What is a Theodolite
The theodolite is an instrument that replaced the compass and the bubble level. A theodolite is a precision instrument used to measure angles in the horizontal and vertical planes. Theodolites are primarily used for surveying applications and have been adapted for special purposes in areas such as metrology and rocket launch technology. A modern theodolite consists of a movable telescope mounted on two perpendicular axes, the horizontal or pivot axis and the vertical axis. When the telescope is aimed at a target, the angle of each of these axes can be measured with great precision.
It is a widely used instrument for measuring horizontal and vertical angles. It is used for prolonging a line, for leveling, and even for indirect distance measurement (tachymetry). Angles up to 20” can be read with a vernier. Accurate theodolites are available that can read angles to within 1 “. They use the optical principle for more precise instruments.
Types of Theodolite
There are two types of theodolites: digital and non-digital (vernier). While non-digital theodolite is rarely used, a digital theodolite has a telescope that is mounted on a base and a screen that displays measurements of horizontal and vertical angles. Performing this type of theodolite reading is convenient because digital displays can replace stepped circles (allowing you to make more accurate measurements).
Uses of Theodolite
Theodolites are primarily used for surveying, but they can also be used in the following applications:
- Navigation.
- Meteorology.
- Laying out building corners and lines.
- Measuring layout angles and straight lines.
- Aligning wood frame walls.
- Forming panels.
- Plumbing columns or building corners.
Components of a Vernier Theodolite
1. Telescope: A telescope is mounted on a horizontal axis (Trunnian axis) and can therefore rotate in the vertical plane. Its length varies from 100 mm to 175 mm and its diameter is 38 mm at the end of the lens. Its function is to provide a line of sight.
2. Vertical circle: A graduated vertical circle with an accuracy of 20 ′ is rigidly connected to the telescope and therefore moves with it when the telescope is rotated vertically. The divisions are in the quadrant system, the 0-0 line is horizontal
3. Vernier frame: It is a T-shaped frame that consists of a vertical arm and a horizontal arm. Using the clamping screws, the vertical frame and thus the telescope can be clamped at the desired angle. Vertical frame is also known as T frame or index frame. The vernier arm is called the index arm. They are equipped with glass magnifiers. The bubble pipe is installed above the horizontal arm.
4. Standards or A-Frame: The frames that support the telescope are shaped like the English letter “A”. This frame allows the telescope to rotate around its trunnial axis in the vertical frame. The T-frame and clamps are also attached to this frame.
5. Top plate: The top plate has the standards on its top. At the bottom, it is attached to an inner axis which rotates in the outer axis of the bottom plate. With the upper clamp, the upper plate can be attached to the lower plate. With tangential screws, it is possible to generate a slight relative movement between the two plates even after tightening. Two diametrically opposed verniers fixed on the upper plate make it possible to read the horizontal divisions of the circle. They are equipped with magnifying glasses.
6. Lower Plate: The bottom plate attached to the outer spindle has a pitch circle on its beveled edge. The graduations are with a precision of 20. It can be clamped in any position with lower pliers. When the upper clamp is locked and the lower clamp is released, the two plates rotate together. When the upper clamp is released and the lower clamp locked, the upper plate rotates on its own. This mechanism is used when measuring the horizontal angle.
7. Plate Level: One or two plate level tubes are mounted on the top plate. When the two-level tubes are provided, they are perpendicular to each other, one of which is parallel to the pivot axis. These planes help to make the vertical axis of the instrument truly vertical.
8. Leveling head: It consists of two parallel triangular plates called tribratch plates. The upper tribratch plate has three leveling screws – each supported by an arm of the tribratch plate. By actuating screws, leveling of the top plate and thus of the telescope can be ensured. The lower base can be integrated into a tripod head.
9. Tripod: The theodolite is always mounted on a tripod. The tripod legs can be solid or framed. At the lower end, the feet are fitted with steel pads in order to have a good grip on the ground. The top of the tripod has an external screw to which the lower tribratch plate can be screwed. When not in use, the tripod head can be protected by a steel cap provided for this purpose.
10. Plumb bob: In the middle of the lower base plate is a hook from which a plumb bob can be hung. It allows the exact centering of the theodolite on a station.
11. Shifting head: It is located under the bottom plate. One plate slides over another over a small area of about a 10mm radius. Both plates can be clamped in the desired position. It facilitates the exact centering of the instruments.
12. Magnetic compass: Some theodolites have a magnetic compass attached to one of the strands. This is useful if the measured values are to be recorded with magnetic north as the meridian.
How to Use a Theodolite
• Mark the location where the theodolite is to be installed with a nail or survey stake. On this basis, you measure angles and distances.
• Set up the tripod ensuring that its height allows you to mount the instrument at eye level. The hole centered on the mounting plates should be above the nail or stake.
• Press the tripod legs into the ground using the brackets on the sides of each leg.
• Mount the theodolite on top of the tripod by screwing it in firmly with the fixing knob.
• Measure the height between the instrument and the ground, which will serve as a reference for the other stations.
• Align the theodolite by adjusting the tripod legs while looking at the height of the viewing window. You can use the leveling buttons to create light signals to get them exactly where you need them.
• Adjust the small sight (called “vertical plumb”) at the bottom of the theodolite using the bottom knobs. This will ensure that the instrument is held on the nail or stake.
• Align the reticle of the main telescope with the point to be measured and use the locking knobs on the side of the instrument to hold it in place.
• Record horizontal and vertical angles by looking through the side viewing tube.
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References-
1.https://www.engineersupply.com/What-is-a-theodolite.aspx