Theodolite

Theodolite

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 180o in 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 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

Working of Vernier Theodolite

Working of a Vernier theodolite starts with setting up and adjustment of theodolite for the measurement of angles.

Temporary Adjustment of Theodolite

Following are the adjustment required for theodolite before using it:

• Setting
• Centering
• Leveling
• Focusing

Setting of Theodolite

Setting means attaching theodolite to the tripod stand. The tripod is unfolded and placed on the ground with all legs in same level. In this operation, we level the tripod by just eye estimation. From the bottom of theodolite, a plumb bob is suspended to set the instrument over station mark.

Centering

Centering is the process in which the vertical axis of the instrument is coincided with the plumb line. The legs of tripod stand should be of equal height on leveled ground and they have cone shape metal shoes which can easily hold the ground without any movement.

 

Leveling of Theodolite

Leveling of theodolite is completed by making the vertical axis of instrument truly vertical. It is done by centering the bubble in the level tube by rotating foot screws provided. The bubbling should be done in all direction of telescope and the centered bubble should not move while taking reading.

Focusing

Focusing is the adjustment of telescope to obtain clear visibility of image through eyepiece and objective lens. This process done by removing parallax error by proper focusing of eyepiece and objective lens.

Measurement of Horizontal Angle using Theodolite

After the temporary adjustment of theodolite, the telescope is located at a point from which we must focus the other point to find horizontal angle between them. Let us say A, B and C are three points on same ground level with some distances as shown in figure below. Now the angle ABC can be determined as below described procedure.

Procedure for Measurement of Horizontal Angle

1. The instrument is located at point B with all temporary adjustments and ranging rods are located at points A and C.
2. Now, set the Vernier A reading to zero and Vernier B reading to 180 by rotating telescope. This should be done by releasing the upper clamp screw and tightening of lower clamp screw.
3. After setting the Vernier, tighten the upper clamp screw and check the reading without any error with the help of upper tangent screw.
4. Whenever the upper clamp is tightly fixed, loosen the lower clamp screw it will allow the telescope to turn without any change in reading. Now focus the ranging rod at location A and bisect it.
5. After bisecting A, release the upper clamp screw and rotate the telescope in clockwise direction towards ranging rod at C and bisect it. Now observe the Vernier reading which will be the horizontal angle between A and C or angle ABC.
6. In the above explained case, the vertical circle is present left side of the telescope, then repeat the same procedure with vertical circle right side of telescope. The average value of angles obtained in face left and face right conditions gives more accurate value of horizontal angle.

Measurement of Vertical Angle using Theodolite

1. After the temporary adjustment of theodolite, set Vernier reading of vertical circle to 0 using clamp screws and focus it to C.
2. Center the bubble using foot screws without any collimation error. Then raise the telescope slowly towards point A as shown in fig. bisect the point A and note down the Vernier reading angle AOC.
3. Similarly bisect the point B from C after which will give angle BOC. Now we can compute the angle AOB.
4. Conduct the procedure in both face left and face right conditions and the average value is considered as accurate vertical angle