Reading a 1:50,000 toposheet without the sheet
A Survey of India toposheet at 1:50,000 is being interpreted in a practical class. Because the sheet cannot be shown, the features are described in words. Spot height 342 m lies at the north-west corner of grid square 45/67. A stream enters that square at its western edge, passes about 500 m south of the spot height, and leaves at the south-east corner. The 320 m and 300 m contours cross the square; on the eastern slope these two contours are 4 mm apart on the sheet, while on the northern slope they are 18 mm apart. The metalled road from the north runs through the square and crosses the stream at a bridge in the south-east. Village Rampur (42 houses, a post office, a weekly market) occupies the interfluve between the stream and a tank with a masonry bund 300 m to its east. The distance on the sheet from the post office to the bridge is 3.7 cm. A reserved forest patch lies on the western edge, and contour bunds and a small canal with distributaries follow the 300 m contour on the eastern side. Two habitations exist in the square: Rampur and a hamlet of eight houses.
1. Convert the map measurements to ground values: the post office–bridge distance, the area of the grid square if it measures 4 cm by 5 cm, and the gradient of the eastern and northern slopes.
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Scale: 1:50,000 means 1 cm on the sheet = 50,000 cm = 500 m = 0.5 km on the ground.
Post office to bridge: 3.7 cm × 0.5 km = 1.85 km (1,850 m). This is the straight-line distance; the road distance would be longer because the road bends to reach the bridge.
Grid square 4 cm × 5 cm: 4 × 0.5 = 2 km; 5 × 0.5 = 2.5 km. Area = 2 × 2.5 = 5 km² (500 hectares).
Eastern slope: contours 4 mm apart. Ground distance = 4 mm × 50,000 = 200,000 mm = 200 m. Vertical interval = 320 − 300 = 20 m. Gradient = 20/200 = 0.10, i.e. 10%, or 1 in 10 - steep, and best shown as a slope of about 5.7°.
Northern slope: 18 mm apart → 18 × 50,000 = 900,000 mm = 900 m. Gradient = 20/900 = 0.022, i.e. 2.2%, or 1 in 45 - gentle.
Interpretation: closely spaced contours mean steep ground, so the eastern side rises sharply within 200 m while the northern side is a long gentle ramp. The eastern slope, being steep, is the more erosion-prone of the two, which fits the canal and bunds running along the 300 m contour nearby.
2. Determine the direction of stream flow, the drainage pattern, and the site advantages that explain Rampur's position.
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Flow direction: the spot height of 342 m is in the north-west and the stream leaves at the south-east, so the land falls from north-west to south-east and the stream flows from west to south-east. The contour test confirms it: a contour crossing a valley forms a V whose apex points upstream, so the apex near the western edge means the water comes from the west. The bridge in the south-east is therefore at the lowest and flattest point in the square.
Drainage pattern: a single consequent stream following the regional slope, joined by short tributaries, gives a dendritic pattern, consistent with weathered gneiss of fairly uniform resistance. Where tributaries join at right angles along joints, the pattern becomes sub-dendritic or trellised in detail.
Rampur's site advantages:
- It sits on the interfluve, above the flood level of the stream but close enough to it for water supply - the classic avoidance of both flood risk and water distance.
- The tank with its masonry bund 300 m east provides assured water for domestic use and irrigation, and the bund is a defensible, permanent feature.
- It commands the bridge and the metalled road, a route focus that explains the post office and the weekly market: a service role for the hamlet of eight houses and for surrounding farmland.
- The gently sloping northern ground gives room for the 42 houses to spread as a nucleated cluster, and the reserved forest on the western edge supplies fuel and timber.
3. A student argues that the canal follows the 300 m contour because it has to cross the stream. Evaluate this claim and state which part of the square is most at risk of flooding.
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The claim is wrong. A contour canal is deliberately aligned along a contour so that it maintains a constant, gentle gradient: water then flows by gravity without needing embankments on one side and cuttings on the other, and without pumping. The canal here runs along the 300 m contour on the eastern side of the square, which is precisely the classic contour-alignment design.
If the canal had to cross the stream, it would need an aqueduct or a level crossing, and it would then lie on the western side of the stream as well. The sheet shows it staying on the eastern side with distributaries running downslope, so no crossing is implied. The student has mistaken the reason for the alignment: the contour is followed to avoid crossings and to command the land below it, not because of a crossing.
Flood risk: the south-east corner, around the bridge. It is the lowest point in the square (below the 300 m contour), it is where the stream leaves after collecting the whole catchment, and the gentle 2.2% northern slope means water arrives there slowly but steadily and spreads. The interfluve where Rampur stands is above this level, which is exactly why the village was sited there.
Wider reading: contour bunds plus the canal indicate a soil- and water-conservation programme; the reserved forest on the western edge protects the steep upper catchment, so any deforestation there would increase the flood peak at the bridge.