Theodolite Surveying - Principles, Working and Types

Theodolite surveying is the most precise surveying technique in the construction industry that enables the measurement of horizontal and vertical angles of a point on the earth's surface easily. Many modern instruments have upgraded from this standard to make measuring more easier.

Let's learn the principle, working, and types of theodolite used in civil engineering surveys.

What is Theodolite Surveying?

A theodolite survey is a surveying method that measures highly accurate horizontal and vertical angles, using a high-precision instrument called a theodolite. This surveying method helps us lay off horizontal angles on site, locate points on line, assist in prolonging survey lines, establish grades, determine the difference in elevation, setting out curves, and many more applications in construction.

Theodolite surveying is more precise and quick compared to other methods like chain, compass, and plane table surveys. It is commonly used in engineering fields for land surveying, route surveying, and construction surveying.

What is a Theodolite ?

Theodolite is an optical instrument mounted with a moveable telescope for the precise measurement of horizontal and vertical angles. In general, among various components of a theodolite, it primarily combines optical plummets to ensure vertical alignment, a spirit level to guarantee horizontal leveling and graduated circles to measure both horizontal and vertical angles. These components work together to allow surveyors to precisely determine the position of points on the Earth's surface.

Principle of Theodolite Surveying

The principle of a theodolite surveying is based on the measurement of horizontal and vertical angles. By rotating the telescope around its horizontal and vertical axes, surveyors can determine the angular position of a target point relative to the instrument. The measured angles, along with known distances, are used to calculate the precise location of points on the Earth's surface.

Types of a Theodolite

Theodolites may be classified as follows:

1. Primary Classification of Theodolite
2. Secondary Classification of Theodolite

1. Primary Classification of Theodolite

Primary classification of theodolite instrument includes:

1. Transit Theodolite: A theodolite is called a transit theodolite, when the telescope mounted can be transited i.e. it can be revolved through a complete revolution about its horizontal axis in the vertical plane. In other words, we can say, the line of sight of the telescope can be rotated 180 degrees in the vertical plane.
2. Non-Transit Theodolite: This type of theodolite cannot be transited. They were used only for small angular measurements. As they are inferior in utility, it has now become obsolete.

2. Secondary Classification of Theodolite

Based on the type of instrument built in a theodolite to measure the angles, theodolites can be classified into:

1. Vernier Theodolite
2. Micro meter Theodolite
3. Modern Theodolites

1. Vernier Theodolite

A Vernier Theodolite is a type of theodolite that uses verniers to measure the vertical and horizontal angles during a survey. 

Vernier Transit Theodolite

In a transit theodolite, there are two verniers. One is located in the vertical circle and moves along with the telescope in the vernier plane. This vernier measures the vertical angle. 

The second vernier is located in the horizontal circle. The horizontal circle with the vernier, moves with the telescope when it moves horizontally in the horizontal plane. Hence, this vernier measures the horizontal angle.

2. Micrometer Theodolite

If a micrometer is provided in the vertical and horizontal circles of the theodolite to measure vertical and horizontal angle, then it is a micrometer theodolite. Micrometer Theodolites use a micro meter screw to make precise readings on the graduated circles, offering high accuracy but requiring more complex operation.

3. Electronic Theodolites

Electronic theodolites are digital theodolites that utilize electronic sensors and digital displays for direct angle measurement. They are highly accurate, fast, and user-friendly. These instruments utilize advanced technology, such as optical encoders, to measure angles electronically. The measured angles are then displayed on a digital screen, eliminating the need for manual reading and significantly reducing the potential for human error.

Theodolite surveying remains a fundamental technique in civil engineering and surveying. While traditional theodolites, such as vernier and micrometer theodolites, have been used for centuries, modern electronic theodolites offer increased accuracy, speed, and ease of use. By understanding the principles of theodolite surveying and the capabilities of different types of theodolites, engineers and surveyors can ensure the precision and reliability of their measurements.

Read More On: Difference Between Vernier Theodolites and Electronic Theodolites