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**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 a_{2}

þ V - S = D tan a_{1}

Substracting equation (ii) from equation (i), we get S = D (tan a_{2} – tan a_{1})

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 a_{2}– tana_{1})

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)

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