SSC JE Civil Engineering 30-Oct-2020 Shift 1 Paper

Question : 141 of 200

Marks: +1, -0

Choose the correct option from among the following with respect to the given statements.
Statement (i): The effect of the curvature of the earth is to cause an object to appear lower.
Statement (ii): The effect of refraction is to cause an object to appear higher.

Only (i) is correct
Both (i) and (ii) are correct
Only (ii) is correct
(i) is correct and (ii) is incorrect

Validate

Solution:

The level line is a curved line and the line of collimation is a horizontal line that is tangential to the level line. Therefore, in case of long sights, horizontal line is not a level line and hence levels obtained by means of leveling instrument, arehave to be corrected for the curvature of earth.

Phenomena	Staff Reading	Nature of Correction	Reduced Level
Refraction	Decrease	Positive	Increases ⇒ object will appear higher
Earth’s Curvature	Increase	Negative	Decrease ⇒ Object will appear lower

Due to earth refraction, the line of sight bends towards the earth, therefore we noted less value of staff of reading i.e. Decrease in staff reading. To understand this refers the following figure:

\SR_1

= Staff Reading without refraction

\SR_2

= Staff Reading with Refraction
It is clear above that:

\SR_2 < \SR_1

⇒ Staff reading reduced due to refraction

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