contouring

CONTOURING

Objectives:
1. To plot the contour of the given field

Instruments required:
1. Transit Theodolite
2. Staff
3. Ranging rods
4. Pegs and hammer
5. Tape
6. Plumb bob

Theory
Introduction
The value of plan or map is highly enhanced if the relative position of the points is represented both horizontally as well as vertically. Such maps are known as topographic maps. Thus, in a topographic survey, both horizontal as well as vertical controls are required. On a plan, the relative altitudes of the points can be represented by shading, hachures, form lines or contour lines. Out of these, contour lines are most widely used because they indicate the elevations directly.

Contour
A contour is an imaginary line on the ground joining the points of equal elevations. It is a line in which the surface of the ground is intersected by a level surface. A contour line is a line on the map representing a contour.
A topographic map presents a clear picture of the surface of the ground. If a map is to a big scale, it shows where the ground is nearly level, where it is slopping, where the slopes are steep and where they are gradual. If a map is to a small scale, it shows the flat country, the hills and valleys, the lakes and water courses and other topographic features.

Contour interval
The vertical distance between any two consecutive contours is called contour interval. The contour interval is kept constant for a contour plan, otherwise the general appearance of the map will be misleading. The horizontal distance between two points on two consecutive contours is known as the horizontal equivalent and depends upon the steepness of the ground. The choice of proper contour interval depends upon the following considerations:
1. The nature of the ground
The contour interval depends up on whether the contour is flat or highly undulated. A contour interval chosen for a flat ground will be highly unsuitable for undulated ground. For every undulated ground, a small interval is necessary. If the ground is more broken, greater contour interval should be adopted, otherwise the contours will come too close to each other.
2. The scale of the map:
The contour interval should be inversely proportional to the scale. If the scale is small, the contour interval should be large. If the scale is large, the contour interval should be small.
3. The purpose and extent of the survey:
The contour interval largely depends upon the purpose and extent of the survey. For example, if the survey is intended for detailed design work or for accurate earth work calculations, small contour interval is to be used. The extent of survey in such cases will generally be small. In the case of location surveys, for lines of communications and for reservoir and drainage areas, where the extent of survey is large, a large contour interval is to be used.
4. Time and expense of field and office work:
If the time available is less, greater contour interval should be used. If the contour interval is small, greater time will be taken in the field survey, in reduction and in plotting the map.

Characteristics of contours:
The following characteristic features may be used while plotting or reading a contour plan: