What is a Contour

 Contour is an imaginary line on the ground surface joining points of equal elevation

or a line on which every point is at the same level above or below a chosen reference

surface. In most maps, the reference surface is sea level. This line on the map

represents a contour and is called contour line.

Therefore, a map showing contour lines is known as Contour map. Contour maps

are one of the most effective means of displaying information about the geologic

structure (i.e. the degree of buckling and faulting of the layers) of an area. A contour

map gives an idea of the altitudes of the surface features as well as their relative

positions in the plan. A map showing structure contours for a certain rock layer

throughout an area is called a structure contour map (Fig. 3.1). Such maps are used

to illustrate the size, shape and location of geologic structures.

Contour lines are drawn as fine and smooth freehand curved lines. Sometimes

they are represented by broken lines. They are inked in either in black or brown

colour. A drawing pen gives a better line than a writing pen and French curves

should be used as much as possible. Every fifth contour is made thicker than the rest.

The elevation of contours must be written in a uniform manner, either on the

higher side or in a gap left in the line. When the contour lines are very long, their

elevations are written at two or three places along the contour. In the case of small￾scale maps, it is sufficient to figure every fifth contour. Therefore, the constant

vertical distance between two consecutive contours is called the contour interval.

The contour interval is constant between the consecutive contours

Methods of Contouring

There are basically two main methods of locating contours; these are the Direct

Method and Indirect Method.

Direct Method

This method requires a lot of time to be invested in searching for points of the same

elevation on the ground surface. This implies that it is very slow and tedious but it is

the most accurate method of contouring, thus suitable for small area and where great

accuracy is required. In this method, the contours to be located are directly traced out

in the field by locating and marking a number of points on each contour. These

points are then surveyed and plotted on plan and the contours drawn through them

(Fig. 3.2).

For a radial line, temporary benchmarks are first established at the centre and near

the ends of the radial lines. The contour points are then located and marked on these

lines and their positions are determined by measuring their distances along the radial

lines. They are then plotted on the plan and the contours drawn by joining all the

corresponding points with the help of a plane table instrument (Fig. 3.3).

3.3.1.2 Indirect Method

In this method, the points located and surveyed are not necessarily on the contour

lines but the spot levels are taken along the series of lines laid out over the area. The

spot levels of the several representative points representing hills, depressions, ridge

and valley lines and the changes in the slope all over the area to be contoured are also

observed. Their positions are then plotted on the plan and the contours drawn by

interpolation. This method of contouring is also known as contouring by spot levels.

Conversion from Planimeter Unit to Field Unit

For a map scale of 1:10,000