Types of Theodolite, Surveying and Levelling | Surveying and Levelling Notes- Agricultural Engg - Agricultural Engineering PDF Download

TYPE OF THEODOLITE
In general, theodolite is divided into three types based on angles, which are vernier, optical and electronic.

1.  Vernier Theodolite
   • Uses vernier scale
2.  Optical Theodolite
   • Uses optical with horizontal and vertical circles made from transparent glasses and graduated scale
3.  Electronic Theodolite
   • Has a screen with digits for angles on front and back of the instrument.
   • The face of the current observation (telescope position) is the side on which the vertical circle is, when viewed from the eyepiece, which is either face left or face right
   • The telescope has its own clamp and tangent screws. (The clamp screws require only finger tip pressure)
4. Basically Transit Theodolite are those in which the telescope can revolve through a complete revolution about its Horizontal axis in vertical plane.
5. Components of Transit theodolite
   Transit theodolite consists of the following parts :
   1. Levelling Head
   2. Lower Plate or Scale Plate
   3. Upper Plate or Vernier Plate
   4. The standard or A Frame
   5. T-Frame or Index Bar.
   6. Plate Levels
   7. Telescope

1. Levelling Head -  Levelling Head consists of upper tribrach and lower Tribrach.  Upper tribrach has three arms, each arm carries a levelling screw for levelling the equipment.  Lower tribrach has got a circular hole through which a plumb bob may be suspended for centering.
Three distinct functions of levelling head are:
 i) to support the main part of the instrument
ii) to attach the Theodolite to the Tripod
iii) to provide a means for levelling the theodolite

2. Lower Plate (Scale Plate) :  Lower Plate which is attached to outer spindle, carries a horizonta graduated circle, it is graduated from 0-360.  Each degree is further divided into 10 minutes or 20 minutes.  Scale plate can be clamped to any position by a clamping screw and a corresponding slow motion screw.
When the lower plate is tightened, the lower plate is fixed to the upper tribrach of the levelling head.  The size of the Theodolite is determined by the size of the diameter of this lower plate.

3. Upper plate or Vernier Plate :  Upper plate is attached to Inner spindle axis.  Two verniers are screwed to the upper plats.  It carries an upper clamp screw and tangent screw.  On clamping the upper clamp and unclamping the lower clamp, the instrument may be rotated on its outer spindle without any relative motion between the two plates.
            On the other hand if lower clamp screw is tightened and upper clamp screw is unclamped, the instrument may be rotated about its inner spindle with a relative motion between the vernier and graduated scale of the lower plate.  This property is utilised for measuring angles.

4. Plate Levels -  Upper plates carries two plate levels placed at right angles to each other.  One of the plate bubble is kept parallel to the trunion axis.  Plate levels can be centred with the help of foot screws.

5. Telescope – Telescope is supported on the pivots of the trunion axis which affords its movement in the vertical plane.