Theodolite | Civil Engineering SSC JE (Technical) - Civil Engineering (CE) PDF Download

Surveying (Part 6)

THEODOLITE

Theodolite is a versatile instrument and can be used for the following purposes :

(i) To measure horizontal angle
(ii) To measure vertical angle
(iii) For astronomical observations
(iv) For levelling work
(v) For foundation layout of buildings, boundaries etc.
(vi) To fix alignment of rail, road, electric poles, telephone poles, etc.

CLASSIFICATION OF THEODOLITE :
Theodolite is primarily classified as :

(i) Transit Theodolite
(ii) Non-transit Theodolite

1. Transit Theodolite : A theodolite is called a transit when its telescope can be revolved through a complete revolution about its horizontal axis in a vertical plane. It is mainly used now a days.

2. Non-transit Theodolite : A theodolite is called a non-transit when its telescope cannot be revolved through a complete revolution about its horizontal axis in a vertical plane.

Theodolie are also classified as :

(i) Vernier theodolites
(ii) Glass arc theodolite

i) Vernier Theodolite : In this type of theodolites, vernieres are provided for reading horizontal and vertical graduated circles.

(ii) Glass are theodolite : In this type of theodolites, micrometers are provided for reading horizontal and vertical graduated circles.

PARTS OF TRANSIT THEODOLITE :
A transit theodolite consists of following essential parts.

(i) Levelling head
(ii) Lower plate (or scale plate)
(iii) Upper plate (or Vernier plate)
(iv) The standards (or A Frame)
(v) T-frame or Index-Bar
(vi) Plate levels
(vii) Telescope
(viii) Vertical Circle
(ix) Tripod
(x) The plumb bob

(i) Levelling head: It consists of a tribarch on a trivet plate with three arms, each carrying a level screw or foot screw.
The three distinct functions of a levelling head are
(a) To support the main part of the instrument.
(b) To attach the theodolite to the tripod.
(c) To provide a means for levelling the theodolite

(ii) Lower Plate : The lower plate is attached to the outer spindal. It is also called the scale plans, having its levelled. The edge is silvered and graduated from 0° to 360° in a clockwise- direction. This scale is a1so called horizontal circle. The size of theodolite is determined by the size of the diameter of the lower plate.

(iii) Upper Plate : It is also called the vernier plate and is attached to the inner axis.

(iv) The Standards : A frames stand upon the upper plate to support the horizontal axis are called standards.

(v) Telescope : The telescope is rigidly fixed but the center and at the right angles to the horizontal axis.

(vi) Vertical Circle : It is rigidly attached to the telescope and moves with it.

(vii) Plumb bob : A Plumb bob is suspended from the hook fitted to the bottom of the main vertical axis to centre the theodolite exactly over the ground station work.

Definitions and other Technical Terms : Following terms are used while making observations with a theodolite.

1. VERTICAL AXIS : The axis about which the theodolite, may be rotated in a horizontal plane, is called vertical axis. Both upper and lower plates may be rotated about vertical axis.

2. HORIZONTAL AXIS (or Trunnmior or Transverse axis) : The axis about which the telescope along with the vertical circle of a theodolite, may be rotated in vertical plane is called horizontal axis.

3. LINE OF COLLIMATION: The line which passes through the intersection of the cross hairs of the eye-piece and optical centre of the objective and its continuation, is called line of collimation.

4. AXIS OF THE TELESCOPE : The axis about which the telescope may be rotated is called the axis of telesope.

5. AXIS OF THE LEVEL TUBE : The axis of the level or bubble tube is the straight’ tangential to the longitudinal curve of the level tube at the centre of the tube. It is called the bubble line

6. CENTERING : It means setting up the theodolite exactly over a station mark. It can done by means of plumb bob.

7. TRANSITING (Plunging or Reversing): The process of turning the telescope in vertical plane through 180° about its horizontal axis, is known as transisting.

8. SWING: It means turning the telescope in a horizontal plane. If the telescope is rotated in clockwise direction it is known as right swing and if the telescope is rotated in at the anti clock wise direction, it is known as the left swing.

9. FACE LEFT OBSERVATIONS : When the vertical circle is at the left of the telescope at the time of observations, observations of the angles, are known as face left observation

10. FACE RIGHT OBSERVATIONS : When the vertical circle on the right of the telescope at the time of observation observations of the angles, are known as face right observation

11. CHANGING FACE : It is the operation of changing the face of the telescope from left  right and viceversa.

12. A ‘MEASURE’: It is the determination of the number degrees, minutes and seconds. grades contained in an angle.

13. A ‘SET’: A ‘set’ of horizontal observation of any angle consists of two horizontal measure i, one on the left to and other on the face right.

14. TELESCOPE NORMAL : A telescope is said to be normal when its vertical circle is :: its left and the bubble of the telescope is up.

15. TELESCOPE INVERTED : A telescope is said to be inverted or reversed when  verticle circle is to its right and the bubble of the telescope is down.