Levelling – Level & Horizontal Surface, Datum, Benchmark, Focusing & Parallax
In levelling, understanding basic concepts and terms is essential for accurate measurement of elevations. These concepts form the foundation of surveying practices. In ITI Draughtsman Civil, students must clearly understand level surfaces, horizontal surfaces, datum, benchmarks, focusing, and parallax.
1. Level Surface
A level surface is a curved surface that is parallel to the mean surface of the earth. Every point on a level surface is at the same elevation.
- It follows the curvature of the earth.
- Used as a reference for measuring elevations.
- Examples include the surface of still water.
2. Horizontal Surface
A horizontal surface is a flat plane that is tangential to the level surface at a given point.
- It is perpendicular to the direction of gravity.
- Used for instrument setup and measurements.
- Unlike level surface, it is a straight plane.
Difference Between Level and Horizontal Surface
| Level Surface | Horizontal Surface |
|---|---|
| Curved surface. | Flat plane. |
| Parallel to earth’s surface. | Tangent to level surface. |
| Used for elevation reference. | Used for measurement and setup. |
3. Datum
Datum is a reference level or surface from which the elevations of points are measured.
- Usually taken as mean sea level (MSL).
- All reduced levels (RL) are calculated with respect to datum.
- Can be assumed for small projects.
4. Benchmark (BM)
A benchmark is a fixed reference point with a known elevation above or below the datum.
Types of Benchmarks
- GTS Benchmark: Established by survey departments with high accuracy.
- Permanent Benchmark: Fixed on permanent structures like buildings.
- Temporary Benchmark: Used for short-term projects.
- Arbitrary Benchmark: Assumed value for convenience.
5. Focusing
Focusing is the process of adjusting the telescope of the levelling instrument to obtain a clear image of the object and crosshairs.
Steps in Focusing
- Adjust the eyepiece to make crosshairs sharp.
- Focus the objective lens on the levelling staff.
- Ensure the image is clear and well-defined.
6. Parallax
Parallax is the apparent shift of the image of an object with respect to the crosshairs when the observer’s eye is moved.
Causes of Parallax
- Improper focusing of eyepiece.
- Improper focusing of objective lens.
Elimination of Parallax
- Focus eyepiece correctly.
- Adjust objective lens until image is sharp.
- Check by moving the eye—no movement of image should be observed.
7. Importance of These Concepts
- Ensures accurate levelling and measurement.
- Helps in proper setup of instruments.
- Reduces errors in surveying work.
- Improves reliability of data.
Role of Draughtsman Civil
A Draughtsman Civil uses levelling data to prepare accurate plans, profiles, and sections. Understanding these basic concepts helps in correct interpretation of survey data.
Conclusion
Concepts such as level surface, horizontal surface, datum, benchmark, focusing, and parallax are fundamental in levelling. Proper understanding ensures accurate measurements and reliable results.
For ITI Draughtsman Civil students, mastering these basics is essential for successful surveying and construction work.