Introduction
A theodolite is the most precise instrument which measures horizontal and vertical angle accurately It is an extremely useful instrument for exact survey work of engineering projects. It consists of telescope by which distant objects can be sighted The system of surveying in which the angles are measured with the help of a theodolite is called theodolite surveying
People have been measuring angles for construction purposes for many centuries Egyptians used groma, an early version of a theodolite, to help build the pyramids Furthermore, there are also records that indicate that the Romans used tools such as dioptra (circular plate that marked angles), for similar purposes. In 1571, Leonard Digges came up with a device which more closely resembled an early theodolite and called it theodolitus. It was a divided circle and square with a compass in the center, according to "Brief History of Turning Angles" at noaa gov, but it lacked a telescope (found in modern versions).
The telescope mounted on the top of the measuring device came into being by the mid 1700s The device also had a horizontal circle with a vertical semi-circle Early theodolites were woks of art, they were hand made out of brass and the angles were scribed by hand. They did, however, have a significant margin for error because they were only as accurate as the individual who scribed the angles. This is important because an error of one second translated into an error of one foot, at a distance of 40 miles. In 1773, Jesse Ramsden invented a mechanical dividing engine that allowed for higher accuracy and production of theodolites. This in turn, resulted in an increase of theodolite availability and placed England at the front of the theodolite production industry Theodolites came to the United States in 1815 in the request of Thomas Jefferson He wanted Ferdinand Hassler, the appointed Superintendent of the Survey of the Coast, to survey America. Theodolites remained pretty much unchanged until the 1950s when electronic distance measurements were adopted.
figure: Theodolite, Transit Classification of Theodolites
Theodolites are primarily classified as transit theodolite and non-transit theodolite.
Transit Theodolite
A theodolite is called a transit theodolite when its telescope can be transited, Le revolved through a complete revolution about its horizontal axis in the vertical plane The line of sight can be reversed by revolving the telescope through 180 in vertical plane The telescope can be rotated in a vertical plane through a complete revolution Le, 360 Almost all theodolite in ose now a days are transit.
Non-transit Theodolite
Theodolites whose telescope cannot be rotated in a vertical plane about as transverse horizontal axis is known as non-transit theodolite But it can be rotated to a certain extent in the vertical plane to theasure the vertical angles, e, angle of elevation and angle of depression Telescope cannot be tramited in non-trant theodolite. Now non- transit theodolite has become obsolete Another classification of the Theodolite can be done so the basis of the type of the scalei fited to the Theodolite and display used.
1. Vernier Theodolite (obsolete)
2. Micrometer Theodolite (obsolete)
3. Optical Theodolite
4. Electronic Digital Theodolite
1.Optical Theodolite
Micrometer-microscope theodolite has been outdated by optical theodolite. In optical theodolite, observer does not have to walk round the instrument to take horizontal and vertical circle readings.
Optical theodolite is fitted with a micrometer for taking horizontal and vertical angles precisely Most of these theodolites can read angles up to 1 In optical micrometer theodolite, it is important to first set the micrometer to zero. Movement of the micrometer knob also moves the micrometer scale
Total reading=Main scale reading Micrometer scale reading
=306°30'+3'40" =306°33'40".
fig optical Theodolite 4. Electronic Digital Theodolite
Automatically reads and records horizontal and vertical angles. Eliminates the manual reading of scales on graduated circles. Electronic theodolites have zero se buttons for quick orientation. Horizontal angles can be turned left or right. Display windows for horizontal and vertical angles are located at both the front and rear of the instrument.
Electronic theodolites are simple to use and less expensive to purchase and repair These simple electronic theodolites are quickly replacing optical theodolites, which replaced the vernier transit theodolites.
Advantages of Electronic digital theodolite
• Circles can be instantaneously zeroed or initialized to any value.
• Angles can be measured with increasing values either left or right.
• Angles measured by repetition can be added to provide a total larger than 360°.
• Mistakes in reading angles are greatly reduced.
• Speed of operation is increased.
• Cost of instrument is lower.
Star of Theodolite
The size of theodolite defined according to the diameter of the main horizontal grated circle og "10 em theodolite means diameter of main graduated canile is 10 cm. The common sizes are 8cm to 12 cm while 14 cm to 25 cm instrument are used for trangulation work Greater accuracy is achieved with larger theodolites as they have big graduated circle with larger divisions hence used where the survey works require high degree of accuracy.
Parts of Transit and Theodolite
Theodolite is a précise instrument consisting of many parts assembled as shown in thei fgure given below Parts of theodolite are described as:
Fig:Parts of Transit and Theodolite
1. Telescope
The telescope is fitted with an object glass, diaphragm and eyepiece Telescope w placed at right angles to the horizontal axis. It can be rotated in a vertical plane about its transverse horizontal axis. A clamp is provided for fixing the telescope is any position in the vertical cicle A slow motion screw is provided to coincide the target point with the cross-hair.
2. Vertical Scale in Zenithal
The vertical circle is a full 360 scale. It is mounted within one of the standards with its centre co-linear with the trunaien axis. It is used to measure the angle between the line of sight (collimation axis) of the telescope and the horizontal. This is known as the vertical angle.
Note that th youde of the instrument where the standard containing the scale is found is referred to as the face of the instrument.
3. Vertical Clamp and Tangent Screw.
In onder to hold the telescope at a particular vertical angle, a vertical clamp is provided. This is located on one of the standards and its release will allow free transiting of the telescope. When clamped, the telescope can be slowly transated using another fine adjustment screw known as the vertical tangent screw.
4. Upper Plate
The upper plate is the base on which the standards and vertical circles are placed Rotation or transiting of the upper plate about a vertical (alidade) axis will also cause the entire standards telescope assembly to rotate in an identical manner. For the instrument to be m correct adjustment, it is therefore necessary that the upper plate must be perpendicular to the alidade axis and parallel to the trunnion axis Also, before the instrument is used, the upper plate must be "levelled" This is achieved by the adjustment of three foot screws and observing a precise tubebubble. This bubble is known as the plate bubble and is placed on the upper plate.
5. The Lower Plate
The lower plate is the base of the whole instrument. It houses the foot screws and the bearing for the vertical axis It is ngidly attached to the tripod mounting assembly and does not move
6. Leveling Head
The leveling had consists of two parallel triangular plates known as tribrach plates The upper tribrach 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 the modern the shifting head is provided to centre the theodolite quickly and accurately. A leveling head has three distinct functions:
i. To support the main part of the instrument To attach the theodolite to the tripod.
ii.To provide a means for leveling the theodolite.
7. Horizontal Scale (or Horizontal Circle)
The botizontal circle is a full 360 scale It is often placed between the upper lower plates with its centre co-linear with the vertical axis. It is capable of independent rotation about the trunnion axis so that any particular direction may arbitrarily set to read zero.
It is used to define the horizontal direction in which the telescope is sighted Therefore, a horizontal angle measurement requires two horizontal scale reading taken by observing two different targets.
The difference between these readings will be the horizontal angle subtended by the two targets at the theodolite station.
8. The Upper Haricotal Clamp and Tangent Screw
The upper horizontal clamp is provided to clamp the upper plate to the horizon circle Once the clamp is released the mstrument is free to traverse through 360 around the horizontal circle When clamped, the instrument can be gradually tramited around the cache by use of the upper horizontal tangent screw. It is the upper clamp and gent screw which are used during a sequence or "round" of borizontal angle moments
9. The Lower Hortontal Clamp and spent Screw
The lower horizinal clamp is provided to clamp the horizontal circle to the lowe plate Once the clamp is released the circle is fror to rotate about the vertical axis When clamped the buriental circle can be gradually rotated using the lower horizontal tangent screw The lower clamp and tangt screw must only be used a the start of a game or "round" of honontal angle moments to set the first adding (if so disered).
10. Fire Ring wil Optical Micrometer.
Modern strms usually have one eyepiece for reading both circles. It is usually located on one of the standards The vortical and berimatal circles require illumination in onder u read them. This is usually provided by small circular mirrors which can be angled and rotated to reflect maximum light onto the circles.
11. Optical plumb
Unlike optical levels, thedolites must be set up over fixed control stations, often defined by wooden pegs and sails. Positioning of the instrument must be achieved to nail head sourcy Modem instruments have an optical plumb to achieve this It consists of an eyepiece set in the lower pline The line of sight through the cypce which is reflected vertically downwards beneath the instrument by means of's prms precily in fine with the vertical axis.