LIWA

Contours

ICSE Class 10 · Geography · Chapter 2

What this chapter covers

Key definitions

Relief is the variation in height and shape of the land surface of an area, including its hills, ridges, slopes and level ground.

Mean sea level is the average level of the surface of the sea, worked out from long records of the rise and fall of the tides, and it is taken as zero for all heights on land.

Index contours are the thicker contour lines, usually every fifth one, that are always labelled with their height so that the reader can count the rest from them.

Contour interval is the constant difference in height between two successive contour lines on a map.

Slope is the inclination of the land surface, that is, how quickly the ground rises or falls over a horizontal distance.

A conical hill is a hill shaped like a cone, with a pointed top and sides that slope evenly in every direction.

Triangulated height is the height above mean sea level of a triangulation station, a fixed point whose position and height have been found by the method of triangulation used by surveyors.

Spot height is the height above mean sea level of a particular point on the ground, found by survey and printed on the map as a dot with the height in metres written beside it.

Relative height is the height of a feature measured above the level of the land immediately around it, not above mean sea level.

Relative depth is the depth of a feature measured below the level of the land immediately around it.

Key points

Questions answered

Define a contour.
A contour is an imaginary line drawn on a map that joins all places having the same height above mean sea level.
What is meant by contour interval? What is its usual value on a 1 : 50,000 Survey of India toposheet?
Contour interval is the constant difference in height between two successive contour lines on a map. On most 1 : 50,000 Survey of India toposheets it is 20 m.
In what colour are contours printed on a Survey of India toposheet?
Brown, the colour used for relief features.
Why can two contours never cross each other?
Each contour joins points of one height only. If two contours crossed, the point of crossing would have two different heights at once, which is impossible. Only an overhanging cliff, which is very rare, breaks this rule.
Two labelled contours read 260 and 360, with four unlabelled contours between them. Find the contour interval.
The difference is 360 - 260 = 100 m and there are 4 + 1 = 5 spaces, so the contour interval is 100 ÷ 5 = 20 m.
How would you recognise a steep slope and a gentle slope on a toposheet?
A steep slope is shown by contours lying very close together, because the land gains height over a short distance. A gentle slope is shown by contours lying far apart, because the land gains height slowly over a long distance.
Describe the contour pattern of a conical hill.
A conical hill is shown by nearly circular closed contours arranged one inside another around a common centre, spaced at almost equal distances, with values increasing towards the centre.
How is a ridge shown by contours, and how does it differ from a hill?
A ridge is shown by long, narrow closed contours with values rising towards the centre line, close together across the ridge and far apart along it. A hill has closed contours that are roughly as wide as they are long.