Contours: An Overview

Contour – Contour is an imaginary line on ground joining points of equal or constant elevations. contours are important to draw the topographical maps in which vertical distances are also shown using the contour lines.
Contour interval: Vertical distance between any two consecutive contours is known as the contour interval. It depends upon the scale of the map, nature of the ground and availability of the fund and time.
Horizontal equivalent/ horizontal interval: It is the shortest horizontal distance between the two consecutive contours.
Contour Gradient: Imaginary line on the surface of the earth, maintains a constant angle to the horizontal.

Characteristics of the contours:

When contours are drawn closer to each other, it shows a steep slope on the ground and when they are far apart it shows the gentle slope on the ground.
When they are parallel straight and equivalent they represent a plane surface.
A contour is perpendicular to a line of the steepest slope.
A contour must close itself in the map or must go out of the boundaries of the map.
A set of ring contours with higher values of contour inside and lower values outside represents a hill and if the higher values are outside and lower values inside then it represents a depression like a pond.
When contours cross a ridge they form a V- shape across them. While if they cross a valley they form a u-shape or may a V shape also difference being, the concavity of the contour lines lies towards the lower contours in case of valley while it is convexity lying towards the another lower value in case of contours of a ridge.
Contour lines must close, not necessarily in the limits of the plan.
The horizontal distance between any two contour lines indicates the amount of slope and varies inversely on the amount of slope.
Widely spaced contour indicates flat surface.
Closely situated contour indicates steep slope ground.

contour showing steep slope terrain
Equally spaced contour indicates uniform slope.

contour showing uniform slope terrain
Irregular contours indicate uneven surface.
Approximately concentric closed contours with decreasing values towards centre indicate a pond.

Pond and its contour
Approximately concentric closed contours with increasing values towards centre indicate hills.

Hill and its contour
Contour lines with U-shape with convexity towards lower ground indicate ridge.
Contour lines with V-shaped with convexity towards higher ground indicate valley.

contour showing ridge line and valley line
Contour lines generally do not meet or intersect each other. If contour lines are meeting in some portion, it shows existence of a vertical cliff.

Vertical cliff and its contour
Contours of different elevations cannot cross each other. If contour lines cross each other, it shows existence of overhanging cliffs or a cave.

overhanging cliff and its contour
The steepest slope of terrain at any point on a contour is represented along the normal of the contour at that point.
Contours do not pass through permanent structures such as buildings.

contour across a permanent structure

Uses of Contours:

To study the general character of the tract of the country without visiting the ground. With the knowledge of characteristics of contours, it is easy to visualize whether country is flat, undulating or mountainous.
To decide the sites for engineering works such as reservoirs, canals, roads and railways etc. on the basis of the economy.
To determine the catchment area of the drainage basin and hence capacity of the proposed reservoir.
To compute the earth work required for filling or cutting along the linear alignment of the projects such as canals, roads, etc.
To find out the inter-visibility of the points.
To trace out a contour gradient for road alignments.
To draw longitudinal and cross- sections to ascertain nature of the ground.

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