Theodolite surveying

Theodolite surveying 

 Centering: 

The setting of a theodolite exactly over a station marks by means of a plumb-bob is known as centering.

Transiting:

The method of turning a telescope about its horizontal axis in a vertical plane through 180° is known as transiting.

. Face left: 

Face left means the vertical circle of the theodolite is on the left of the observer at the time of taking readings.

Face right: 

Face right means the vertical circle of the theodolite is on the right of the observer at the time of taking readings.

. . Telescope normal: 

The face left position is called as Telescope normal. Telescope inverted: The face right position is called as Telescope inverted.

• Changing face:

The operation of bringing the vertical circle from one side of the observer to the other side is known as changing face.

Swinging the telescope:

It indicates the turning of a telescope in a horizontal plane. Adjustment: Adjustment of a theodolite means the operation of tightening or loosening of moveable parts to prepare the instrument for accurate measurement. It also includes other operations meant for this purpose. There are two types of adjustments for a theodolite - Temporary Adjustment & Permanent Adjustment.

Permanent adjustment:

The adjustment of a surveying instrument that is made infrequently and not at each setup.

Temporary Adjustment: 

An adjustment, such as leveling or focusing, made to a surveying instrument at each setup.

Axis of telescope: This axis is an imaginary line passing through the optical centre of the object glass and the optical centre of the eye-piece. .

Axis of bubble-tube: 

This axis is an imaginary line tangential to the longitudinal curve of the bubble tube at the middle point.

Least count of the vernier:

This is the difference between the value of the smallest division of the main scale and that of the smallest division of the vernier scale

Let (n − 1) small divisions of the main scale be divided into n-small divisions in the vernier scale.

 Then, n × v= (n - 1)d

V = (n-1)d/n

Where, v = value of smallest division of vernier scale.

 d = value of smallest division of main scale. 

 So,             Least count =  d- v = d-{(n-1)d/n} = d/n

Closing error:

In a closed traverse the algebraic sum of latitudes must be equal to zero and so should the algebraic sum of departures.

But due to the errors in field measurements of angles and lengths, sometimes the finishing point may not coincide with the starting point of a closed traverse. The distance by which a traverse fails to close is known as closing error.

Closing error = √(ΣL)²+(ΣD)² , 

where L = latitude and D= Departure.

 Components of a transit theodolite

 (1) Trivet 

(2) Foot Screws

 (3) Tribrach

 (4) Levelling head

 (5) Spindles

 (6) Lower plate

 (7) Upper plate 

(8) Plate bublle

 (9) Standard or ‘A’ frame

 (10) The telescope 

(11) Vertical circle

 (12) Index lear or T frame

 (13) Altitude bubble 

(14) Compass.

Temporary adjustment of transit theodolite: 

Setting of the theodolite over a station at the time of taking any observation is called temporary adjustment. This is very essential for every set up of the instrument. This includes the following steps:

(1) Setting the theodolite over the station 

(2) Approximat leveling by tripod stand 

(3) Centring

(4) Levelling

(5) Focussing the eye -- piece

(6) Focussing the object glass

(7) Setting the Vernier.

Swinging the telescope :

This indicates turning of the telescope in a horizontal plane. It is called “right swing 'when the telescope is turned clockwise and ‘left swing 'when the telescope is turned anticlockwise.

Transiting:

Transiting is the method of turning the telescope about its horizontal axis in a uertical plane through 180°.

Changing face:

Changing face is the process of bringing the vertical circle from one side of the observer to the other.

Measurement of horizontal angle:

 There are two methods of measuring horizontal angles

 (1) Repetition Methods

 (2) Reiteration. Procedure: .

(1) Repetition Methods: 

Let suppose it is desire to measure the angle A from the following fig.

Procedure:

* Set up the theodolite at Station A. 

* Bisect the point B with vertical hair of theodolite and move telescope in clockwise direction theodolite at point C.

* Note this circle reading in the book and fix the circle reading fixed. 

* Now release the circle reading and rotate the telescope again in clockwise direction till it bisect again point C. 

* similary get 3rd and 4th Repetition and note the circle reading after 4th Repetition in the book.

* change the face of telescope and repeat the above steps.