What Is Transit Theodolite | Theodolite Parts

Introduction of Transit Thelotie.

Distance between two points (in a small area) is measured in chain survey.

Horizontal angles between stations and objects are obtained in a compass survey with an accuracy of half a degree. Points on the ground are located with the help of angles and/or distances measured or calculated.

Plane table survey helps in the plotting work of drawings in the field itself while simultaneously measuring the distances and viewing the direction of the location of objects.

Level gives only the elevations of various ground point locations. However, all these observations are limited for covered area, accuracy, time, sophistication, money, and use of many other latest devices.

Theodolite is widely used to obtain both horizontal and vertical angles between various station points at different elevations with the highest precision (1 to 20 seconds).

Traversing is also easy. Quick and more accurate. Further, horizontal and vertical distances, slopes.

Location of a meridian, positions of stars, etc. can also be deter pined using a theodolite.

Trigonometric leveling and tachymetric surveying are the two immediate examples of utilizing theodolite work, in addition to traversing.

Nowadays, more advanced instruments, like a total station, are also used for various types of measurements.

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What is Transit Theodolites?

Theodolite mainly consists of three units, i.e., a telescope to sight various objects and capable of moving in horizontal and vertical directions; a horizontal circle with markings from 0° to 360° and a vertical circle to record telescopic movements in the upward and downward directions.

It will have a base to support on a tripod stand. As per below, the figure shows a transit theodolite with its salient parts.

Transit is the operation where the telescope can be reversed or revolved through 180° in the direction of the vertical plane about its horizontal axis.

The vertical circle can be on the left side, or right side of the telescope, and the observations are named accordingly as face left or faced right observations.

A telescope may be rotated in all directions, left, right, upwards or downwards, to observe the positions of various stations (signals) in the field.

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Theodolites Parts & Details

Transit Theodolite Parts Figure.

Vertical Circle

• Vertical circle rotates with the telescope movement and measures vertical angle observations to the station points. It is fitted to the trunnion axis.
• The index frame consists of a clipping arm (vertical): and an index arm (horizontal).
• Two extremities of the index arm, main scales C and D, from 0° to 90° graduations, measure vertical angles.
• Two verniers are also accompanying the main scales to read the parts of a degree.

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Altitude Bubble

• In addition to the plate bubble, some older instruments are fitted with another bubble tube at the top of the standard supporting the vertical circle.
• This is the altitude bubble, and it is linked to the vertical circle and to an altitude bubble adjusting screw on the standard, similar to a slow-motion screw.
• When the altitude bubble is central, it indicates that the vertical circle is properly zeroed and vertical angles read off the vertical circle will give the correct values.
• The altitude bubble must be centered by its adjusting screw immediately before taking a vertical angle reading.
• The altitude bubble was sometimes read through a coincidence prism reading system, instead of being a simple open bubble tube.
• Like the similar systems used on some levels, these give an image of the two ends of the bubble tube, as in as per below figure

Altitude Bubble

• Most modem instruments are fitted with self-zeroing vertical circles (automatic vertical indexing) which make use of gravity-operated liquid compensators and these avoid the need to center an altitude bubble before reading a vertical angle.
• However, the instrument must be leveled up with care to ensure that the working range of the compensator is not exceeded.

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Horizontal Axes

• It is also called the trunnion axis or transverse axis (as per above figure Point 3). It’s the axis where the telescope could be rotated in a vertical plane.
• It’s the axis about which the telescope along with vertical circle rotates in a vertical plane. This horizontal axis is also known as the trunnion axis.

Vernier Arm

• A theodolite has two verniers A and B placed on the opposite sides of the upper plate (i.e., they are placed at a difference of 180°).
• For ordinary work usually, the vernier A is read, whereas for precise work both the verniers A and B are read and the mean of the two readings is used.
• This practice minimizes the error due to eccentricity and imperfection in subdivisions which might exist in the circular scale.
• The main scale and vernier of a typical theodolite as graduated are shown in as per below figure. The main scale is graduated from 0° to 360° in degrees and minutes.
• Each degree part is tested and divided into three equal parts. Hence, the minimum reading that can be read from the main scale is 20′.
• The vernier scale is graduated into minutes and seconds. Each minute division is divided into three equal parts. Hence, the least reading that can be read from the vernier scale is 20″.

Vernier Scale Graduation 

• To read an observation, first, determine the value of the scale reading in degrees and minutes up to the last scale division passed by the vernier zero (index).
• Add to this the vernier reading. A vernier reading is obtained by locating the vernier line, which coincides with the main scale line.
• The number of that vernier line is then multiplied by the least count of the vernier to get the reading. For example, the reading shown in Fig. 4.3 is 150°40′ t 1’40” =150°41’40”.
• It may be noticed that on the scale, there are two more graduations behind the vernier index.
• These are used for permanent adjustment of the theodolite. Since different arrangements of verniers will be found on different theodolites, a transit man should be careful while determining the characteristics of the vernier on the instrument.

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Plate Bubble

• One or two plate levels are mounted on the upper plate. If two levels are provided, they will be at right angles to each other, one of them being parallel to the trunnion axis.
• The bubble of a plate level can be centered with the help of the foot screws.
• These levels are also useful in making the vertical axis of the instrument truly vertical.

Leveling Head

• The leveling head is provided with three or four leveling screws. These should be of the fine pitch.
• The distance of screws from the vertical axis of the instrument governs the delicacy of action. The greater the distance, the smaller will be the tilt caused by turning a screw by one turn.
• A four-screw head is compact but leads to uneven pressures on screws which result in their excessive wear. The three-screw arrangement is free from these objections.
• Moreover, it has an important advantage of being more rapidly level.

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Clamping Nut

• Clamping nut for fixing in trip pod table.

Vertical Axes

• It’s the axis Where the telescope could be rotated in a horizontal plane (as per above figure Point 9).
• This is the axis about which the instrument rotates in a horizontal plane. It passes through the centers of the inner and outer spindles of the upper and lower plates.

Telescope

• An internal focusing telescope (having a supplementary double concave lens) mounted in a short tube, which can be moved to and fro between the objective and the diaphragm, is used for focusing the objects.
• The eyepiece, an objective glass, a diaphragm (with horizontal and vertical crosshairs) and a focusing screw are the primary parts of the body of the telescope, for viewing the objects.
• A telescope is mounted on a spindle corresponding to horizontal or trunnion axis. It is supported by U-frame, i.e., two standards of ‘A’ shape, resting on the horizontal upper plate.
• Altitude bubble is attached to the standards of the frame for setting the telescope axis truly horizontal. A clamp and a tangent screw (for finer movements) are used to properly bisect the object (signal or station point) and fix the telescope in a vertical plane.

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Vertical Circle Clamp Screw

• The Vertical Circle plate carries a lower clamp screw and a corresponding slow motion or tangent screw with the help of that it could be fixed accurately in almost any desired position for clamp and tangent screw.
• When the clamp is tightened, the lower plate is fixed to the upper tribrach of the leveling head. on turning the tangent screw, the lower plate can be rotated slightly.
• Usually,  the size of a scale plate, i.e… a10 cm theodolite or 12cm theodolite, etc.

Standard (Frame)

• Standards or A-Frame: The frames supporting the telescope are in the form of English letter A. They are known as standards or A-frame.
• The frame allows the telescope to rotate on its trunnion axis in a vertical plane. The T-frame and the clamps for the vertical circle are also fixed to this frame.

Line of Sight

• It’s an imaginary line joining the intersection of cross-hairs to the optical center of the objective and its continuation.

Axis of Plate Bubble

• It’s a straight line tangential to the longitudinal curve of  this plate level tube at its center
• When a bubble is centered, it is horizontal

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Upper Plate

• Upper plane: It supports standards at its upper surface. On the lower side, it is attached to an inner spindle which rotates in the outer spindle attached to the lower plate (As per below figure).
• The upper plate can be clamped to the lower plate using upper clamp screws.
• A small movement of the upper plate is possible even after clamping, using tangent screws.

Upper Plate

• Two diametrically opposite verniers (A and B) are fixed to the upper plate. They are provided with magnifiers

Lower Plate

• The lower plate of the instrument. attached to the outside spindle carries a graduated ring during its beveled edge.
• The graduations are divided into 360° and each degree is further divided into 20′ intervals.
• It could be clamped at any desired position utilizing lower clamps.
• If the upper clamp is locked and a lower clamp is loosened the two plates rotate together on the outer spindle without causing any change in the graduated circle read-ing.
• If the upper clamp is loosened and the lower clamp is locked, the upper plate rotates on its inner spindle with relative motion between the two plates. This property is utilized in measuring horizontal angles.

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Lower Plate Clamping

• The lower plate carries a lower clamp screw and a corresponding slow motion or tangent screw with the help of that it could be fixed accurately in almost any desired position for clamp and tangent screw.
• When the clamp is tightened, the lower plate is fixed to the upper tribrach of the leveling head. on turning the tangent screw, the lower plate can be rotated slightly.
• Usually,  the size of a scale plate, i.e… a10 cm theodolite or 12cm theodolite, etc.

Tribrach

• It is the bottom-most assembly which is screwed on to the top of the tripod.
• At its base is the tribrach which contains three or four screws and a circular bubble.
• This bubble is used to put the horizontal circle into a horizontal plane. A locking device holds the leveling head and tribrach together.
• With the help of foot screws, the instrument can be leveled. i.e., the vertical axis can be made truly vertical.
• The various parts of a transit theodolite are discussed below.

Foot Screw

• Leveling Screws The leveling head is provided with three or four leveling screws. These should be of the fine pitch.
• The distance of screws from the vertical axis of the instrument governs the delicacy of action.
• The greater the distance, the smaller will be the tilt caused by tuning a screw by one turn.
• A four-screw head is compact but leads to uneven pressures on screws which result in their excessive wear. The three-screw arrangement is free from these objections.
• Moreover, it has an important advantage of being more rapidly level.

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Trivet

• The centering device or also known as the movable head is placed immediately below the trivet stage, but sometimes it is placed above the tribrach.
• The latter arrangement has the advantage that centering may be done after the instrument is leveled, and is therefore not likely to be disturbed by any subsequent leveling.
• Centering should always be accurate within 2 mm, otherwise short lines introduce unacceptable large angular errors into the measurements.
• Trivet is also called the baseline

Tripod Top

• Theodolite is utilized by placing it on a tripod. It consists of three solids or framed legs.
• The legs are provided with pointed steel shows to get a good grip with the ground.
• The top of the tripod is provided with an external screw to which on which lower plate of theodolite can be screwed.
• When not in use tripod head screw is protected with a steel cap.

Plumb Bob

• A hook is provided to the bottom of the inner axis Where plumb bob could be suspended.
• It eases precise centering of the theodolite on a station.

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Normal Least Count of Theodolite 20 Second 

Transit Theodolite PPT