Tangential Method: Tacheometry | Civil Engineering SSC JE (Technical) - Civil Engineering (CE) PDF Download

Surveying (Part 8)

Tangential Method of Tacheometry: In tangential Tacheometery, horizontal and vertical distance from the instrument to the staff position, are computed from the observed vertical angle of two targets fixed at a known distance S on the staff.

DEPENDING UPON THE VERTICAL ANGLES, THREE CASE MAY BE ARISE :

1. Both vertical angles may be elevation angles.
2. Both vertical angle may be depression angles.
3. One of the angle may be an elevation angle and the other may be a depression angle.

Case 1 : Distance and elevation angle :

Let A and B represent two targets fixed S metres apart on a staff held vertical at D.
Let O represent the trunnion axis of the theodolite.

þ V = D tan a₂

þ V - S = D tan a₁

Substracting equation (ii) from equation (i), we get S = D (tan a₂ – tan a₁)

CASE 2. Distance and depression angle

 þ  V = D tana 2

    þ V - S = D tana 1

Substracting equation (ii) from equation (i), we get

S = D (tan a₂– tana₁)

Note : The most common method of tacheometry is a fixed hair stadia method, using a staff held vertically.
 

THE SUBSTANCE BAR : The substance bar is an instrument usd for measuring horizontal distances in the area where direct chaining becomes difficult due to undulations or other obstructions.
Comput

 
(where a is very small) 

(where a is very small)