THEODOLITE

Introduction:

In surveying, theodolite is an instrument used to measure horizontal and vertical angles with high precision. It is also used for levelling, measuring horizontal and vertical distances and calculating bearing. The line of sight of theodolite can be rotated through 180 degrees in vertical plane about its horizontal axis. The theodolite was invented by Jonathan Sisson in 1720.

Types of Theodolite:

There are two types of Theodolite: Transit Theodolite and Non-Transit Theodolite.

Transit Theodolite: A Theodolite is called a transit Theodolite when its telescope can be transited i.e the line of sight can be reversed by revolving the telescope 180 degrees along the vertical plane.

Non-Transit Theodolite: In this type of Theodolite, the line of sight can not be revolved in the vertical plane.

Description:

Parts of a Theodolite and their importance:

Telescope: It is one of the most important components of the Theodolite. It is used to sight distant objects and for focusing. It is mounted perpendicularly at the centre of the horizontal axis(or trunnion axis). It can be either rotated in a horizontal plane or in a vertical plane.

Vertical Circle: It is another essential part of the Theodolite. A circular graduated ring is attached to the horizontal. With the help of vertical circle, we can measure the vertical angle of the line of sight with the horizontal axis.

Index frame or T frame: It is T-shaped and is centered on the horizontal axis of a telescope in front of the vertical circle. Two verniers are provided to read the vertical circle. Clip screw is used for adjustment. At the top of the index frame, an altitude bubble is placed.

Transit level head: It consists of two triangular parallel plates and these plates are called tribrach plates. The leveling head is used for the following purposes:

It provides a bearing for the outer hollow spindle.
Attaching the instrument to its tripod
Levelling the instrument.

Plumb Bob: A plumb bob is suspended from the hook fitted to the bottom of the vertical axis. It is used for the temporary adjustment of the theodolite which includes:

Centering the instrument exactly over the station point.
Levelling
Elimination of parallax.

Compass: It is a very important part of the theodolite as it is used to measure the angle between the normal line and the line of sight.

Lower plate: This carries the circular scale graduated from 0° to 360° and half degrees and third of a degree, and a tapered spindle which works in the outer conical bearing. A well supported bracket is fitted to the spindle carrying clamping screws for upper plate and lower plate.

Upper plate: The upper plate can be rotated relative to the lower plate about the spindle as axis. It carries two verniers marked A and B, which are used for taking readings accurately up to 20 degree on the lower graduated circle. Furthermore, this plate carries a level tube too and two vertical standards for supporting the telescope.

Tripod: Perhaps the most important part of the theodolite. It is used to support the theodolite while taking angular measurement during a survey.

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How to use a Theodolite:

Mark the point at which the theodolite will be set up with a stake. This point is the basis for measuring angles and distances.
Set up the tripod making sure the height of the tripod allows the instrument to be eye-level. The centered hole of the mounting plate should be over the stake.
Drive the tripod legs into the ground using the brackets on the sides of each leg.
Mount the theodolite by placing it on to the tripod and screw it in place with the mounting knob.
Measure the height between the ground and the instrument. This will be used a reference to other stations.
Level the theodolite by adjusting the tripod legs and using the bulls-eye level.
Adjust the small sight (the vertical plummet) found on the bottom of the theodolite using the knobs. This ensures that the instrument remains over the stake.
Aim the crosshairs in the main scope at the point to be measured. Use the locking knobs on the side of the theodolite to keep it aimed on the point. Record the horizontal and vertical angles using the viewing scope found on the theodolite’s side.

Price of a Theodolite:

The price of a theodolite is normally between 400-600 dollars.

Where to buy:

Theodolites are easily available on many online shops.