Derive the expression to find the distance and elevation of the object when the base of the object is inaccessible?

When the base of the object is inaccessible we use an indirect method of surveying.

A tachometer is used to measure the distance between the two points with the use of an analytical lens. There are two methods

1. Stadia method: In this method diaphragm of the tachometer are provided with two stadia hair .the difference between staff intercept gives stadia reading.
2. Tangential Method: in this method, the tachometer is not provided with stadia hairs, the reading is taken by single horizontal hair.

D=(f/L)S + (f + d)

D=horizontal distance between tachometer and stadia rod.

f/L = multiplying constant

S= Staff Intercept

Elevation formula:

v = KSsin(2©)/2 + Ccos(2©)

© = Theta= Angle of elevation or depression.

K & C = Tachometric Constant

substance method and tangential method is also used to find the elevation between two points.