The theodolite | What is the use of theodolite in surveying?
The theodolite is a precision optical instrument for measuring angles between designated visible points in the horizontal and vertical planes.
One of the fundamental weaknesses of American engineer transportation is the difficulty of accurately reading graduate zones with verniers.
As a result, many iterations are required to accurately measure or outline an angle.
European manufacturers, for many decades, have been producing precision instruments called theodolites for angular measurement.
These compact devices have internal optical devices that make the zones more readable More accurately possible with American equipment.
And they can be read much faster with less chance of blunder. Accuracy varies from 0.1-second are to 0.1 minutes (6 seconds) readability in some good tools.
With a theodolite in general, high accuracy can be achieved in the shortest time required by the iterative method used with American-style transit.
Theodolites were originally used in the United States and Canada for this reason.
General features of theodolite
Theodolites differ in transport in several ways (Figures 6-18 and 6-19). In overall appearance, they are significantly lighter and more compact than engineered shipping.
They usually do not have a built-in compass box on the Allied plate. Telescopes are small and usually lack spirit bubble tubes.
There is no circle or vernier window in the alidade, but a small reading microscope is attached to the iris beside the telescope, through which the graduated circle is read.
Theodolites are generally characterized by an ambient, dust-resistant, and moisture-resistant, and light-colored finish to minimize the effects of direct sunlight.
Theodolite is usually mounted on a three-screw leveling head instead of a four-screw leveling head, used for American transport. The circular bull’s-eye bubble vial is used for roughness;
The highly sensitive plate bubble tube is embedded in Allied for accurate leveling. The origin of the instrument is called the Tribrach.
It is designed with a release mechanism so that theodolite can be easily removed from the tripod and exchanged with an electronic distance measurement instrument (EDMI), target, or reflector.
Disturbing the ground floor and centering position on the survey center (see Figure 6-20).
This is known as forced centralization. Another characteristic of theodolite is the optical plummet.
This is a small telescope sight mounted in a vertical hole through the spindle and set to fit With azimuth or standing axis of alidade.
It is viewed during the setup of the instrument by the horizontal IPs on the side of the alidade or the bottom of the instrument.
After grounding the theodolite, the optical plummet shows the position of the axis of the tack or other markings on the survey station.
Since the device does not swing, it is not completely affected by air.
Theodolite is placed with a regular plumb bob and after being grounded, the position is checked and precisely adjusted with an optical plummet.
Some experienced surveyors prefer to install theodolite only with an optical plummeted; This method is described in the next section.
Theodolite amidase is usually fitted to a leveling head with a smooth steel cylinder and rotates freely around the azimuth axis on precise ball bearings (see Figure 6-21).
The horizontal and vertical zones are made of glass; They are graduated precisely with very thin, sharply defined lines engraved on their surfaces.
The optical system, including microscopes with prisms and/or mirrors, allows zones to be read quickly and accurately.
Replication and Direction Tools There are many different models of theodolites. Two common
Types Recurrent Theodolite and Directional Theodolite.
Generally, the direction type is more accurate than the repeat type. The repetitive theodolite, like a carriage, has two independent upper and lower movements with corresponding clamps and tangential screws.
Some repetitive tools, including only one clamp and tangent screw, have been fitted with a lever that can change the clamp and tangent operation from one motion to another.
The angles are basically, rotate with the carriage. Directional Theodolite has only the upper movement, with the same clamp and tangent screw connecting the alidade to the leveling head.
The slight friction between the circle and the leveling head prevents the circle from turning but can be turned freely on the allied bearings.
In some directional instruments, the circle can be rotated and oriented relative to the leveling head using a special finger-operated wheel.
In general, however, the circle cannot be set exactly to zero when rotating or measuring the angle.
With directional theodolites, the angles cannot be measured by the transverse method. The horizontal angle is usually measured as the difference between the initial and final direction of the alidade and the two corresponding readings of the circle.
This method is described in the next section. Internal optics are designed so that each reading represents an average of two values on the opposite sides of the circle, offsetting any eccentricity errors. (This is equivalent to the average readings of A and B Verniers in an engineer’s carriage.)
Setting and leveling theodolite
Theodolite must be carefully removed from its case, either by holding the attached carrying handle or by lifting it to grasp the criteria. It must be securely fastened to the tripod (see Figure 6-22a).
The threaded centering screw attached to the bottom of the tripod head is attached to the nerd handle;
The tool is placed in the center of the tripod head and the tripod centering screw is fastened to its base.
When the screw is loose, theodolite can be switched laterally on the tripod head for precise positioning on the survey point.
If the plumb bob is used, theodolite can be set one way over the station 
Engineer Transit (see Appendix A). Once the tool is grounded, an optical plummeted is used to check the position of the tool (see Figure 6-22b).
First, optical plummet pieces are focused. Then, if it is not focused precisely on its side hairs or bulls-eye circle point, the focusing screw is loosened.
However, When the point is viewed through an optical plummet, it is switched on until the instrument is positioned exactly. This must be accomplished without turning the leveling head into an azimuth
Throws the instrument out of level.
When precisely pointing, the tripod centering screw must be tightened to securely attach the instrument to the tripod. If relaying is required, the optical plummet should be used again to check for the center.
This centralization and leveling process is repeated until both are satisfied.
It is important to remember that the optical plummet is accurate only when the instrument is level.
When work on a particular survey station is completed, some surveyors prefer to remove theodolite from the tripod, and in the event, it is transported to the next station.
They usually set the tripod on point before mounting the equipment. The plumb bob is first used to focus the tripod on the point, with its head held horizontally by the eye.
The tool is then lifted from its case and securely fastened to the tripod. A plumb bob is removed from the tripod and the process of leveling and centering using an optical plummet continues.
Leveling the equipment
Theodolite is first roughly grounded with three leveling screws (see Figure 6-23) by centering the bubble in a circular Bulls-Eye spirit vial.
The alidade is then rotated so that the tubular spirit vial on the top plate is parallel to the imaginary line passing through the centers of any pair of leveling screws (see Figure 6-23).
The bubble in the tube is centered by adjusting the two screws (“thumbs-in, thumbs-out, the bubble follows the left thumb”).
Next, the alidade is rotated 90 ° and the bubble is concentrated in the tube with a screw that was not used before. (When the bubble rotates clockwise it moves toward the screw and vice versa.)
With the plate level vial, this process is repeated for an additional 90 ° revolutions of the instrument until the bubble is concentrated in all positions.
Setting and leveling theodolite or total station without plumb bob while theodolite Embedded in an adjustable-leg tripod, it is possible to set one stage faster without the use of a regular plumb bob.
One method relies only on the optical plummet. First, the instrument is positioned over the eye, while the footplate is positioned approximately (see Figure 6-24a).
When viewed through an optical plummet, the instrument may be several centimeters or about 0.1 feet at this time (see Figure 6-24b).
The optical plummet is focused on the point by adjusting the three leveling screws. But the circular vial bubble is still in the middle. That bubble is now centered by adjusting the length of the tripod legs (the bubble moves from the shorter leg and the longer leg).
Finally, the instrument is accurately grounded using a tubular plate level vial as described previously (see p. 3) Figure 6-24c).
And, Another method to install without a plumb bob involves the use of a special telescopic centering rod that can be attached directly to the leveling head.
The rod is attached to the bottom surface of the tripod head and mounted onto the supporting plate.
The tripod is mounted on the rod clamp loosely stationary and the lower end of the rod is placed directly at the survey point.
However, The circular bubble on the rod is centered by adjusting the tripod legs; When the bubble is perfectly centered, the tripod plate is horizontal.
Theodolite is then attached to the plate and the tubular plate is positioned exactly with the bubble vial.
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