According to Indian standards error in feet for ordinary levelling for location and construction surveys is given as 0.1×?M.

Correction for curvature = \(\frac{d^2}{2R} = \frac{2.4^2}{2*6370}*1000 = 0.452m\) Correction for refraction \( = \frac{1}{7} * C_c = \frac{0.452}{7} = 0.064m.\)

Horizontal distance between axis and staff is D = f1 + d = f*s/i + (f+d) = k.s + c, Where, D=k. s+c is distance equation, c is additive constant of instrument, k is multiplying constant or stadia interval factor.

The above statement is true because it involves simple calculations when compared to Rise and fall method, which can be completed with minimal effort.

The optical principles of surveying telescope are based on fact that all parallel rays of light reaching a convex lens are bent when y leave it in such a manner that y intersect at a common point, called focus and that all rays passing through anor point called optical centre pass through geometric centre of lens without bending.

Turning point is a point on which both minus sight and plus sight are taken on a line of direct levels. It is also called a change point.

Horizontal plane through a point is a plane tangential to level surface at that point. It is, refore, perpendicular to plumb line through point.

When it is necessary to carry a levelling across a river ravine or any obstacle requiring a long site between two points so situated that no place for level can be found from which lens of foresight and backsight will be even approximately equal, special method that is reciprocal levelling must be used. It is used to obtain better accuracy.

Profile levelling is a direct levelling object of which is solely to determine elevations of points at measured intervals along a given line. It is done in order to obtain a profile of surface along that line.

According to Indian standards, error in feet for precise levelling for benchmarks of widely distributed points is given as 0.017×?M.