9. Contouring in Surveying

 

CONTOURING IN SURVEYING

1. Definition

A contour is an imaginary line joining points of equal elevation (RL – Reduced Level) above a given datum (usually Mean Sea Level).

Contouring is the process of locating and plotting these contour lines on a map to represent the relief (shape) of the ground surface.

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2. Purpose of Contouring

Contouring is used to:

• Show topography of land

• Plan roads, railways, canals, buildings

• Find catchment areas

• Decide alignment of engineering projects

• Calculate earthwork (cut and fill)

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3. Important Terms

Term	Meaning
Contour Interval (CI)	Vertical distance between two successive contours
Horizontal Equivalent	Horizontal distance between contours
Index Contour	Thick contour line with RL marked
Datum	Reference level (usually MSL)
Spot Level	Elevation of a single point
Benchmark (BM)	Fixed reference point with known RL

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4. Characteristics of Contour Lines

1. Contours never cross each other (except overhangs).

2. Contours form closed loops.

3. Close spacing → steep slope

4. Wide spacing → gentle slope

5. Even spacing → uniform slope

6. Contours are perpendicular to direction of steepest slope.

7. Contours cross rivers forming a V-shape pointing upstream.

8. Contours cross ridges forming a V-shape pointing downhill.

9. Circular contours:

   • Higher values inside → hill

   • Lower values inside → depression

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5. Methods of Contouring

A) Direct Method

Contours are traced directly in the field by locating points of equal elevation.

Used for:

• Small areas

• High accuracy works

Instruments: Level, staff, tape

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B) Indirect Method

Spot levels are taken and contours are drawn later by interpolation.

Types:

1. Grid method

2. Cross-section method

3. Radial line method

Used for:

• Large areas

• Quick surveys

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6. Interpolation of Contours

Interpolation means finding the position of a contour line between two known points.

Example

If point A = 102 m
Point B = 112 m
Contour interval = 2 m

Contours between A and B:
104, 106, 108, 110

Using proportional division:

$$\text{Distance to contour} = \frac{\text{Required rise}}{\text{Total rise}} \times \text{Distance AB}$$

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7. Numerical Example

Problem

Two points A and B are 20 m apart.
Their reduced levels are:

$$RL_A = 100.0 \text{ m}, \quad RL_B = 110.0 \text{ m}$$

Contour interval = 2 m

Find the positions of contour lines between A and B.

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Step 1: Find total rise

$$= 110 - 100 = 10 \text{ m}$$

Contours to be drawn:

$$102, 104, 106, 108$$

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Step 2: Find distance of each contour from A

Using proportionality:

$$d = \frac{\text{Rise to contour}}{\text{Total rise}} \times AB$$

For 102 m:

$$d = \frac{2}{10} \times 20 = 4 \text{ m}$$

For 104 m:

$$d = \frac{4}{10} \times 20 = 8 \text{ m}$$

For 106 m:

$$d = \frac{6}{10} \times 20 = 12 \text{ m}$$

For 108 m:

$$d = \frac{8}{10} \times 20 = 16 \text{ m}$$

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Final Answer

Contours will be located from point A at:

4 m, 8 m, 12 m, and 16 m along line AB.

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8. Uses of Contour Maps in Civil Engineering

• Route selection for:

  • Roads

  • Railways

  • Canals

• Site selection for:

  • Dams

  • Reservoirs

  • Buildings

• Estimating:

  • Earthwork

  • Storage capacity

• Studying:

  • Drainage pattern

  • Watershed area

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9. Advantages of Contouring

• Clear representation of ground

• Easy to understand terrain shape

• Helps in planning & design

• Saves cost in construction

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10. Disadvantages

• Time-consuming

• Needs skill and experience

• Errors in leveling affect whole map