 1 in 20.
Judgement: 1 in 20 is far too steep for a metalled road, which is normally held to about 1 in 30 (3.3 per cent), and far too steep for a railway, which needs roughly 1 in 100 to 1 in 150. A 5 per cent climb straight up a spur means heavy cutting and filling, culverts at every nala, and rapid erosion of the fill slopes in a monsoon-dominant climate.
The direct route is defensible only as a short steep cart track. The answer should give the arithmetic before the verdict, and state the acceptable limit used.
2. Evaluate the longer spur route and recommend an alignment.
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Longer route: 10.0 cm × 0.5 = 5.0 km, crossing two contours over 4.0 cm × 0.5 = 2.0 km, a 40 m rise in 2,000 m. Gradient = 40 ÷ 2,000 = 0.02 = 2 per cent, or 1 in 50 - within road limits and close to ideal.
Comparison. The direct route saves 1.8 km but climbs at 5 per cent. The spur route costs about 1.8 km of extra running surface and earthwork, but avoids deep cutting and vulnerable fill. In a flood-prone, erosion-sensitive belt, the deciding factor is maintenance over the life of the road, not construction cost: a 5 per cent grade will need repairs after every monsoon, and repairs recur while construction happens once.
Recommendation: adopt the spur route, with a minor realignment so it touches both Rampur and Bela rather than bypassing them. Justify by stating the saving in gradient, the reduced earthwork, and the lower whole-life cost - and note that the extra 1.8 km also opens the hamlets on the spur, which the direct route would leave isolated.
3. Where would you site a check dam, and what field checks are needed before either the dam or the road is finalised?
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Check dam: at the gorge in grid square 4621. A gorge gives a short dam wall for a given storage height, so earthwork and cost per unit of water stored are low; the broad basin upstream provides catchment and storage volume; and a site on a straight reach with firm abutments reduces scour risk. Verify it lies upstream of the settlement and does not submerge the market at Bela.
Field checks, in order:
- Date of the sheet - surveyed 1972–73, so the market, bus stand and any new bridge or road must be resurveyed; a 'spur' on the map may now be a built-up area.
- Ground truth the contours and gradient with a total station or GPS, and confirm the contour interval interpretation.
- Land ownership and acquisition, plus forest or protected-area status.
- Hydrology: catchment area, rainfall intensity, silt load - a check dam silts up, so plan desilting - and downstream water rights.
- Route constraints: bridges across the river, and a ruling-gradient survey if a rail alignment is ever intended.
State plainly that a toposheet gives a hypothesis; only field survey confirms it.