The Theodolite is the most accurate instrument designed for the measurement of horizontal and vertical angles. When compared with a regular chain survey, compass, or plane table, a theodolite survey turns out more precise and speedy.
Theodolite is an instrument used for laying off horizontal angles, locating points on the line, prolonging survey lines, establishing grades, determining differences in elevation, setting out curves, etc.
This Theodolite survey is required for building construction work. In the field of civil engineering, workers require this to measure each and all works that can be executed in satisfactory ways.
What Is Theodolite Surveying
Transit Theodolite is a measurement instrument used in surveying to specify the horizontal and vertical angles with the tiny low telescope that may change positions within the horizontal and vertical planes.
It is an electronic machine that looks at the type of a tiny telescope. It is greatly utilized for the measurement of vertical and horizontal angles for scaling functions and within the housing industry.
The precision with which these angles may be measured ranges from 5 mins to 0.1 secs. It is used in triangulation networks.
Theodolites are used in every place from construction sites to major road points. It measures angles utilizing age-old principles of pure mathematics and assists surveyors in defining precise locations.
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Types of Theodolite
Theodolite is classified as follows,
A. Based On The Movement Of The Telescope On The Horizontal Axis In A Vertical Plane
- Transit Theodolite
- Non-Transit Theodolite
B. Based On An Arrangement To Measure The Angles
- Vernier Theodolite
- Micrometer Theodolite
- Electronic digital Theodolite
Based On The Movement Of The Telescope On The Horizontal Axis In A Vertical Plane
1. Transit Theodolite
In the case of a Transit theodolite (or simply a transit), the line of sight can be reversed by revolving the telescope through 180° in the vertical plane.
In transit theodolite, an Internal focusing telescope is used. Transit theodolites are mainly used for surveying.
2. Non – Transit Theodolite
In the case of non-transit theodolite, the telescope cannot be revolved around the horizontal axis in a vertical plane completely. It can be rotated in a vertical plane for some limited angle.
These types of theodolites have now become obsolete.
B) Based On An Arrangement To Measure The Angles
1. Vernier Theodolite
The theodolite in which the vernier is fitted to measure the angles is called vernier theodolite. It can measure an angle up to 2O”.
2. Micrometer Theodolite
The theodolite in which a micrometer is equipped to measure the angles is called micrometer theodolite. It can measure an angle up to l”. It conveys more accuracy.
3. Electronic Digital Theodolite
In electronic digital theodolite, the reading of the angle is received in digital form. When E.D.M. (Electronic distance measuring) instrument is connected to the electronic digital theodolite, it becomes a total station.
It is essential to know about theodolite parts and their functions before utilizing them to minimize errors during theodolite surveying.
The following are parts of theodolite and their functions,
- Trunnion Axis
- Vernier Frame
- Vertical Circle
- Plate Levels
- A-frame Or Standards
- Upper Plate (Vernier Plate)
- Upper Clamp Screw
- Lower Clamp ScrewTripod
- Lower Plate (Scale Plate)
- Leveling Head
- Clip Screw
- Altitude level tube
- Plumb Bob
- Shifting Head
- Two Spindles Or Axes
- Tangent Screw
- Foot Screw
- Spirit Level
It is an essential part of the theodolite. It is mounted rigidly on the horizontal axis (trunnion axis).
The telescope may be an internal focusing type or an external focusing type. In most of the transits, an internal focusing telescope is utilized.
It can be rotated about the horizontal axis in a vertical plane. It can be clamped in a vertical plane by a vertical clamp screw.
2. Trunnion Axis
The trunnion axis in the theodolite is the horizontal axis about which the telescope of a theodolite can be rotated.
3. Vernier Frame
Vernier frame can also be termed as a t-frame or index frame in various cases, as it includes two arms vertical and horizontal. The vertical arm enables to lock of the telescope at the desired level and the horizontal arm is helpful to take the measurements of vertical angles.
4. Vertical Circle
It is a graduated circle rigidly connected to the trunnion axis of the telescope, and it rotates with the telescope. Vertical angles are measured on a vertical circle.
The figure indicates a vertical circle and telescope. The circle is divided into four quadrants each quadrant being numbered from 0° to 90°.
When the telescope is in a horizontal position, 0° – 0° on the vertical circle remains on the horizontal axis, and 90° – 90° on the vertical circle remains on the vertical axis.
5. Plate Levels
Plate levels are carried by the upper plate which is at right angles to each other with one of them parallel to the trunnion axis.
These plate levels enable the telescope to settle in an exact vertical position.
6. A-frame Or Standards
Two standards resembling the letter (A) are mounted on the upper plate. The axis called the trunnion axis of the telescope in theodolite is supported on A-frame.
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7. Upper Plate (Vernier Plate)
The upper plate is connected to the inner axis. It carries two verniers with magnifiers placed 180° apart. The upper plate supports the standards.
It holds up an upper clamp screw and a corresponding tangent screw for accurately adjusting it to the lower plate.
On clamping the upper clamp and unclamping the lower clamp the instrument can rotate on its outer axis without any relative movement between the two plates. It is named lower motion.
On clamping the lower clamp and unclamping the upper clamp, the upper plate and the instrument can rotate on the inner axis with relative motion between the vernier and the scale. It is named upper motion.
8. Upper Clamp Screw
The upper plate carries an upper clamp screw and a corresponding tangent screw. The upper plate can be fixed to the lower plate by tightening the upper clamp screw.
The upper plate can be slightly rotated for adjustment with the help of the upper tangent screw (upper slow-motion screw).
9. Lower Clamp Screw
The lower plate carries a lower clamp screw and a lower tangent screw. When the lower clamp screw is tightened, the lower plate is fixed to the upper tribrach plate and it can be rotated slightly for adjustment with the help of the lower tangent screw.
Theodolite is always utilized by mounting it on a tripod. The legs of the tripod either be solid or framed. At the lower end, the legs are equipped with steel shoes to get a decent grip on the ground.
The top of the tripod is contributed with an external screw to which the lower tribrach plate can be screwed. When not in use tripod head may be conserved with a steel cap, given for this intention.
11. Lower Plate (Scale Plate)
The lower plate is connected to the outer spindle. The lower plate carries a horizontal circle at its beveled edge. The graduations are marked from 0° to 360° in a clockwise direction, with the smallest division of 20″.
The lower plate carries a lower clamp screw and corresponding slow motion or tangent screw. When the lower clamp screw is tightened the lower plate is fixed to the upper tribrach plate and it can be rotated slightly for adjustment with the help of the tangent screw.
12. Levelling Head
The leveling head usually consists of two parallel triangular plates known as tribrach plates. The upper tribrach plate has three arms, each carrying a leveling screw.
The lower tribrach plate has a circular hole through which a plumb bob may be suspended. In some instruments, four leveling screws are provided in place of three leveling screws.
In modern theodolites, centering of theodolite is done by optical plummet. A leveling head has three distinctive functions:
- To support the main part of the instrument.
- To attach the theodolite to the tripod.
- To provide a mean for leveling the theodolite
13. Clip Screw
A clip screw is fitted at the lower end of the clipping arm for slightly rotating the index arm for adjustment.
When the telescope is moved in the vertical plane, the vertical circle moves relative to the verniers and thus, the readings are taken. For adjustment bases, but, the index arm can be rotated slightly with the help of a clip screw.
14. Altitude Level Tube
Many theodolites are provided with altitude-level tubes fitted over the telescope. It is utilized to test the horizontality of the trunnion axis.
The bubble of the altitude level tube can be centered by a clip screw.
15. Plumb Bob
It is suspended starting from the hook fitted at the very bottom of the inner axis. It has the function to center the instrument just over the station point.
Many theodolites are given with a compass which may be in the form of a circular box compass or through compass or tubular compass. Bearings are taken with the help of a compass. It is fitted to the A-frame.
17. Shifting Head
An arrangement of shifting head is made for quick and accurate centering of the theodolite.
By this arrangement, the theodolite can be shifted in the horizontal plane with respect to the tripod head, to bring the plumb bob exactly over the station peg.
18. Two Spindles Or Axes
The internal spindle or axis is solid and conical. The outer spindle or axis is hollow and ground conical in the interior.
The inner spindle is similarly called the upper axis since it carries the vernier or upper plate. The outer spindle takes the scale or lower plate. Both the axes have a common axis which forms the vertical axis of the instrument.
19. Tangent Screw
With each Clamping Screw, there is a tangent screw existing in the instrument to give fine movement. The tangent screws work just after their clamping screws get tightened.
Therefore when the upper clamp screw has been tightened, a slight movement of the upper plate can be generated by the upper tangent screw; when the lower clamp screw has been tightened, a slight movement of the lower plate can be generated by the lower tangent screw and also for vertical clamp screw.
20. Foot Screw
These are means for leveling the instrument. The lowest part of the foot screw is conserved in the trivet utilizing a ball and socket arrangement and the upper threaded part passes through the threaded hole in the tribrach plate.
It is the bottom-most assembly that is screwed onto the top of the tripod. At its base is the tribrach which includes three or four screws and a circular bubble.
This bubble is utilized to settle the horizontal circle into a horizontal plane. A locking device carries the levelling head and tribrach together.
With the support of foot screws, the instrument can be leveled. For example, the vertical axis can be made truly vertical.
22. Spirit Level
A spirit level, bubble level, or simply a level, is designed to specify whether a surface of the theodolite is horizontal (level) or vertical (plumb).
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Size Of Theodolite
The diameter of the graduated circle on the lower plate indicates the size of the theodolite.
For common surveying works, theodolites of 8 cm to 12 cm size are utilized. For triangulation survey and additional accurate survey works theodolites of larger size are utilized.
For the Indian triangulation survey, a theodolite of 91.4 cm (36″) diameter was utilized.
Uses Of Theodolite
Theodolite Uses as follows,
- To measure horizontal angles accurately.
- To measure vertical angles accurately.
- To decide true north by astronomical observations.
- To know the difference in elevation of two points.
- To measure the height of the building, tower, and depth of a valley.
- To measure the distance between two points.
- For the alignment of road, railway. bridges, dams, canals, etc. on the ground.
- For the alignment of the tunnel on the ground.
- For survey works of mining.
- To measure slopes.
- To prolong (extend) a survey line.
- To measure deflection angle between two lines.
- For tachometer survey.
- For triangulation survey.
- To prepare topographic maps.
Temporary Adjustment Of A Theodolite
Temporary adjustments are the adjustments that are needed to be made at each setting of the instrument before obtaining observations. These adjustments are also called station adjustments.
The following three temporary adjustments are required:-
- Setting up and centering
- Elimination of Parallax.
1. Setting Up And Centering
- Place the tripod over the station. The legs of the tripod should be spread so that they make an angle of 60° horizontal.
- Take out the instrument from the box. Lift the instrument from the base and screw it firmly on the tripod head.
- Adjust the height of the tripod so that the telescope is at an effective height.
- Suspend a plumb bob from the hook beneath the internal spindle.
- Approximate centering is done utilizing the tripod legs. The tripod legs are moved radially or circumferentially for centering.
- Sometimes the instrument and the tripod have to be moved bodily for centering to bring the plumb bob over the station mark.
Accurate leveling of the theodolite is done by using leveling screws or foot screws regarding the plate levels.
The procedure of leveling with three-foot screws is as follows;
- Turn the upper plate until the longitudinal axis of the plate level is roughly parallel to a line joining any two of the leveling screws (A and B).
- Hold these two leveling screws between the thumb and the first finger and turn them uniformly so that the thumbs move either towards each other or away from each other until the bubble is central.
- Turn the upper plate through 90°, for example, until the axis of the level passes over the position of the third leveling screw (C).
- Turn this leveling screw (C) until the bubble is central.
- Return the upper plate through 90° to its original position. Rotate screws A and B inwards or outwards till the bubble is middle.
- Turn back again through 90°. Rotate the leveling screw C till the bubble is central.
- Repetition the above steps till the bubble is central in both positions.
3. Elimination Of Parallax
A parallax is a situation happening when the image formed by the objective is not in the plane of the cross–hairs.
Unless parallax is eliminated, accurate sighting is impossible. Parallax can be eliminated in two ways :
- Focusing of eye-piece
- Focusing of objective
What is a theodolite used for?
Uses Of Theodolite
1. To measure horizontal angles accurately.
2. To measure vertical angles accurately.
3. To decide true north by astronomical observations.
4. To know the difference in elevation of two points.
5. To measure the height of the building, tower, and depth of a valley.
6. To measure the distance between two points.
What is the principle of theodolite?
The principles of the theodolite are as Follows
a beam of light travels in a straight line, and that when you know the length of one side of a triangle and the angles of the corners as measured by reflecting beams of light then you can precisely map features on the ground both horizontally and vertically.
Can a theodolite measure distances?
theodolite is with a built-in distance meter (distancer), and so itcan measure angles and distances at the same time. Today’s electronic total stations all have an opto-electronic distance meter (EDM) and electronic angle scanning.
How theodolites are used by surveyors?
Theodolite, basic surveying instrument of unknown origin but going back to the 16th-century English mathematician Leonard Digges; it is used to measure horizontal and vertical angles. In its modern form it consists of a telescope mounted to swivel both horizontally and vertically.
What are the disadvantages of theodolite?
Errors due to leveling.
Inaccurate centering of theodolite instrument over station mark point.
Error due slipping of lower plate of theodolite instrument. …
Taking reading wrong from vernier scale.
Errors occur due to parallax.
Errors due to not holding ranging rod vertically.
The Theodolite is the most accurate instrument designed for the measurement of horizontal and vertical angles. When compared with a regular chain survey, compass, or plane table, theodolite survey turns out more precise and speedy.
Theodolites survey is used in every place from construction sites to major road points. It measures angles utilizing age-old principles of pure mathematics and assists surveyors in defining precise locations.
How to Use a Theodolite
1. Set up theodolite on tripod stand
2. Set horizontal circle scale to matching zero.
3. Bisect first point with telescope which angle or distance has to be measure.
4. Now, rotate telescope to second point and fix the horizonal circle clam screw.
5. Read the reading on horizontal Vernier in degree minute and second accuracy.
6. Calculate angle and distance between two points.
Types of Theodolite
Theodolite has the following types,
A. Based On The Movement Of The Telescope On The Horizontal Axis In A Vertical Plane
Non – Transit Theodolite
B. Based On An Arrangement To Measure The Angles
Electronic digital Theodolite
Transit Theodolite is the most accurate instrument designed for the measurement of horizontal and vertical angles. When compared with a regular chain survey, compass, or plane table, theodolite survey turns out more precise and speedy.
How Many verniers are there in transit theodolite?
The transite theodolite consite of two verniers marked A and B, which are used for taking readings accurately upto 20″ on the lower graduated circle.
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