Introduction to Contours, Surveying and Levelling | Surveying and Levelling Notes- Agricultural Engg - Agricultural Engineering PDF Download

Definition:

A line joining points of equal elevations is called a contour line. It facilitates depiction of the relief of terrain in a two dimensional plan or map.

Characteristics of contour:

The principal characteristics of contour lines which help in plotting or reading a contour map are as follows:

1. The variation of vertical distance between any two contour lines is assumed to be uniform.
2. The horizontal distance between any two contour lines indicates the amount of slope and varies inversely on the amount of slope. Thus, contours are spaced equally for uniform slope
3. The steepest slope of terrain at any point on a contour is represented along the normal of the contour at that point. They are perpendicular to ridge and valley lines where they cross such lines.
4. Contours do not pass through permanent structures such as buildings.
5. Contours of different elevations cannot cross each other (caves and overhanging cliffs are the exceptions). 
6. Contours of different elevations cannot unite to form one contour (vertical cliff is an exception). 
7. Contour lines cannot begin or end on the plan.
8. A contour line must close itself but need not be necessarily within the limits of the map.
9. A closed contour line on a map represents either depression or hill . A set of ring contours with higher values inside, depicts a hill whereas the lower value inside, depicts a depression (without an outlet).
10. Contours deflect uphill at valley lines and downhill at ridge lines. Contour lines in U-shape cross a ridge and in V-shape cross a valley at right angles. The concavity in contour lines is towards higher ground in the case of ridge and towards lower ground in the case of valley . 
11. Contours do not have sharp turnings.

Contour Interval:  

The vertical distance between any two consecutive contours       is known as a contour interval. For example, if the various   consecutive contours are 100m, 98m,96 m etc., then the contour interval is 2m. This interval depends upon,

• The nature of the ground
• The scale of the map and
• The purpose of  survey

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Contour intervals for flat country are generally small, eg. 0.25m, 0.5m,  0.75 m etc. For a steep slope in hilly area is greater, eg. 5m, 10m,  15 m etc.

Again ,for a small-scale map, the interval may be of 1m,2m,3m etc. and for large scale map,it may be of 0.25m,0.50m,0.75m etc.

It should be remembered that the contour interval for a particular map is constant. 

Contouring:

The process of locating these contour lines on the surface of the earth is known as contouring.