Theodolite has many parts which need to be adjusted every time while surveying. It is important to know about theodolite parts and their functions before using them 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 180o in a 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
Telescope
A telescope is a focusing instrument that has an object piece at one end and an eyepiece at the other end. It rotates about the horizontal axis in the vertical plane. The graduations are up to an accuracy of 20’.
Vertical Circle
A vertical circle is fitted to a telescope and moves simultaneously with a telescope. It has graduation in each quadrant numbered from 0 to 90degrees.
Index Frame
It is also called a t-frame or vernier frame. It consists of two arms vertical and horizontal. The 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 that support the telescope and allow it to rotate about a vertical axis. Generally, these are in letter A-shape. So, standards are also called A-frame.
The Upper Plate
This is also called a vernier plate. The top surface of the upper plate gives support to the standards. It also consists of an upper clamping screw with respect to tangents screw which helps to fix 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 the upper tangent screw. The upper [late also consists of two verniers with magnifiers which are arranged diagonally. It is attached tow the inner spindle.
The Lower Plate
This is also called a 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 the lower clamp screw is loosened and the 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 tribrach plates. The upper one is known as the 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 a lower tribratch plate and is attached to the tripod stand.
The Shifting Head
The shifting head also contains two parallel plates which are moved one over the other within a 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 an 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 an 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 that helps to attach the theodolite by tribratch plate in a fixed position.
Plumb Bob
A Plumb bob is a tool having a cone-shaped weight attached to a long thread. The weight is hanged using thread from the center of the tripod stand and centering of theodolite is done.
Magnetic Compass
Simpler theodolites may contain a circular compass box in the center of the upper plate. When we select north as a reference meridian it will be useful.