1.18. Theodolite Surveys

Theodolite is the most versatile instrument used in survey work. It is a multi-purpose equipment which can be used for measuring the angles, prolongation of a straight line, determination of levels and determination of distances. In general, it is used for measuring the vertical and horizontal angles of given objects. Inception of a theodolite was done basically for providing the horizontal and vertical controls in survey work, i.e., the coordinates. For mapping a larger area, locations of ground controls are essentially required which can be determined using angles and other observation.

A theodolite is a critical surveying instrument that helps surveyors measure angles accurately in both horizontal and vertical planes. It serves multiple purposes, including establishing straight lines and determining various measurements essential for mapping. The device's main function is to help create precise points in a larger area, facilitating accurate geographical representations through coordinates.

Think of a theodolite like a pair of binoculars that can not only help you see faraway objects clearly but also help you know the exact angle and distance to them. Just like you might use binoculars to look at landmarks on a mountain range and find their distances, surveyors use theodolites to measure angles and distance to create accurate maps of land.

There are various models of theodolites, such as: (a) Vernier theodolite (b) Optical theodolite (c) Electronic theodolite (or Total Station)

The types of theodolites include:

• Vernier Theodolite: This is a traditional model used for measuring angles with good accuracy using vernier scales.
• Optical Theodolite: This type uses lenses for precise angle measurements, providing improved accuracy.
• Electronic Theodolite (Total Station): This is a modern version that uses digital technology to read and display measurements, making the data easier to collect and error-prone readings less likely.

If we compare types of theodolites to cameras, the Vernier theodolite is like a basic film camera where you need manual settings to take a good photo. The Optical theodolite is more advanced, much like a DSLR that helps focus better and take clearer pictures, while the Electronic theodolite is similar to your smartphone camera that automatically adjusts settings and displays information quickly.

Figure 1.39 shows a sectional view of a typical vernier theodolite. The main parts of a vernier theodolite are: (a) Telescope (b) Upper plate and Lower plate (c) Vertical circle (d) Plate bubble tube (e) Vernier frame (f) Altitude bubble (g) Levelling head (h) Shifting head (i) Tripod (j) Plumb bob

A vernier theodolite consists of several key parts:

• Telescope: Provides a line of sight for measuring angles.
• Upper and Lower Plates: Allow for measurement adjustments.
• Vertical Circle: Used for measuring vertical angles.
• Vernier Frame: Holds the telescope and supports angle measurements.
• Levelling Head: Ensures the instrument is perfectly level.
• Plumb Bob: A weight that helps accurately center the instrument over a point.

Imagine building a model city with different components; the telescope is like your eyes looking at a specific building, the plates are the base and support of the model, and the plumb bob is like a string that helps you ensure everything is situated correctly and straight. All parts are essential for making sure that your model accurately represents the layout of a real city.

Technical terms associated with theodolites include centering, line of collimation, axis of the telescope, vertical axis, horizontal axis, transiting, swinging, and changing faces.

Each technical term plays a crucial role in using a theodolite:

• Centering: Ensuring the theodolite is positioned directly above the survey mark.
• Line of Collimation: The line of sight through the telescope.
• Transiting: Making the telescope point directly opposite.
• Swinging: Moving the telescope horizontally.
• Changing Faces: Switching between left and right observations to eliminate errors.

Consider these terms like setting up a stage for a performance. When centering, you want to be directly in the center of the stage to see the action clearly (centering). The line of collimation is like the imaginary line you follow to focus on the main act (line of collimation). Transiting is similar to turning your head to get a better view (transiting), while changing faces is looking to the left and right to ensure you're taking the best view of everything happening on stage.

Theodolite is generally used for accurately measuring horizontal and vertical angles. For this, the theodolite is centered on the desired station point, levelled and telescope is focused. This process of centering, levelling and focusing is called temporary adjustment of the instrument...

To measure horizontal angles:

1. Set up the theodolite over the station and level it.
2. Focus the telescope to the desired object, bisecting it with the crosshairs.
3. Record readings from the main and vernier scales.
4. Repeat measurements to ensure accuracy using methods like reiteration and repetition to confirm consistency.

Think of measuring angles like taking your aim with a bow and arrow. You need to ensure your stance is correct (centering), your bow is level (levelling), and your focus is sharp on the target (focusing) to hit your intended mark. Similarly, making precise adjustments with theodolite helps ensure your angle readings are accurate.

Theodolite can be used to prolong a given line, such as in alignment surveys. The procedure to prolong a line AB up to Z is given below...

To prolong a line using a theodolite:

1. Position the theodolite at point A and level it.
2. Bisect point B, then set another point C in line with it.
3. Shift to B, take a sight on C to establish D, and repeat until point Z is reached.

This process is similar to playing a game of connect-the-dots. If you're drawing a straight line from A to Z, you need to carefully go point-by-point (A to B, then C, D, and so forth) ensuring each line is straight and properly aligned. The theodolite helps you keep the line straight without any deviations.

The theodolite can be used as a level, if the motion of the telescope in vertical plane is clamped such that the vertical circle verniers, C and D, read zero value...

A theodolite can serve as a leveling tool by clamping its telescope in a horizontal position, allowing surveyors to measure height differences between two points by reading the leveling staff at both locations.

Imagine trying to ensure that a foundation of a building is level. Just like workers check that the ground is flat and even for a stable foundation, surveyors use a theodolite to confirm that different parts of an area are level before construction.

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