Levelling in Surveying
Levelling is an important part of surveying used to determine the height of different points on the earth’s surface. It helps in finding the difference in elevation between points and is widely used in construction projects such as roads, buildings, canals, and drainage systems. In ITI Draughtsman Civil, levelling is a fundamental topic for understanding ground profiles and preparing accurate drawings.
1. Definition of Levelling
Levelling is the process of measuring vertical distances of points with respect to a reference level, usually known as the mean sea level (MSL). It is used to determine the elevation or reduced level (RL) of various points.
2. Objectives of Levelling
- To determine the difference in elevation between points.
- To establish points at a required height.
- To prepare contour maps.
- To design roads, railways, and drainage systems.
- To calculate earthwork quantities.
3. Terms Used in Levelling
- Level Surface: A curved surface parallel to the earth’s surface.
- Horizontal Line: A straight line tangential to the level surface.
- Datum: A reference level from which elevations are measured.
- Reduced Level (RL): Height of a point above or below datum.
- Bench Mark (BM): A fixed reference point with known RL.
- Back Sight (BS): First reading taken on a known point.
- Fore Sight (FS): Last reading taken before shifting the instrument.
- Intermediate Sight (IS): Reading taken between BS and FS.
4. Instruments Used in Levelling
- Level Instrument: Used to establish a horizontal line of sight.
- Levelling Staff: A graduated rod used for taking readings.
- Tripod Stand: Supports the level instrument.
5. Types of Levelling
(a) Simple Levelling
- Used to find the difference in elevation between two points.
(b) Differential Levelling
- Used when points are far apart or obstacles exist.
(c) Fly Levelling
- Used to transfer benchmark from one place to another.
(d) Profile Levelling
- Used for preparing longitudinal section of roads and canals.
(e) Cross Section Levelling
- Used to measure levels across a line.
6. Methods of Levelling
(a) Height of Instrument (HI) Method
- Calculates height of instrument first.
- RL = HI – Staff Reading
(b) Rise and Fall Method
- Determines rise or fall between consecutive points.
- More accurate and used for checking errors.
7. Procedure of Levelling
- Set up the level instrument on tripod.
- Level the instrument using leveling screws.
- Take back sight reading on benchmark.
- Take intermediate and foresight readings.
- Record all readings in level book.
- Calculate reduced levels.
8. Errors in Levelling
- Instrumental errors due to faulty equipment.
- Personal errors due to wrong readings.
- Natural errors due to wind, temperature, or refraction.
Precautions
- Ensure proper leveling of instrument.
- Keep staff vertical while taking readings.
- Check readings carefully.
- Use stable ground for setup.
9. Applications of Levelling
- Construction of roads, buildings, and canals.
- Preparation of contour maps.
- Drainage and irrigation planning.
- Land development and layout design.
Role of Draughtsman Civil
A Draughtsman Civil uses levelling data to prepare profiles, sections, and contour maps. Accurate levelling ensures proper design and execution of construction projects.
Conclusion
Levelling is a basic and essential surveying technique used to determine elevations of points. It plays a key role in planning and construction work.
For ITI Draughtsman Civil students, understanding levelling concepts, methods, and instruments is important for accurate surveying and successful project execution.