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. It can be found by putting a ruler between the contour points, on the map, and the convert the distance into field equivalent using the scale of the map.
Contour Gradient
The slope of the imaginary line between two contour points on different contours, drawn considering both the horizontal and vertical distances, maintains a constant angle to the horizontal.
Characteristics of the Contours
- Two contours of different elevation do not cross each other with only exception in the contours of a overhanging cliff.
- Contours of different elevations do not combine or overlap to each other with exception in the contours of a vertical cliff.
- 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 contours are equally spaced they represent a uniform slope, and 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.
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.
