Theodolite and its components and functions

Theodolite: - It is an instrument used for measuring horizontal and vertical angles in surveying. This is very important instrument in surveying and can be used in large survey works.

There are basically two types of theodolite:-

Transit theodolite: - Transit theodolite is one which telescope revolve through 180° in a vertical plane about horizontal axis. (It means revolving in direction of Y axis) Refer below image. 

Non-Transit theodolite: - When its telescope cannot be rotated through 180° in a vertical plane about horizontal axis. These types are obsolete in these days. These kind of theodolite can be classified in two types:-

1. 1.      Vernier theodolite: - In this theodolite vernier are used for taking the readings. Least count of theodolite is 20 second
   2.          Optical theodolite:- It is fitted with optical system by which we can read both horizontal and vertical angles. Least count of these theodolite is 1 second.
   
   
   Note:- Size of theodolite is measured from the size of its lower plate.

   MAINS PARTS OF THEODOLITE:-

   

         1) Telescope: - It is used for sighting the object and mounted on the trunnion axis or horizontal axis or horizontal spindle. As you can see in the above picture item number 1(telescope) is mounted with item number 2(Trunnion axis).

•     This telescope can be rotate in vertical plane about horizontal axis for sighting the object.
•     Telescope is majorly internal focusing type

•       As we can see in telescope image an additional double concave lens is fitted inside the small tube which can move forward and backward between diaphragm and objective with the help of focusing screw.
•       By moving the focusing lens, the image of the object formed by the objective is brought in the plane of cross hairs of the diaphragm.
•      Internal focusing type prevent any dust to enter into the telescope.

 2) Vernier Frame: - It is also called Index frame or T frame. It consists of a vertical leg called clipping          arm. And a horizontal bar called index frame. As we can see in the below image that at the two  end of index frame, Vernier C and Vernier D are fitted. Its vertical leg is called T frame provided with fork and screw at its lower end.

        3) Vertical clamp screw: - It is used to clamp the vertical circle and telescope at any desired vertical angle and rotation of telescope about horizontal axis is prevented. We can use vertical tangent screw after tightening this clamp so that small rotation can be given for proper sighting.

      4) Altitude bubble: - Altitude bubble is attached to index arm. It is used to make the index arm truly horizontal during temporary adjustment. Altitude bubble is more sensitive than plate bubble because while we are taking vertical angle, altitude bubble will be at center

       5) Upper plate: - It is also called lower plate it supports the standard at its upper surface  At its lower face, upper plate is attached to vertical spindle known as the inner spindle .Upper plate can be clamped to the lower plate with the use of upper clamp screw

 

            5) Lower plate: - It is also called main scale plate or horizontal circle. It is mounted on the hollow tapered spindle or outer spindle. Size of theodolite id governed by the size of lower plate

        The lower plate is graduated in degrees from 0 to 360 with least count is 20’. These graduation increase in clockwise direction. The horizontal circle reading are taken by means of Vernier A and B. The least count of Vernier is 20”

            When Both upper and lower clamp screw clamped then there will be no movement in the theodolite.

      When lower clamp screw is tightened but upper clamp screw is lose then instrument will rotates              along upper plate and lower plate will be fixed.

      When lower clamp screw is lose and upper clamp screw is tightened, then both upper and lower             plate will move together.

 6) Plate level: - A level tube called plate level is mounted on upper plate. This bubble is centered with the help of levelling screw.

  In some instruments, there are two plate levels. These plate level are mounted horizontally ta right angle to each other on the upper plate. One of the plate levels is parallel to the trunnion axis and the other is right angle to it.

This plate level are used to make the vertical axis of the instrument truly vertical.

 

These all are the main components of theodolite.