Give Introduction of Contouring

Contouring is the science of representing the vertical dimension of the terrain on a two dimensional map. We can understand contouring by considering a simple example.

Let us assume that a right circular cone of base 5m diameter and vertical height 5m is standing upright on its base. Let the base be resting on a horizontal plane at zero level as shown in Figure 1.
contouring-1

At zero level, the outline of the cone will be a circle of 5m diameter. This circle is the contour line at 0m elevation for the cone. We draw this first contour line on paper to a convenient scale.

Let us now slice the cone at 1m height from the base. This will produce another circular outline corresponding to the diameter of the cone at 1m elevation. Let us draw this second circle on our contour map using the same scale. The second circle being smaller in diameter than the first will appear as a concentric circle within the first circle.

Similarly, we continue to draw the outline of the cone at 2m, 3m, 4m and 5m levels on our contour map. Our contour map for the conical object is now ready. The circles on the map are called contour lines. (see figure-2)
contouring-2

Like the cone in our example, hills project upwards from ground level. The contour map of a hilly terrain will be similar to that of the cone, except that instead of perfect circles, the contour lines would be of
irregular shapes. The important point of similarity to note here is that hilly terrain would be represented by contour lines with increasing elevation towards the centre.

In contrast to this, a pond or depression would be represented by contour lines with decreasing elevation towards the centre.

We at engineeringcivil.com are thankful to Mr Ramasesh Iyer for submitting this useful information to us.

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Explain Terms Used in Contouring

Define Contour Line
A Contour line is an imaginary outline of the terrain obtained by joining its points of equal elevation. In our example of the cone, each circle is a contour line joining points of same level.

Define Contour Interval (CI)
Contour interval is the difference between the levels of consecutive contour lines on a map. The contour interval is a constant in a given map. In our example, the contour interval is 1m.

Define Horizontal Equivalent (HE)
Horizontal equivalent is the horizontal distance between two consecutive contour lines measured to the scale of the map.
contouring-3

Gradient
Gradient represents the ascending or descending slope of the terrain between two consecutive contour lines. The slope or gradient is usually stated in the format 1 in S, where 1 represents the vertical component of the slope and S its corresponding horizontal component measured in the same unit.

The gradient between two consecutive contour lines can also be expressed in terms of Tan Q(theta) as follows:
Tan Q (theta) = CI / HE … both measured in the same unit.

We at engineeringcivil.com are thankful to Mr Ramasesh Iyer for submitting this useful information to us.

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What is the Difference Between Contour Interval and Horizontal Equivalent

There are three main differences between contour interval and horizontal equivalent as follows:

S.No Contour Interval Horizontal Equivalent
1 It is based on vertical levels Represents horizontal distance
2 No measurement or scaling is required since the contour levels are indicated on the contour lines The distance must be measured on the map and converted to actual distance by multiplying with the scale of the map
3 In a given map the contour interval is a constant The horizontal equivalent varies with slope. Closer distance indicates steep slope and wider distance gentle slope

We at engineeringcivil.com are thankful to Mr Ramasesh Iyer for submitting this useful information to us.

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What are the factors governing Selection of Contour Intervals?

The survey leader has to decide an appropriate contour interval for his project before start of survey work. The following factors govern the selection of contour interval for a project:

S.No Factor Select High CI like 1m, 2m, 5m or more Select Low CI like 0.5m, 0.25m, 0.1m or less
1 Nature of ground If the ground has large variation in levels, for instance, hills and ponds If the terrain is fairly level
2 Scale of the map For small scale maps covering a wide area of varying terrain For large scale maps

showing details of a small area

3 Extent of survey For rough topographical map

meant for initial assessment only

For preparation of detailed map for execution of work
4 Time and resources

available

If less time and resources are

available

If more time and resources are available

We at engineeringcivil.com are thankful to Mr Ramasesh Iyer for submitting this useful information to us.

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What are the Characteristics of Contours?

Contours show distinct characteristic features of the terrain as follows:

i) All points on a contour line are of the same elevation.

ii) No two contour lines can meet or cross each other except in the rare case of an overhanging vertical cliff or wall
iii) Closely spaced contour lines indicate steep slope

iv) Widely spaced contour lines indicate gentle slope

v) Equally spaced contour lines indicate uniform slope

vi) Closed contour lines with higher elevation towards the centre indicate hills

vii) Closed contour lines with reducing levels towards the centre indicate pond or other depression.

viii) Contour lines of ridge show higher elevation within the loop of the contours. Contour lines cross ridge at right angles.

ix) Contour lines of valley show reducing elevation within the loop of the contours. Contour lines cross valley at right angles.

x) All contour lines must close either within the map boundary or outside.

We at engineeringcivil.com are thankful to Mr Ramasesh Iyer for submitting this useful information to us.

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