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

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.

By subtracting fore sight value from height of instrument, value of reduced level for next set of reading can be obtained.

Correction for refraction is correction for curvature/7. Therefore, here (0.07849 * (2.48)²)/7 = 0.069 m.

Levelling is a branch of surveying objects of which is to establish points at a given elevation or at different elevations with respect to given or assumed datum. It deals with measurements in a vertical plane.

The correction difference in elevation between any two points is given by taking a difference of correct staff reading. Therefore, here Ha ? Hb.

A barometric levelling makes use of phenomenon that difference in elevation between two points is proportional to difference in atmospheric pressures at se points. A barometer, refore, may be used and readings observed at different points would yield a measure of relative elevations of those points.

Due to refraction, ray of light bends downwards in form of a curve with its concavity towards earth surface, thus decreases staff readings.

The establishment of a line of sight, refore, involves following two essential conditions, real image must be formed in front of eyepiece, plane of image must coincide with that of cross hairs.

When observations are taken from P apparent difference in elevation between P and Q is 2.748 ? 1.824 = 0.924. When observations are taken from Q apparent difference in elevation between P and Q is 1.606 ? 0.928 = 0.678. Hence true difference in elevation is (0.924 +0.678)/2 = 0.801 mts. Error in observation = 0.924 ? 0.801= 0.123 m. Error due to curvature and refraction is 0.069 mts. Therefore error in collimation is 0.123 + 0.069 = 0.192 m.