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:
SR1 = Staff Reading without refraction SR2 = Staff Reading with Refraction It is clear above that: SR2<SR1 ⇒ Staff reading reduced due to refraction