**Contents**hide

**What is Tacheometric Surveying?**

Tacheometric surveying is a branch of angular surveying in which the vertical and horizontal distances are achieved by optical means.

Optical means is done with the help of two special kinds of instruments such as stadia rod and transit theodolite.

On the other hand, traditional surveying methods such as traverse surveying or chain surveying require the surveyor to take linear measurements on the site by a chain or a tape. These are relatively time consuming processes and also tiresome.

**Tacheometric Surveying Instruments**

Tacheometry is done with the help of Stadia rod and Tacheometer. The Instruments used in Tacheometric surveying are given below.

- Tacheometer
- Stadia Rod
- Anallatic Lens

**Tacheometer**

Tacheometer is defined as a special diaphragm which is fitted in a transit theodolite. Tacheometer is a major instrument of Tacheometric surveying. Telescope of the Tacheometer contains horizontal hairs (is known as stadia hair) as well as the regular crosshair. The types of stadia diaphragms are given below.

In general, the telescopes used in Tacheometric surveying are three types given below.

- Stadia theodolite – simple external focusing telescope
- Tacheometer – external focusing anallatic telescope
- The internal focusing telescope

**Features of Tacheometer **

The features of the Tacheometer are given below.

- The multiplying constant should have a value of 100.
- The value of the additive constant should be zero.
- The telescope should have strong magnification properties.
- The axial horizontal line should be equal-distant from the lower and upper stadia hairs.

**Stadia Rod**

Up to 100 metres distance (small distance), a level staff may be used for Tacheometric surveying. But for more distances a stadia rod is required. Stadia rod having 3m to 5m length. The smallest division is generally 5 mm.

**Anallatic Lens**

Anallatic lens is an additional and special lens which is located between the eyepiece and the object glass of the telescope in order to eliminate the additive constant (f+d). This is done to make the declaration for the distance between levelling staff position and instrument station more simplified. The lens is only located in an external focusing telescope.

**Different Methods of Tacheometric Surveying**

The different methods of Tacheometric Surveying are given below.

- Stadia Rod Method A. Fixed Hair Method B. Movable Hair Method or Subtense Method
- The Tangential Method

The brief description of the Stadia Method is given below.

**Stadia Method**

In tacheometry, ‘Stadia Method’ is the most widely used method so we will describe the principle of Stadia Method.

**Principle of Stadia Method **

The Principle of Stadia Method is the same as the Principle of Isosceles Triangles ‘the ratio of the perpendicular to base is constant’.

In the above figure, the central ray is shown as OZ. Here 2 rays are equally inclined to the central ray. X_{1}Y_{1}, X_{2}Y_{2} and XY are staff intercepts i.e. difference between upper and lower stadia reading.

Apparently, OZ_{2}/X_{2}Y_{2} = OZ_{1}/X_{1}Y_{1} = OZ/XY = constant K = 0.5cot(𝞪/2)

This constant depends on the angle 𝞪. Let, the constant K is to be 100. It means the distance between the point O and the staff will be 100 times the staff intercept.

**Errors in Tacheometric Surveying**

There are three types of errors occurring in Tacheometric surveying.

- Instrumental Errors
- Sighting Errors
- Natural Errors

**Instrumental Errors**

Instrumental errors occur due to faulty modification in the tacheometer or inexact graduations on the stadia rod.

**Sighting Errors**

Sighting errors occur due to faulty centering and levelling of instruments as well as inaccurate stadia rod reading.

**Natural Errors**

Natural errors occur due to high wind velocity, expansion of instruments due to temperature and unequal refraction.

**Advantages of Tacheometric Surveying**

The advantages of Tacheometric surveying are given below.

- Speed of surveying is quick.
- As compared to chain and tape surveys, Tacheometric surveying is less time consuming.
- Cost of Tacheometric surveying is low.
- Accuracy of Tacheometric surveying is good for compilation of topographic maps and hydrographic surveys etc.

**FAQs**

#### What is a tacheometric survey?

Tacheometric surveying is a branch of angular surveying in which the vertical and horizontal distances are achieved by optical means. Optical means is done with the help of two special kinds of instruments such as stadia rod and transit theodolite.

#### What is the principle of tacheometric surveying?

Principle of Tacheometric Surveying is the same as the Principle of Isosceles Triangles ‘the ratio of the perpendicular to base is constant’.

#### What are the common errors in tacheometric surveys?

There are three types of errors occurring in Tacheometric surveying. 1. Instrumental Errors 2. Sighting Errors 3. Natural Errors

#### What are the advantages of tacheometric surveying?

The advantages of Tacheometric surveying are given below. 1. Speed of surveying is quick. 2. As compared to chain and tape surveys, Tacheometric surveying is less time consuming. 3. Cost of Tacheometric surveying is low. 4. Accuracy of Tacheometric surveying is good for compilation of topographic maps and hydrographic surveys etc.

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