Theodolite Surveying and Its Types

  

Theodolite surveying is measuring horizontal and vertical angles with a theodolite.

A theodolite is a very accurate instrument to measure the horizontal and vertical distances between two points. It can also be used to extend a line, indirectly measure distances, as a level, and as a tachometer. It is also known as a "Universal Instrument" due to its vast range of applications.

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Types of Theodolite:

There are generally two types of Theodolites:

1. Transit Theodolite: A transit Theodolite is one in which the telescope positioned in the instrument can be rotated 360 degrees around its horizontal axis in a vertical plane.
2. Non-Transit Theodolite: It is the opposite of Transit Theodolite. In this type of Theodolite, the telescope can not be resolved through a complete revolution about its horizontal axis in a vertical plane. It can be rotated to a certain extent to take vertical angles.

Theodolites can also be classified into two categories based on the Scale used in Theodolite:

1. Vernier Theodolite: It comes with a Vernier Scale. In typical surveying activities, Vernier Theodolites are most usually utilized.
2. Micrometre Theodolite: equipped with a micrometer scale

• The diameter of the primary Scale is used to define the size of theodolites; for example, a 10 cm theodolite has a diameter of 10cm.

• 8 cm to 12 cm theodolites is commonly employed in surveys.

Essential Parts of the Theodolite & Their Functions

To understand the instrument, it is necessary to know the parts it is composed of. These parts are given in as follows:

1. Telescope: The theodolite telescope is mounted on a horizontal spindle. To see the objects, they can be rotated about the horizontal axis. The telescope is of the internal focusing variety, which means that the objective lens is fixed in place, and a second double concave (focusing lens) is moved between the diaphragm and the objective.
2. Vertical Circle: The vertical circle is securely attached to the telescope and moves in tandem. It is divided into four quadrants, each with a reading of 0° to 90° in the proper directions. The vertical circle also includes a scale commonly used to take vertical angular measurements.
3. Clamp Screws: The vertical clamp screw is used to secure the telescope and vertical circle at any angle requested. It prevents the telescope from rotating about its horizontal axis. These are primarily found in the instrument's lower plate and are utilized to rotate the instrument about its horizontal axis. There are two clamp screws: the lower clamp screw, which is used to rotate the entire instrument, and the upper clamp screw, which is used to fix the Vernier A and Vernier B to a specific degree (usually 0° and 180°) by turning the upper half of the instrument.
4. Plate Bubble: Two plate bubbles are positioned at right angles on the upper surface of the Vernier plate. One plate bubble is kept parallel to the horizontal axis of the Theodolite and is used to level the instrument horizontally. Another plate bubble is attached to the Theodolite's vertical axis and is used to level the device vertically.
5. Trivet: The rivet is the instrument's lowest point. It is made out of a circular plate with a central, threaded hole in the center for properly attaching the instrument to the tripod support. This plate is also known as the Base Plate. The foot screws are fastened with a ball and socket arrangement above this plate.
6. Foot Screws: These are also known as leveling screws and are used to level the instrument on the ground. There are three-foot Screws that are rotated in a specific manner to level the instrument.
7. Tangent Screws: The instrument is made up of two tangent screws, one in the lower plate and one in the upper plate. The lower tangent screw is used to move the crosshairs very slightly in order to correctly bisect the ranging rod placed at the spot, and the upper tangent screw is used to shift the scale reading very slightly. Both screws are for precise measurements.
8. Vernier Scales: There are two Vernier scales: Horizontal Scale and Vertical Scale. The horizontal Scale is used to take horizontal angles and is attached to the instrument's lower plate, while the vertical Scale is used to take vertical angles and is set on the vertical circle.

Important Definition related to Theodolite Surveying

Telescope Normal

When the vertical circle is to the left of the surveyor and the bubble tube on the telescope is upward, the Theodolite is called telescope normal.

Telescope Inverted

When the vertical circle is to the surveyor's right and the bubble tube on the telescope is downward, the Theodolite is called telescope normal.

Transit

Transit of a theodolite refers to the process of rotating a theodolite telescope by 180 degrees about its horizontal axis (i.e., in the vertical plane).

The telescope will now aim in the opposite direction. It's also known as reversing or plunging.

Swinging

Swinging or swinging of the telescope refers to the process of rotating a theodolite telescope about its vertical axis (i.e., in the horizontal plane).

A right swing indicates a clockwise rotation of Theodolite, while a left swing indicates an anticlockwise revolution.

Changing Face

Changing face refers to moving the telescope from the left to the right face position or vice versa.

The face can be altered by reversing the Theodolite (transit) and rating it by 180-degree (swing) operations.

Temporary Adjustments of a Theodolite

Temporary adjustments of a theodolite refer to the activities necessary at each setup of the Theodolite at a station before taking observations. It includes the following:

Setting up

Theodolite installation procedure:

• Position the tripod over the necessary station.

• Spread the tripod legs so that they form a 60-degree angle with the horizontal.

• With some force, press the shoe of each tripod leg into the ground.

• Remove the Theodolite from its packaging. Remove it from the base and secure it to the tripod.

Centering

The Theodolite is centered so that it is exactly above the station. A plumb Bob and a nail are used to center the piece. The nail is positioned precisely where the device will be put. The Plumb Bob is hung from a hook that is attached beneath the instrument. The tripod legs should then be adjusted to place the dangling plumb bob exactly over the point.

Leveling

Theodolite leveling is done to make the Theodolite's vertical axis really vertical or the horizontal plane fully horizontal.

This can be accomplished by aligning the horizontal bubble tube with any two of the foot screws and then moving the foot screws in the opposite direction to center the bubble.

The bubble is then moved to perpendicular its initial position after being placed in the center. The third foot screws are then moved in either direction to center the bubble. These steps should be carried out multiple times until the bubble is completely centered. In the case of the vertical bubble tube, the same process should be followed.

Focussing the Eye-Piece

Theodolite eyepiece is focused on making the diaphragm's crosshairs crisp and distinct. It is dependent on the surveyor's vision.

• Point the theodolite telescope at the sky or place a white paper in front of the objective lens.

• Rotate the eyepiece in or out gently until the crosshairs show clear.

Focussing the Objective

Theodolite's goal is to get the image of the item into the plane of crosshairs (diaphragm). It is accomplished with the aid of a focusing screw. It is determined by the object's distance.

Measurement of a Horizontal Angle by Theodolite

Horizontal angles can be done by two methods:

1. Repetition Method
2. Reiteration method.

Repetition Method

The repetition method is used mostly in measuring horizontal angles.

Procedure:

• Centering occurs once the instrument has been appropriately leveled.
• After cantering, Vernier A is set to 0° and Vernier B to 180° by loosening the upper clamp screw. Vernier A & B are precisely placed into their readings by rotating the upper tangent screw.
• The upper clamp screw is then tightened, followed by the bottom clamp. At one point, the instrument is rotated to bisect the ranging rod. The bottom tangent screw is used to dissect the range rod precisely.

• Next, the bottom clamp screw is tightened, followed by the upper clamp screw. The ranging rod at the other point is bisected by rotating the telescope clockwise about its horizontal axis. The reading at Scale should be recorded.
• These steps should be repeated three times, with the average value taken.

Measurement of a Vertical Angle by Theodolite

This is another essential anti-function of a Theodolite, the only manual instrument using which vertical angles should be taken. The procedures for taking vertical measurements are as below:

Procedures:

• The instrument's leveling and Cantering of the instrument are done following the methods mentioned above.
• The Vernier A & Vernier B of the vertical Scale will be fixed at 0° and 180°, respectively. 
• The telescope is targeted toward the point from which the vertical angle is to be taken. Tangent screws are used for accurate bisection of the ranging rod at that point.
• Then the telescope is rotated in a vertical plane to bisect the point at some specific elevation.
• After the bisection of the point, the telescope is to be fixed, and the reading is taken, which is the required angle of elevation.

Measurement of a Deflection Angle by Theodolite

While surveying, changes in the direction of the survey line may often occur, due to identifying some obstacles, such as buildings, factories, etc. The deflection angle is the angle formed between the extension of the previous survey line and the new survey line. Deflection angles hold a great value in Surveying operation, as it determines the angle, at which the existing survey line is to be deflected.

The procedure for determining the Deflection angle by theodolite is given below:

Procedure:

• The leveling and Cantering of the instrument are to be done.
• A point Is fixed on the extension line, and another point is fixed on the new survey line. The instrument is placed in a position from where both points are visible.
• The upper clamp is loosened, and the Vernier A & B are fixed at 0° and 180°, respectively. Then the upper clamp is tightened.
• By loosening the lower clamp screw, the point at the extension line is bisected. The lower clamp is then fixed.
• The instrument is turned clockwise by loosening the lower clamp screw, and the point situated at the new survey line is bisected. Tangent screws are used for accurate bisection.
• The reading is to be taken.

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Measurement of Magnetic Bearing by Theodolite

Theodolites are also used for measuring magnetic bearings. The procedure is as follows:

Procedure:

• The instrument needs to be centered and leveled.
• The technique, as mentioned above, fixes Vernier A and B at 0° and 180°, respectively.
• The device must be equipped with a trough compass or circular compass to identify the north direction. The instrument is then set in the north direction by loosening the lower clamp screw. This is referred to as Orientation.
• The point is then bisected using the top clamp screw. The obtained value is the magnetic Bearing of that place.

Ranging a Line using a Theodolite

Theodolites can also be used for ranging a line accurately.

Procedure:

• The instrument must be leveled and centered.
• The telescope is pointed at the point to be measured. 
• Following that, the observer instructs the follower to position a ranging rod between the two points by issuing appropriate commands. When the crosshairs perfectly bisect the ranging rod, the ranging rod should be fixed at that location, completing the ranging operation.