Tacheometry is a branch of angular surveying in which the horizontal and vertical distances of points are obtained by instrumental observations.

§ The chaining operation is completely eliminated in such survey.

§ The accuracy of tacheometry is less than that of chaining.

§ The tacheometry is very rapid and convenient.

§ The tacheometry is adopted in rough and difficult terrain (e.g. steep and broken ground, deep ravines, stretches of water or swamp etc.) where ordinary leveling and chaining are either impossible or very difficult.

§ The primary object of tacheometry is the preparation of contoured maps or plans.

§ It is also widely used on hydrographic survey, location survey for roads, railway, reservoirs etc.

§ Under favourable conditions, the error will not exceed 1/1000.

§ It can be used for checking the more precise measurement.

**What are the instruments used in tacheometry?**

The instruments used in tacheometry are –

1. Tachometer

2. Leveling staff or stadia rod.

**Tacheometer**

**An ordinary transit theodolite fitted with a stadia telescope i.e. a telescope fitted with a stadia diaphragm is called ‘Tacheometer’.**

Stadia diaphragm consists of two additional horizontal cross hairs are fixed one above and the other below the middle horizontal hair equally spaced.

The type of stadia diaphragm commonly used are shown in following figure-

**The kinds of telescope fitted in tacheometer are-**

a) The external - focusing telescope

b) The internal- focusing telescope

c) The external – focusing analytics telescope

(I.e. telescope fitted with an anallatic lens)

**CHARACTERISTICS OF TACHE**

**OMETER:-**

Following are the essential characteristics of a tacheometer –

a) The value of multiplying constant (f/i) should be 100.

b) The telescope should be fitted with an anallatic lens to make additive constant (f + d) equal to zero.

c) The telescope should be powerful having a magnification of 20 to 30 diameters.

d) The aperture of the objective should be 35 to 40 mm in diaphragm in order to have a sufficiently bright image.

e) The axial horizontal line should be exactly midway between other two lines.

f) The magnifying power of the eyepiece should be greater

render staff graduation clearer at a long distance.

**LEVELLING STAFF OR STADIA ROD**:-

For small distances (say up to 100m) ordinary levelling staff is used.

For long distances, a stadia rod is used stadia rod is usually of one piece but for easy transport.

It may folding type. It is 10cm to 15cm wide, 1cm in thickness 3 to 4 m in length. The stadia rod used for small distances have least count as 5mm and long distances as 1 cm graduation should be bold and distinct .

The following figure should some common pattern of stadia rods.

**WHAT IS THE PRINCIPLE OF TACHEOMETRY?**

The principle of tacheometry is based on the property of isosceles triangles, where the ratio of the distance of the base from the apex and the length of the base is always constant.

O

_{1 }a_{1}a_{2, }O_{1}b_{1}b_{2, }and_{ }O_{1}c_{1}c_{2}are all isosceles triangles where D_{1, }D_{2 }and D_{3}are the distances of the bases from the apices and S_{1,}S_{2 }and S_{3 }are the length of the bases (staff intercepts).
So, according to the stated principle,

The constant f/i is known as the multiplying constant.

Where,

F = focal length of objective.

i = stadia intercepts.

(Written by: Dipali .P. Pounikar.)

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