Contours

Contour interval means the difference of height between two consecutive contours. For example, if one thick contour shows the height 300 metres and the next thin contour depicts the height of 320 metres. Then, the contour interval is 20 metres.

The vertical distance between any two contour lines is called vertical interval.

The actual distance between two points on two contour lines is called horizontal equivalent.

The ratio between the vertical height and the horizontal distance of a slope of land measured along the ground is called gradient.

Spot height

Vertical interval

Triangulation point

Relative height

Spot height

Relief refers to actual configuration of land which includes its altitude and slope. The relief features found on the earth's surface include mountains, valleys, hills, plains and plateaus.

The three methods used to show the earth's relief features are-

1. hachures
2. hill shading
3. contours

Contour lines are imaginary lines which are drawn on a map to join places having the same height above the mean sea level.

Contour lines are imaginary lines joining places which have the same height above mean sea level.

Hill shading and hachuring only give an impression of the slope but do not indicate the heights of the land above sea level, whereas the contours show both impression of the slope as well as the heights of the land above the sea level.

The contour lines are shown by brown colour as thick brown lines and thin brown lines. Contour lines show:

1. Elevation of land: The height of a contour is indicated by figures inserted in a break in the line.

2. Steepness of its slope: When the contours are very close, they represent steep slopes. When they are farther apart, they represent gradually increasing slopes. Absence of contour lines indicate that the land is flat i.e., a low land.

3. Shape of land at various heights: The contour lines indicate the shape of land. For example, almost circular contours whose value decreases inside represent a lake.

V.I. stands for vertical interval. It is the vertical distance between any two contour lines.

H.E. stands for horizontal equivalent. It is the actual distance between two points on two contour lines.

Two contours never intersect each other because each line represents a different height, and we can't have two different heights at the same point.

Rivers and streams are shown on the map with blue colour. Generally water flows from higher altitude to lower altitude. Since the contours indicate the elevation of land, we can use them to find the direction of a river or stream.

The rivers and streams usually flow from contours representing greater elevations to contours representing lower elevations. The course of a well-developed river can be divided into three stages - upper, middle and lower course; each of which can be recognised with the help of contour lines.

1. Kiln
2. Tank with Embankment
3. Permanent huts
4. Temple
5. Lighthouse
6. Triangulated station
7. Benchmark
8. Post office
9. Overhead tank
10. Police station
11. Hospital
12. Powerline with pylons surveyed