Contouring – Direct and Indirect Methods
Contouring is the process of representing ground surface features by means of contour lines. These lines connect points of equal elevation and help in understanding the shape of the land. In ITI Draughtsman Civil, contouring methods are broadly classified into Direct and Indirect methods.
1. Direct Method of Contouring
Definition
In the direct method, contour points are located directly in the field by using levelling instruments. Points having the same reduced level (RL) are identified and marked on the ground.
Procedure
- Set up the levelling instrument at a suitable position.
- Select a contour value (e.g., 100 m).
- Move the staff around the area to find points with the same RL.
- Mark these points on the ground.
- Join the points to form contour lines.
Advantages
- Highly accurate method.
- Contours are obtained directly on the ground.
Disadvantages
- Time-consuming and labor-intensive.
- Costly method.
- Not suitable for large areas.
Uses
- Small areas requiring high accuracy.
- Detailed engineering works.
2. Indirect Method of Contouring
Definition
In the indirect method, levels of selected points are measured first, and contour lines are drawn later by interpolation on the map.
Procedure
- Select points over the area at regular intervals.
- Determine the reduced levels (RL) of these points.
- Plot these points on a map.
- Draw contour lines by interpolation.
Advantages
- Faster and more economical.
- Suitable for large areas.
- Less fieldwork required.
Disadvantages
- Less accurate compared to direct method.
- Requires careful interpolation.
Types of Indirect Method
(a) Grid Method
- Area is divided into squares.
- Levels are taken at grid intersections.
(b) Cross-Section Method
- Levels are taken along lines at regular intervals.
- Used for roads and canals.
(c) Radial Line Method
- Levels are taken along lines radiating from a central point.
- Suitable for hilly areas.
3. Comparison Between Direct and Indirect Methods
| Direct Method | Indirect Method |
|---|---|
| Contours located directly in field. | Contours drawn after field measurements. |
| Highly accurate. | Less accurate. |
| Time-consuming. | Faster and economical. |
| Suitable for small areas. | Suitable for large areas. |
4. Importance of Methods
- Helps in selecting suitable method based on area and accuracy.
- Improves efficiency in survey work.
- Ensures proper representation of terrain.
Role of Draughtsman Civil
A Draughtsman Civil uses contour data to prepare maps and plans. Understanding both methods helps in accurate plotting and design of engineering projects.
Conclusion
Contouring methods are essential for representing ground features. The direct method provides high accuracy, while the indirect method is faster and more practical for large areas.
For ITI Draughtsman Civil students, understanding both methods is important for efficient surveying and construction planning.