Identify the correct statement(s) about a change point in a levelling operation. Statement I : It is also known as turning point. Statement II : The plus sight is taken on the change point on one set of instrument to ascertain the elevation of the point. Statement III : The minus sight is taken on the change point to establish the new height of the instrument.
Statement I and III only
The question is asking about the correct statements regarding a change point in a levelling operation. Let's evaluate each statement:
- Statement I: It is also known as turning point.
This statement is correct. In levelling operations, a "change point" is commonly referred to as a "turning point." It is a point where the level of the instrument is changed while keeping continuous levelling operations.
- Statement II: The plus sight is taken on the change point on one set of instrument to ascertain the elevation of the point.
This statement is incorrect. At a change point, a plus sight is not taken to ascertain the elevation. Instead, a minus sight is usually taken on the backsight rod (on the CP) to determine the height of the instrument, and the foresight rod reading is used to transfer this height to the next setup.
- Statement III: The minus sight is taken on the change point to establish the new height of the instrument.
This statement is correct. The minus sight (backsight) reading is taken on the change point to establish the new height of the instrument (HI). After setting up the instrument at the new location, the HI can be calculated by adding the backsight reading to the known elevation of the change point.
Thus, the statements that correctly describe a change point in levelling operations are Statement I and III.
Important Questions from Levelling
The expression for sensitivity of the bubble tube (α) can be taken as, ______
Where n = No. of divisions, s = Net staff reading, D = Distance, R = Radius of curvature, l = Length of one division
A vertical line which is perpendicular to the level line is called:
In levelling between two points A and B on the opposite sides of a river, the level was first set up near A and the staff readings on A and B were 2.645 m and 2.30 m respectively. The level was then moved near B and set up; the respective staff readings then were 1.085 m and 1.665 m on A and B respectively. What is the true difference of level between A and B?
A level, when set up $20$ m from peg A and $70$ m from peg B, reads $0.750$ m on a staff held on A and $2.065$ m on a staff held on B, keeping the bubble at its centre while reading. If the reduced levels of A and B are $100.500$ m and $101.800$ m respectively, what is the collimation error per $100.0$ m?
In levelling back, sight is also called as ______.
