Question

The levelling method in which correction for curvature and refraction can be eliminated is

• A. Simple levelling
• B. Compound levelling
• C. Differential levelling
• D. Reciprocal Levelling

Hint:

The principle of "reciprocity" is common in surveying to eliminate errors.
By taking measurements in both the forward and backward directions and averaging them, you cancel out systematic errors that are dependent on the direction or setup.
This is the core idea behind reciprocal levelling.

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
The question asks to identify a specific levelling technique that is designed to cancel out the errors caused by the Earth's curvature and atmospheric refraction.

Step 2: Detailed Explanation:
Errors in levelling are caused by:
1.

Curvature of the Earth: The Earth is round, but the line of sight of a level is a horizontal line.
This causes the reading on the staff to be higher than it should be.
2.

Atmospheric Refraction: The line of sight is bent or refracted as it passes through air of varying density.
This partially counteracts the curvature error.
These combined errors are significant over long distances and are difficult to calculate precisely.

Reciprocal Levelling is a procedure used to find the difference in elevation between two points that are far apart, with an obstacle like a river or valley between them.
The method involves two setups:
- First, the level is set up near point A, and readings are taken on staves at A and B.
- Second, the level is moved and set up near point B, and readings are again taken on staves at A and B.
By taking the average of the two differences in level calculated from these two setups, the systematic errors due to curvature, refraction, and any instrumental collimation error are automatically cancelled out.

Step 3: Final Answer:
The method that eliminates correction for curvature and refraction is Reciprocal Levelling.
This corresponds to option (D).