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
- Stadia method: In this method diaphragm of the tachometer are provided with two stadia hair .the difference between staff intercept gives stadia reading.
- 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
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.