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

In rise and fall method, first we must calculate difference between B.S and F.S. We get 1.142 ? 2.121 = -0.979 m, which is negative. It means we have to subtract it from given B.M for obtaining R.L i.e., R.L = 400 ? 0.979 = 399.021 m.

Due to irregular holding of staff, values which are obtained by levelling instrument might lead to decrease in R.L.

Here, V+S = Dtan?1, V = Dtan?2, S = D(tan?1 ? tan?2). Therefore, D = S/(tan?1 ? tan?2).

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 m. Therefore true elevation of Q is 126.386 ? 0.801 = 125.585 m.

The object glass provides a real inverted image in front of eyepiece which, in turn, magnifies image to produce an inverted virtual image.

In isosceles triangle ratio to perpendicular to base is constant. Thus, this method principle is used in stadia method.

From definition of level surface and a horizontal line it is evident that a horizontal line departs from a level surface because of curvature of earth.

The surveyors telescope is an adaptation of Kepler?s telescope. It employs two convex lenses. The one nearest to object is called objective and or near eye is called eyepiece.

Combined correction for curvature and refraction is given by 0.06728 d². Therefore, here 0.06728 (10)² = 6.728 m.