The first step of any construction work is surveying and thus it is an important branch of civil engineering. In general, surveying can be defined as the art of determining the relative position of various features above, on or beneath the surface of the earth using different instruments and preparing the maps. One of the important types of such surveying instruments is the Theodolite.
In general, theodolite may be defined as the optical survey instrument that is used to measure the angles between the specified points both in the vertical and horizontal plane.
In other words, theodolite may be defined as the instrument fitted with a small telescope that can move freely in the horizontal and vertical plane to measure the horizontal and vertical angles in surveying. It is extensively used in traversing, land surveying and other infrastructure construction.
Theodolite- An Overview
Theodolite is an optical instrument that consists of a small moveable telescope mounted on it. The telescope is free to rotate around both the horizontal and vertical axis. Theodolite is thus capable of delivering the angular readouts that indicate the orientation of the telescope. The angles thus measured can be used for the positioning of points and preparation of plans and maps. Usually, for the non-transit type theodolite, the rotation of the telescope is restricted to a limited arc. On the other hand, for the transit theodolite, the telescope is short enough to rotate through the zenith.
Figure: Theodolite (Schematic Diagram)
Technical Terms Used in Theodolite
Vertical axis: The axis about which the theodolite rotates in a horizontal plane.
Horizontal axis: The axis about which the theodolite rotates in a vertical plane.
Centring: Process of setting up the theodolite exactly over the ground station point.
Transiting: The process of turning the telescope in the vertical plane.
Swing: Continuous motion of the telescope about the vertical plane.
Face left observation: Vertical circle is on the left at the time of observation.
Face right observation: Vertical circle is on the right at the time of observation.
Changing face: Operation of changing the face of the telescope.
A set: It consists of two horizontal measures, one on the face left and other on the face right.
Fundamental Axes of Theodolite
1. Vertical axis
2. Horizontal or trunnion axis
3. Line of collimation
4. Axis of plate levels
5. Axis of altitude level
Working Mechanism of Theodolite
Theodolite works by the combined mechanism of optical plummets also referred to as the plumb bobs, a spirit or the bubble level and the graduated circles to determine the vertical and horizontal angles. The optical plummets or the plumb bobs ensure that the theodolite is placed as close to exactly vertical above the point of the survey. The spirit level ensures that the theodolite is exactly levelled to the horizontal. Two types of graduated circles are provided namely the vertical graduated circle and horizontal graduated circle for measuring the angles.
Parts of a Theodolite in Surveying
A theodolite consists of a small telescope that is mounted on it. The telescope consists of sight on the top of it that is used to align the target. The theodolite also consists of a focusing knob that is used to make the object clear. The telescope of the theodolite is fitted with an eyepiece that the user looks through to find the target being sighted.
An objective lens is also fitted on the telescope on the opposite end of the eyepiece. The objective lens is provided to sight the object, and also with the help of the mirrors inside the telescope, allows the object to be magnified. The base of the theodolite is threaded for easy mounting on a tripod. Figure 3 shows the different parts of the theodolite.
Figure: Parts of a Theodolite
Types of Theodolite in Surveying
The theodolites can be classified on the following basis:
1. Based on the Constituent Parts
Depending upon the constituent parts of a theodolite, the theodolite can be classified into digital theodolite and non-digital theodolite. The non-digital theodolite is the ancient or the traditional type of theodolites. This type of theodolite is manual and is rarely used nowadays. The digital theodolites are simply the advancement of the non-digital theodolites.
The digital theodolites consist of a telescope that is mounted on a base and an electronic readout screen that is used to display horizontal and vertical angles. Digital theodolites are commonly used because the digital readouts take the place of traditional graduated circles and this creates more accurate readings.
2. Primary Classification of Theodolite
The primary classification categorizes the theodolite into transit and non-transit type theodolites. The theodolite whose telescope can be transited i.e. revolved through a complete revolution about its horizontal axis in the vertical plane is known as a transit type theodolite.
On the other hand, the theodolite whose telescope cannot be transited is known as non-transit type theodolite. The transit type theodolites are the most commonly used type of theodolite.
Applications of Theodolite in Surveying
The major application areas of theodolite can be listed as follows:
3. Surveying and its applications
4. Measurement as well as laying out the angles and straight lines
5. Aligning of walls
6. Formation of Panels
7. Plumbing building corners, column etc.
8. Tacheometric Surveying
9. Finding the difference in the level
10. Ranging Curves
Uses of Theodolite in Surveying
Some of the uses of theodolite in surveying can be listed as follows:
1. To determine the vertical and horizontal angles.
2. To determine the difference in elevation between the points.
3. To locate the points on a line.
4. To prolong or extend the survey lines.
5. To set out the grades and ranging of curves.
Temporary Adjustment of Theodolite in Surveying
The temporary adjustment of the theodolite mainly includes a set of operations that are carried out to make it ready for taking the observations. The temporary adjustment includes the following series of steps:
1. Setting up:
The first step of the temporary adjustment is the setting up of the theodolite. It includes the process of fixing the theodolite onto a tripod along with approximate levelling and centring over the station mark.
Centring is the process of bringing the vertical axis of the theodolite exactly over the station mark using a centring plate. The centring plate is also known as a tribrach.
It is the process of levelling of the base of the instrument to make the vertical axis vertical usually with an in-built bubble-level.
Focusing includes the removing parallax error by proper focusing of the objective and the eye-piece.
Using of a Theodolite in Surveying
The theodolite can be used for reading the observations by following the below-mentioned steps:
1. First of all the point at which the theodolite is to be set is marked with a stake or surveyor’s nail. This point acts as the basis for measuring the angles and the distances.
2. The tripod is then set up over the marked point. The height of the tripod must be adjusted suitably. The centred hole of the mounting plate should be over the marked point.
3. The legs of the tripod are driven into the ground using the brackets present on the side of each leg.
4. The theodolite is then mounted on the theodolite. It is done by placing the theodolite on top of the tripod and screwing it in place with the help of the mounting knob.
5. The height between the ground and the instrument is then measured to use as the reference for other stations.
6. The theodolite is then duly adjusted.
7. The vertical plummets by using the knobs are then adjusted to ensure that the instrument remains over the marked point.
8. The crosshairs are aimed in the main scope at the point to be measured using the locking knobs present on the side of the theodolite. The horizontal and the vertical angles are recorded using the viewing scope found on the theodolite’s side.
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