Short Answer:

The methods of contouring are the different ways used to collect ground elevation data and draw contour lines. The two main methods are the direct method and the indirect method. In the direct method, contour points are located and connected directly on the field. In the indirect method, elevations are first measured at regular intervals and contours are drawn later.

The direct method gives accurate results but is slow, while the indirect method is faster and more commonly used for large areas. These methods help prepare contour maps for construction, drainage, and land planning.

Detailed Explanation:

Methods of contouring

Contouring is an important part of land surveying that helps in representing the elevation and shape of land surfaces using contour lines. The goal is to find points of the same elevation and draw lines through them. This process can be done using two main methods of contouring: the direct method and the indirect method. Each method has its own steps, tools, and applications depending on the required accuracy, time, and area covered.

1. Direct Method

In this method, the surveyor goes into the field and directly locates points that lie on a specific contour level. These points are marked and then joined to create contour lines.

Steps involved:

Select the contour interval.
Use leveling instruments (like a dumpy level or total station) to identify points that lie at the desired elevation.
Mark and record these points.
Connect the points of the same elevation to form the contour lines.

Advantages:

Very accurate because actual contour points are recorded on the field.
Useful for small areas and important construction sites.

Disadvantages:

Time-consuming and requires more labor.
Not practical for large or rough terrains.

2. Indirect Method

This method is commonly used because it is faster. Instead of finding exact contour points in the field, the surveyor takes spot levels on a grid or cross-section, and contour lines are later drawn by estimating in between values.

Types of Indirect Method:

Grid Method:

The area is divided into squares or rectangles of known size.
Elevations are taken at each grid corner.
Contours are drawn by interpolating between points.

Cross-Section Method:

Sections are taken at regular intervals along straight lines.
Levels are measured along each line.
Suitable for road or canal surveys.

Radial Line Method:

Used when surveying from a central point.
Lines are drawn out like spokes of a wheel.
Elevations are measured at intervals along each line.

Advantages:

Faster and more efficient, especially for large areas.
Requires fewer personnel and is easier to manage.

Disadvantages:

Less accurate compared to the direct method.
Requires careful interpolation to avoid errors.

When to use which method:

Direct method: For small areas where high accuracy is needed (like building sites).
Indirect method: For large, open areas like farmland, valleys, or town planning.

Conclusion

The methods of contouring include the direct and indirect approaches, each suitable for different types of surveying work. The direct method gives high accuracy but takes more time, while the indirect method is faster and more commonly used. Understanding both methods helps in choosing the right approach for drawing reliable and useful contour maps in civil engineering.