Shantilal shah GEC,bhavnagar

Tacheometric Surveying

Guided By prof.

Tachometry Tachometry is a branch of surveying in which

the horizontal distances and the difference in elevations are determined by optical means without the use of a chain or tape.

Tacheometry is also known as tachymetry or telemetry.

The method is more rapid though less accurate as compared with chaining

Situation 1. In rough country, both horizontal and vertical

measurements are tedious and chaining is in accurate, slow and difficult.

2. when obstacles such as steep and broken ground, stretches of water or swamps are net with.

3. In locating contours and filling in detail in a topographic survey, this method is usually the quickest & best.

4. when area to be surveyed is very large and accuracy required is less.

Characteristics 1. horizontal angles and vertical elevations are

measured with tacheometry.2. The horizontal and vertical distance between

two points is calculated from the observations taken by tacheometer.

3. For tacheometric survey, only tacheometer and stadia rod are required.

4. Use of chain is completely eliminated.5. Accuracy is less compared with chaining or

differential levelling.

Applications1. Preparation of topographic maps which require

both horizontal distances and elevations.2. Survey work in difficult terrain where direct

methods are inconvenient.3. Filling details in a traverse.4. location surveys for highways, railways, canals,

etc.5. hydrographic surveys.

Stadia method 1. In this methods, stadia interval is

kept constant.2. In this methods, the staff intercept

on the staff varies depending upon the horizontal distance between the instrument station and the staff.

3. This method is most commonly used in practices as it is convenient to take the staff readings speedily.

4. Tacheometer and stadia rod are used.

1. In this method, the stadia hairs are adjusted by micrometer screws such that upper hair bisects the upper target and the lower hair bisects the lower target.

2. In this method, the staff intercept is kept constant.

3. This method is generally not used, as it is inconvenient to measure the stadia interval.

4. substance theodolite and target staff are used.

Movable hair methods

Fixed hair methods

Both angles are angles of elevation

D = S

(tan ∝1 - tan ∝2)

V = S tan ∝2

(tan ∝1 - tan ∝2)

Both angles are angles of depression

D = S (tan ∝1 + tan ∝2)

V = S tan ∝2

( tan ∝1 + tan ∝2)

Both angles are angles of depression

D = S

(tan ∝2 - tan ∝1)

V = S tan ∝2

(tan ∝2 - tan ∝1)

1. As two vertical angles have to be measured, it takes more time as compared with stadia method.

2. The error will occur if the instrument gets disturbed between the two observations which will cause error.

3. There may be changes in atmospheric refraction in the period between the two observation which will cause error.

4. The readings are not easily reduced to the horizontal distance and vertical intercept.

Disadvantages of tangential method

Error in tacheometry Like in other survey methods, tacheometry can also have the following errors :1. Instrumental errors 2. Personal errors 3. Errors due to natural causes

High degree of accuracy in tacheometric observations can be achieved by : ● Taking proper care in manipulating the instrument.● Limiting the length of sights.● Using instruments of good quality.● working in favourable atmospheric condition.

Standards of precision 1. The error in single horizontal distance(D)

should not exceed 1 in 500.2. The linear error of closure in traversing

can be computed from c√p metres, where c varies from 0.03 to 0.06 and P is the perimeter of the traverse in meters.

3. The permissible closing error in levelling in a bench mark may be taken as 0.003√l metres, where l is the total length traversed.

THANK YOU

STUDENT’SGK