Technical Terms Used in Levelling
Levelling is an important process in surveying used to determine the elevation of different points on the ground. To understand levelling operations clearly, it is essential to know the technical terms used in levelling. These terms are commonly used during fieldwork and calculations. For ITI Surveyor trainees, knowledge of these terms is fundamental.
1. Level Surface
A level surface is a curved surface that is everywhere perpendicular to the direction of gravity. Example: surface of still water.
2. Horizontal Surface
A horizontal surface is a plane that is tangential to the level surface at a given point.
3. Datum
Datum is a reference level from which elevations are measured. The most common datum is Mean Sea Level (MSL).
4. Elevation or Reduced Level (RL)
Reduced Level is the height of a point above or below the datum.
5. Benchmark (BM)
A benchmark is a fixed reference point of known elevation.
- GTS Benchmark: Established by Survey of India
- Permanent Benchmark: Fixed permanently in a locality
- Temporary Benchmark: Fixed temporarily for project work
- Arbitrary Benchmark: Assumed value for convenience
6. Back Sight (BS)
The first staff reading taken on a known point (usually a benchmark) after setting up the instrument.
7. Fore Sight (FS)
The last staff reading taken before shifting the instrument to another position.
8. Intermediate Sight (IS)
Staff readings taken between back sight and fore sight.
9. Change Point (CP)
A point where both fore sight and back sight readings are taken when the instrument is shifted.
10. Height of Instrument (HI)
The height of the line of sight above the datum.
HI = RL + BS
11. Line of Collimation
An imaginary line passing through the optical center of the telescope and crosshairs.
12. Plane of Collimation
The horizontal plane containing the line of collimation.
13. Rise
When the next point is higher than the previous point, it is called rise.
14. Fall
When the next point is lower than the previous point, it is called fall.
15. Parallax
Parallax is the apparent movement of the image relative to the crosshairs. It must be eliminated by proper focusing.
16. Levelling Staff
A graduated rod used to measure vertical distances.
17. Line of Sight
The line joining the eye of the observer and the object being observed.
18. Turning Point
Another name for change point where instrument is shifted.
Importance of These Terms
- Helps in understanding levelling operations
- Ensures accurate field measurements
- Useful in calculations and plotting
- Forms the base for advanced levelling techniques
Conclusion
Technical terms in levelling are essential for proper understanding and execution of surveying work. These terms are widely used in fieldwork and calculations.
For ITI Surveyor trainees, mastering these terms is important for performing accurate levelling and achieving professional skills in surveying.