Table of Contents
Theodolite
Learn about the theodolite, a precision instrument used in surveying to measure horizontal and vertical angles. Explore its different types, such as transit and non-transit theodolites, and understand key terms like centering, transiting, and changing face. Discover the various adjustments, including temporary and permanent, and gain insights into essential components like the telescope, bubble tube, and diaphragm.
what is theodolite
The theodolite is a precise instrument primarily used for accurately measuring horizontal and vertical angles, with a precision of up to 10″ or 20″, depending on its least count. Due to its versatility, the theodolite is often referred to as a universal instrument.
The following are a few of the purposes for which the theodolite is utilized:
1. Measuring horizontal angles
2. Measuring vertical angles
3. Measuring deflection angles
4. Measuring magnetic bearings
5. Measuring the horizontal distance between two points
6. Finding the vertical height of an object
7. Finding the difference in elevation between various points.
8. Ranging a line
Theodolites may be of two types
- transit theodolite, and
- non-transit
In a transit theodolite, the telescope can rotate through a full 360-degree revolution around its horizontal axis in the vertical plane.
In a non-transit theodolite, the telescope cannot complete a full 360-degree revolution in the vertical plane. However, it can rotate to a limited extent to measure angles of elevation or depression.
Theodolites are further classified as:
1. Vernier theodolites: Equipped with a vernier scale.
2. Micrometer theodolites: Fitted with a micrometer for more precise readings.
The size of a theodolite is defined by the diameter of its primary horizontal graduated circle. For example, in a “10 cm theodolite”, the diameter of the main graduated circle is 10 cm, In engineering survey, 8 cm to 12 cm theodolites are generally used.
Definitions
1. Centring
The process of positioning a theodolite precisely over a station mark using a plumb-bob is called centring. The plumb-bob is hung from a hook attached beneath the vertical axis of the instrument.
2. Transiting
The act of rotating the telescope around its horizontal axis within the vertical plane by 180Β° is known as transiting. In simpler terms, transiting causes a change in face of the telescope.
3. Face Left
Face left implies that the vertical circle of the theodolite is located on the observer’s left side while taking measurements.
The measurement taken in this position is referred to as a face-left observation.
4. Face Right
Face right refers to the situation where the vertical circle of the theodolite is positioned on the right side of the observer while taking readings.
The reading taken when the vertical circle of the theodolite is positioned on the observer’s right side is called a face-right observation.
5. Telescope Normal
The face-left position is also called ‘telescope normal’ or ‘telescope direct’. This position is sometimes referred to as bubble up.
6. Telescope Inverted
The face-right position is also referred to as ‘telescope inverted’ or ‘telescope reversed’. This position is sometimes referred to as bubble down.
7. Changing Face
Changing face refers to the process of moving the vertical circle of the theodolite from one side of the observer to the opposite side.
8. Swinging the Telescope
This refers to the rotation of the telescope in a horizontal plane. It is termed ‘right swing’ when the telescope rotates clockwise and ‘left swing’ when it rotates counterclockwise.
9. Line of Collimation
It is an imaginary line that passes through the intersection of the cross-hairs at the diaphragm and the optical center of the objective lens, extending in a straight line. This line is called the line of sight.
10. Axis of the Telescope
This axis is an imaginary line passing through the optical center of the objective lens and the optical center of the eyepiece. It is known as the line of collimation.
11. Axis of the Bubble Tube
It is an imaginary line that is tangent to the longitudinal curve of the bubble tube at its midpoint. This line is known as the axis of the bubble tube.
12. Vertical Axis
It is the axis around which the telescope rotates in the horizontal plane and is referred to as the horizontal axis.
13. Horizontal Axis
It is the axis around which the telescope rotates in the vertical plane. It is also known as the turnnion axis.
14. Temporary Adjustment
The process of positioning the theodolite over a station when taking any observation is called temporary adjustment. This adjustment is required for every setup of the instrument. (See Sec. 9.5 for a detailed description.)
15. Permanent Adjustment
When the desired alignment between the fundamental lines of a theodolite is disrupted, certain procedures are followed to restore this relationship. This adjustment is known as permanent adjustment. (This is described in detail in Sec. 9.21.)
16. Least Count of the Vernier
This is the difference between the value of the smallest division on the main scale and the smallest division on the vernier scale. It is the smallest value that can be measured by a theodolite.
17. Magnification or Magnifying Power of Telescope
The magnifying power of a telescope is the ratio of the focal length of the objective lens to that of the eyepiece.
18. Diaphragm
The diaphragm is a brass ring with cross-hairs or a glass disc containing finely engraved lines. It is positioned by turning four capstan-headed screws and can be adjusted up, down, or sideways as needed. It is mounted in front of the eyepiece. The cross-hairs may be made of spider web or fine platinum wire; they may also be in the form of a fine scratch mark engraved on glass.
19. Sensitiveness of Bubble Tube
The ability of a bubble tube to show a very small deviation of the bubble from its horizontal position is termed as the sensitiveness of the bubble tube. It depends on the following factors:
(a) The radius of curvature of the inner surface of the tube,
(b) The diameter of the bubble tube,
(c) The length of the bubble tube, and
(d) The viscosity of the liquid used inside the tube.
Sensitiveness is expressed in terms of the angle by which the axis of the bubble will be tilted for a deviation of the bubble by one small division.
Transit Theodolite
The following are the key components of a theodolite:
1. Trivet
It is a circular plate with a central threaded hole for securing the theodolite to the tripod stand using a wing nut. This is also referred to as the base plate. Three foot screws are attached to the plate through a ball-and-socket mechanism.
2. Foot Screws
These are meant for levelling the instrument. The lower part of the foot screws is fixed in the trivet using a ball-and-socket arrangement, while the upper threaded part passes through the threaded hole in the tribrach plate.
3. Tribrach
It is a triangular plate with three foot screws at its corners.
4. Levelling Head
The trivet, foot screws, and tribrach together form a structure known as the levelling head.
5. Spindles
The theodolite comprises two spindles or axesβone inner and one outer. The inner axis is solid and conical, while the outer axis is hollow. The two spindles are coaxial.
6. Lower Plate
The lower plate is connected to the outer axis and is also called the scale plate. It is bevelled, and the scale is graduated from 0 to 360Β° in a clockwise direction. Each degree is subdivided into two, three, or four divisions; consequently, the value of one small division may be 30′, 20′, or 15′, respectively.
The lower plate is equipped with a clamp screw and a tangent screw to control its movement. When the clamp screw is tightened, the plate is fixed to the outer axis. For fine adjustment of the lower plate, the tangent screw is rotated as needed. The size of the theodolite is specified based on the diameter of the lower plate.
The trivet, foot screws, and tribrach together form a unit known as the levelling head.
7. Upper Plate
The upper plate is fitted with vernier scales A and B and is attached to the inner axis. Its movement is controlled by the upper clamp screw and the upper tangent screw. When the clamp screw is tightened, the vernier scales are locked to the inner axis, and fine adjustments are made by rotating the tangent screw.
8. Plate Bubble
Two plate bubbles are fixed at right angles to each other on the upper surface of the vernier plate, with one bubble aligned parallel to the horizontal axis of the theodolite. Occasionally, only one plate bubble is provided on the vernier plate. These bubbles are used for leveling the instrument while measuring horizontal angles.
9. Standard or A-frame
Two frames (shaped like the letter ‘A’) are provided on the upper plate to support the telescope, the vertical circle and the vernier scales. These frames are known as standards or A-frames.
10. Telescope
The telescope is mounted between the standards at right angles to the horizontal axis and can rotate around its horizontal axis in a vertical plane. It is equipped with a focusing screw, a clamping screw, and a tangent screw.
11. Vertical Circle
The vertical circle is firmly attached to the telescope and moves along with it. It is divided into four quadrants, with each quadrant graduated from 0 to 90Β° in opposite directions. The ‘zero’ marks are located at the ends of the horizontal diameter of the vertical circle.
The line joining the ‘zero’ marks, corresponds to the line of collimation. The subdivisions of the vertical circle are similar to these of the horizontal circle. The vertical circle can be clamped or finely adjusted with the help of the clamping screw and the tangent screw provided along with the telescope.
12. Index Bar or T-frame
The index bar is mounted on the standard in front of the vertical circle. It features two verniers (C and D) at the ends of its horizontal arm. The vertical leg of the index bar is equipped with a clip screw at its lower end, allowing the altitude bubble to be centered.
13. Altitude Bubble
A long sensitive bubble tube, called the altitude bubble, is mounted on top of the index bar. The bubble is centered using the clip screw while measuring the vertical angle. A mirror is attached above the bubble to aid in observation when the instrument is set up at an elevated height.
14. Compass
Occasionally, a circular box compass is mounted on the vernier scale between the standards. In modern theodolites, an adjustable trough compass or tubular compass can be attached to the standard using a screw. The compass is provided for taking the magnetic bearing of a line.
Principle Of Compass Surveying
This guide provides an overview of compass surveying, focusing on the principles of traversing, the tools involved, and their practical applications. It covers different methods like chain, compass, theodolite, and plane table traversing, and explains how to measure bearings and distances accurately using a prismatic compass. Perfect for anyone looking to understand the fundamentals of this surveying technique Principle Of Compass Surveying.
How to Use a Theodolite? (Complete Guide) For Civil
Theodolite is one of the most essential surveying instruments widely used in civil engineering and construction projects. It is primarily used to measure horizontal and vertical angles with high precision. This blog post is a complete guide on how to use a theodolite effectively.