Theodolite has many parts which needs to be adjusted every time while surveying. It is important to know about theodolite parts and their functions before using it to minimize errors during theodolite surveying.
Theodolite is an instrument used in surveying to measure horizontal and vertical angles. It is also used for leveling, indirect measure of distances and prolonging a line etc. The line of sight of theodolite can be rotated through 180oin vertical plane about its horizontal axis.

Parts of Theodolite and their Functions

Following are the parts of a theodolite:
  • Telescope
  • Vertical circle
  • Index frame
  • The standards
  • The upper plate
  • The lower plate
  • The leveling head
  • The shifting head
  • Plate level
  • Tripod
  • Plumb bob
  • Magnetic compass
Theodolite Parts and Functions

Telescope

A telescope is a focusing instrument which has object piece at one end and eye piece at the other end. It rotates about horizontal axis in vertical plane. The graduations are up to an accuracy of 20’.

Vertical Circle

Vertical circle is fitted to telescope and moves simultaneously with telescope. It has graduation in each quadrant numbered from 0 to 90degrees.

Index Frame

It is also called as t-frame or vernier frame. It consists two arms vertical and horizontal. Vertical arm helps to lock the telescope at desired level and horizontal arm is useful to take the measurements of vertical angles.

The Standards

The standards are the frames which supports telescope and allow it to rotate about vertical axis. Generally, these are in letter A-shape. So, standards are also called as A-frame.

The Upper Plate

This is also called as vernier plate. The top surface of upper plate gives support to the standards. It also consists an upper clamping screw with respect to tangents screw which helps to fixing it to the lower plate.
When the upper clamping screw is tightened both upper and lower plates are attached and moved together with some relative motion because of upper tangent screw. The upper [late also consists two verniers with magnifiers which are arranged diagonally. It is attached tow inner spindle.

The Lower Plate

This is also called as scale plate. Because it contains a scale on which 0 to 360 readings are graduated. It is attached to the outer spindle and consists lower clamping screw. If lower clamp screw is loosened and upper clamp screw is tightened, both plates can rotate together.
Similarly, if lower clamping screw is tightened and upper clamp is loosened then, only upper plate is movable and lower plate is fixed with tribratch plate.

The Leveling Head

The leveling head contains two parallel triangular plates called as tribratch plates. The upper one is known as upper tribratch plate and is used to level the upper plate and telescope with the help of leveling screws provided at its three ends. The lower one is called as lower tribratch plate and is attached to the tripod stand.

The Shifting Head

Shifting head also contains two parallel plates which are moved one over the other with in small area. Shifting head lies below the lower plate. It is useful to centering the whole instrument over the station.

Plate Level

Plate levels are carried by the upper plate which are right angles to each other with one of them is parallel to trunnion axis. These plate levels help the telescope to settle in exact vertical position.

Tripod

Tripod is nothing but a stand on which theodolite is mounted. It should place in such a way that theodolite should be in exact leveled position. The tripod has legs with steel shoes at their ends. These hold the ground strongly without any movement when placed.
Tripod has an external screw which helps to attach the theodolite by tribratch plate in fixed position.

Plumb Bob

Plumb bob is tool having a cone shaped weight attached to a long thread. The weight is hanged using thread from the center of tripod stand and centering of theodolite is done.

Magnetic Compass

Simpler theodolites may contain circular compass box in the center of upper plate. When we select north as reference meridian it will be useful.