Theodolite, its types and essential parts

Articles > Theodolite, its types and essential parts

What is Theodolite?

Theodolite is a very commonly used instrument that has a fundamental importance in the field of surveying and structural engineering. A theodolite is used to measure all sorts of vertical and horizontal angles. The work of land survey is almost impossible without the use of a theodolite. Apart from the work of land surveying, theodolites are widely being used in locating points on the line, differentiating the levels, prolonging survey lines, etc. Their use in the construction of buildings and infrastructures, meteorology, and rocket launching is growing very rapidly.

Uses of a Theodolite:

A theodolite is a measuring instrument, which is used to measure the horizontal and vertical angles.
A theodolite is used to locate points on a line.
It is used to find the difference among levels.
It is used to prolong the survey lines.
it is used to range the curves.
A theodolite is also used to set out grades.
A theodolite is used in Techometric surveying.

Types of Theodolite:

There are many types of theodolites, which may be classified into two groups:

One is Transit theodolite, and The other is a non-transit theodolite

(1). Transit Theodolite:

The line of the sight in a transit theodolite can be reversed if the telescope is reversed through 180° in a vertical plane.

(2). Non-Transit Theodolites:

They are also known as Y-theodolites and plane theodolites. In such types of theodolites, the reversal of sight line and telescope is not possible.

Digital Theodolite:

The values of the observation are provided directly on the viewing panel of the digital theodolite. The precision of a digital theodolite varies in order of 1” to 10”.

Vernier Theodolite:

It is the most commonly used theodolite, in which the principles of a Vernier Caliper are utilized while taking out observations. The precision values of this type of instrument vary in the order of 10" to 20".

Total station Theodolite:

This is a modern electronic instrument. All the parameters can be obtained which are required to be observed in the survey procedure. The viewing panel of the instrument displays the observation values. The precision of this type of instrument ranges in the order of 0.1" to 10".

Different Parts of Theodolites:

There are several types of theodolites based on their distinctive specifications and function. Each type has a different mode of operation. However, some fundamental functions are common in all types of theodolites inherently. In this article, we will discuss the parts of a Vernier Theodolite, because it is the most common type of theodolite, widely used in all sorts of surveying all over the world. The salient parts of a vernier theodolite are as follow:

Leveling Head:

The top leveling Head of a theodolite is its lowermost part, which consists of two parallel and horizontal panels that are separated by screws. These screws are known as the traveling screws. In the lower plate, there is a large thread-hole in the center. This thread hole is called trivel or foot-plate. It enables the instrument to be installed on the tripod stand and get it screwed. The central aperture of trivel gives a way for hanging a plumb bob. There is a tribrach in the form of a plate in the upper part of the head. There is a tapered-bearing at the center of the tribrach. It has three arms. Each carries a leveling screw. It gives support to the upper portion of the instrument. The main objective of the leveling head is to provide a balanced level to the instrument.

The Shifting Head:

shifting head of a theodolite consists of a pair of horizontal-plates and one annular treaded ring. One of the plates is installed beneath the lower plate but above the tribrach and the other plate is placed above the tribrach and below the upper plate. The annular treaded ring is also put above lower plate and below the tribrach. The annular treaded ring is used to tighten and untighten the whole of the instrument. After the completion of the leveling, the shifting head of the theodolite is used for the exact centering of the instrument.

3. The Lower Plate:

The lower plate is a horizontal circular plate monolithically constructed with an outer spindle. A scale is engraved at its beveled edge with divisions in degrees and minutes increasing in a clockwise direction. It provides the main scale reading of a horizontal angle and a means to fix or unfix the whole of the instrument.

The Upper Plate:

It is a horizontal circular plate monolithically constructed with the inner spindle. It is fitted with two diametrically opposite vernier scales designated as A and B. Functions of upper plates are to support a pair of magnifiers for the verniers, a pair of plate levels, a pair of support frames for the telescope, and a means to fix or unfix the upper plate of the instrument with its lower plate.

The Plate Levels:

A pair of tube-levels is placed horizontally at the right angle above the upper plate to determine the vertical axis of the instrument. The identification of the vertical axis is made for the sake of leveling the instrument.

The Standard (or a Frame):

Two standards resembling the letter A are attached on the upper plate. This provides the bearings of the pivots of the telescope allowing it to rotate on its trunnion axis in the vertical plane. The vernier frame and arm of vertical circle clamp are also attached to it.

The Vernier Frame:

Also called T-frame or index frame, consists of a vertical leg known as clipping arm and a horizontal bar called the index arm engraved with verniers C and D at its ends. Each of the verniers at C and D are having two scales that increase in opposite directions. It is used as a seat for altitude bubble and also provides vernier reading for vertical angle measurement

The Telescope:

The telescope of a theodolite is identical in structure and uses, as in the case of a dumpy level. But, in theodolite, the telescope is mounted on a horizontal spindle called the horizontal axis or the trunnion axis to rotate it also in the vertical plane.

The Vertical Circle:

The vertical circle is attached to the trunnion axis. It is engraved with a scale reading vertical angle in degrees and minutes. The vertical circle is divided into four quadrants each reading 0° to 90° with 0° - 0° either along vertical or in horizontal. It provides the main scale reading for the vertical angles.

The Altitude Bubble:

A sensitive level tube placed on the vernier frame is called an altitude bubble. It is used to make the horizontal axis truly horizontal.

The Screws:

A theodolite instrument has number of screws as its component parts. These are classified into different types depending on their functions.

Leveling Screws: These are present in the leveling head of a theodolite in between trivet and tribrach. These work in threaded holes in the tribrach arms and their lower ends rest in recesses in the trivet. These screws are used for leveling the instrument i.e., to make plate level axis truly horizontal.
Lower plate Clamp Screw: The clamp screw attached to the lower plate of a theodolite is called lower plate clamp screw. When it is tightened, the outer spindle gets fixed with the tribrach, and, thus, the lower plate gets fixed in position.
Upper plate Clamp Screw: The clamp screw attached with the upper plate of a theodolite is called upper plate clamp screw. When it is tightened, the inner spindle gets fixed with the outer spindle and, thus, the upper plate gets fixed in position. The manipulation of the upper plate and lower plate clamp screws provide three conditions:
- When both the upper plate clamp screw and the lower plate clamp screw are tightened, the instrument gets fully fixed.
- When the upper plate clamp screw is tightened and the lower plate clamp screw is opened, the instrument rotates on its outer axis, there is no relative motion between the two plate and the readings in the horizontal vernier scales do not change.
- When the lower plate clamp screw is tightened, and the upper plate is opened, the instrument rotates on the inner axis with outer axis fixed. The readings in the horizontal vernier scales change.
Vertical plate Clamp Screw: It is present on a frame fixed with standard and above the shaft of trunnion axis. It is used to clamp the telescope in any plane and hence at any desired vertical angle.
Tangent Screws: With each clamping screw, there is a tangent screw present in the instrument to provide fine movement. The tangent screws work only after its clamping screws get tightened. Thus when the upper clamp screw has been tightened, small movement of the upper plate can be made by the upper tangent screw; when the lower clamp screw has been tightened, small movement of the lower plate can be made by the lower tangent screw and similarly for vertical clamp screw.

The Tripod Stand:

Tripod stand is a stand with three shaft-legs. The theodolite is mounted on the stand in a fixed manner. The head of the stand has motile bearings which smoothly move the theodolite around without misplacing. The legs of the tripod can be moved apart and closed, so as to move the theodolite up and down. The level of the theodolite can be fixed with the help of the legs of the tripod stand.

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Theodolite, its types and essential parts

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