Different Types of Levelling
Levelling is an important process in surveying used to determine the elevation of points relative to a reference datum. Depending on the nature of work and accuracy required, different types of levelling methods are used. For ITI Surveyor trainees, understanding these types is essential for practical fieldwork.
Classification of Levelling
Levelling can be broadly classified into two categories:
- Direct Levelling
- Indirect Levelling
1. Direct Levelling
Direct levelling is the most commonly used method in which elevations are determined directly using a levelling instrument and staff.
Types of Direct Levelling
a) Simple Levelling
Used to find the difference in elevation between two points.
b) Differential Levelling
Used to determine elevation of distant points by taking multiple setups.
c) Fly Levelling
Used to transfer benchmark from one location to another quickly.
d) Profile Levelling
Used to obtain longitudinal section of a ground surface for roads, railways, etc.
e) Cross-Section Levelling
Used to measure ground levels perpendicular to a main line.
f) Reciprocal Levelling
Used when measurements are taken across obstacles like rivers or valleys.
g) Precise Levelling
Used for high-accuracy work with special instruments and procedures.
2. Indirect Levelling
Indirect levelling involves calculating elevations using angles and distances rather than direct measurement.
Types of Indirect Levelling
a) Trigonometric Levelling
Used to determine elevation using vertical angles and distances.
b) Barometric Levelling
Uses atmospheric pressure to determine elevation differences.
c) Stadia Levelling
Uses staff intercepts and instrument constants to calculate distances and elevations.
Comparison of Levelling Types
| Type | Accuracy | Use |
|---|---|---|
| Simple Levelling | Moderate | Short distances |
| Differential Levelling | High | Long distances |
| Fly Levelling | Low | Quick work |
| Profile Levelling | High | Roads, railways |
| Reciprocal Levelling | High | Across obstacles |
| Trigonometric Levelling | Moderate | Long distances |
Importance in Surveyor Trade
- Used in construction projects
- Helps in road and railway alignment
- Required for contour mapping
- Essential for drainage and irrigation planning
Precautions
- Choose appropriate method based on requirement
- Ensure accurate instrument setup
- Take correct staff readings
- Maintain proper records
Conclusion
Different types of levelling are used depending on the purpose and accuracy required. Direct levelling is commonly used for general work, while indirect methods are used for special cases.
For ITI Surveyor trainees, understanding these methods is essential for performing accurate and efficient levelling operations in the field.