Introduction to Levelling
Levelling is one of the most important operations in surveying used to determine the elevation of points on the earth’s surface. It helps in finding the difference in height between two or more points and is essential for construction, drainage, road alignment, and other engineering works. For ITI Surveyor trainees, understanding levelling is a fundamental requirement.
Definition of Levelling
Levelling is the process of determining the relative heights or elevations of different points with respect to a reference level known as a datum.
Datum
A datum is a reference surface or level from which elevations are measured. Commonly used datum is Mean Sea Level (MSL).
Objective of Levelling
- To determine elevation of points
- To establish points at required heights
- To prepare contour maps
- To assist in construction projects
Types of Levelling
1. Simple Levelling
Used to find the difference in elevation between two points.
2. Differential Levelling
Used to determine elevation of distant points using multiple setups.
3. Fly Levelling
Used to transfer benchmark from one place to another quickly.
4. Profile Levelling
Used for longitudinal sections such as roads and railways.
5. Cross-Section Levelling
Used to measure ground levels perpendicular to a line.
Instruments Used in Levelling
- Dumpy Level
- Auto Level
- Levelling Staff
- Tripod Stand
Basic Terms in Levelling
- Benchmark (BM): A fixed reference point of known elevation
- Back Sight (BS): First reading taken on a known point
- Fore Sight (FS): Last reading taken before shifting instrument
- Intermediate Sight (IS): Readings taken between BS and FS
- Reduced Level (RL): Elevation of a point with respect to datum
- Line of Collimation: Line of sight of the instrument
Methods of Levelling
1. Height of Instrument Method (HI Method)
Height of instrument is calculated and RL of points is determined.
2. Rise and Fall Method
Difference in elevation between consecutive points is calculated.
Importance of Levelling
- Used in construction of roads, bridges, and buildings
- Helps in drainage and irrigation planning
- Required for contour mapping
- Essential for land development projects
Precautions in Levelling
- Ensure proper leveling of instrument
- Take accurate staff readings
- Keep instrument stable
- Avoid parallax error
- Record data carefully
Application in Surveyor Trade
Levelling is widely used in surveying for determining heights and preparing plans. It is an essential skill for fieldwork and construction activities.
Conclusion
Levelling is a fundamental surveying process used to determine elevations of points. Understanding its concepts, methods, and instruments is essential for accurate surveying work.
For ITI Surveyor trainees, mastering levelling techniques is important for successful careers in surveying and civil engineering.