A contour is defined as an imaginary line passing through the points of equal elevation. Thus contour lines on a plan illustrates the configuration of the ground. The method of representing the relief of the ground by the help of contour is called contouring. The vertical distance between two consecutive contours is called contour interval. Every 5^th contour which is 5 times of the contour interval is the index contour which is generally darkened in the contour and is known as Index Contour. The least horizontal distance between two consecutive contours is called the horizontal equivalent.

Methods of contouring

There are two ways of contouring. They are namely

1. The Direct method

2. The Indirect method

1. The direct method:

In this direct method, the equal elevated points are joined. For this, firstly the points with the same elevations are found out by setting out the instrument at a point and by hit and trial method of searching the points which gives the same required staff reading.

2. The indirect method:

In this method, some suitable guide points are selected and surveyed, the guide points need not necessarily be on the contours. There are some of the indirect methods of locating the ground points:

a. By squares

b. By cross-sections

c. By tacheometric method

Contour Interpolation

The process of drawing contours proportionately between the plotted ground points or in between the plotted contours is called interpolation of the contours. Interpolation of contours between points is done assuming that the slope of ground between two points is uniform. It may be done by anyone of following methods.

• Estimation
• Arithmetic calculation
• Graphical method

Contour Characteristics

• Two contour lines do not intersect each other except in the case of overhanging cliff.
• A contour line must close onto itself not necessarily within the limits of a map.
• Contours of different elevations do not unite to form one contour except in the case of a vertical cliff.
• Two contour lines do not unite to form a single one except in the case of perpendicular cliff.
• Contours drawn closer depict a steep slope and if drawn apart, represent a gentle slope.
• Contours equally spaced depict a uniform slope. When contours are parallel, equidistant and straight, these represent an inclined plane surface.
• A set ring contours with higher values inside depict a hill whereas a set of ring contours with lower values inside depict a pond or a depression without an outlet.
• When contours cross a ridge or V-shaped valley, they form sharp V-shapes across them. Contours represent a ridge line, if the concavity of higher value contour lies towards the next lower value contour and on the other hand these represent a valley if the concavity of the lower value contour, lies toward the higher value contours.
• The same contour must appear on both the sides of a ridge or a valley.
• Contours do not have sharp turnings.