Theodolite

There are many precision instruments out there. Each one of them has its specific uses. In this article, we will talk about one such instrument. Namely, theodolite. We will tell you what it is, its function, how it works, its parts, and how to use it.

What Is It?

Let’s start with the definition first. What is it exactly? Put it simply, it is a precision instrument used to measure angles in the vertical and horizontal planes. The basic surveying instrument has no known origin, said to be related to Leonard Digges, a 16th-century mathematician from England.

The precision instrument consists of many parts, which we will explain one by one in the later section. Traditionally, it is used for land surveying. Nowadays, theodolite is used in various fields, including building and infrastructure construction and even meteorology and rocket launching.

Function

The function of a theodolite is to help to measure angles in the vertical and horizontal angles.

How Does It Work?

The precision instrument works by combining optimal plummets, a bubble level, and graduated circles to find horizontal and vertical angles in surveying.

The optical plummet ensures that the instrument is placed as near as possible to exactly vertical above the survey point. The bubble or spirit level ensures that the instrument is level to the horizon. The graduated circles, one horizontal and one vertical, allow the user to survey for angles.

Theodolite Parts

theodolite

Leveling Head

This part may consist of either a tribrach plate (a plate that has three arms carrying a leveling screw) or two circular plates or parallel plates.

The leveling head has three functions:

  • As a bearing for the outer spindle.
  • As the mean of attaching the theodolite to the tripod
  • As the mean of leveling the precision instrument.

Lower Plate

The lower part consists of a horizontal circle that has whole circle graduation (that is, from 0 degrees to 360 degrees in a clockwise direction). 

Perpendicular to it is the outer hollow spindle, a part that encloses the inner spindle. The lower plate is locked to the leveling head. It can move using a tangent screw and clamp screw.

Spindles

There are two spindles (also called axes or centers). One of the spindles is inside the other. These two spindles are placed in a way that they are co-axial, thus forming the instrument’s vertical axis.

The outer spindle is hollow and its interior has a conical shape. This is to accommodate the inner axis, which is conical and solid.

Upper Plate

Also known as the vernier plate, the upper plate is attached to the inner axis. It has tangent and clamp screws, allowing it to be clamped with the lower plate.

If both plates are clamped together and the lower clamp loosened, the instrument can be orated about the outer axis. If the upper plate is loosened and the lower plate is clamped, it can be rotated about the inner axis.

Standards

These are the frames that not only support the telescope but also allow it to rotate about the vertical axis. In general, the standards have an A-like shape. This is why they are often referred to as A-frame.

Level Tube

Two-spirit levels are fixed on the vernier’s plate upper surface. These spirits are at angles to each other with one of them placed parallel to the horizontal axis.

Compass

A compass box is attached to the theodolite. The box is usually either trough or circular type. A trough compass is screwed to one of the standards or attached beneath the scale plate while a circular compass is mounted on the upper plate.

Telescope

This is the focusing instrument used to see an object. The telescope can be rotated about horizontal axis in the vertical plane. It has graduations which have an accuracy of up to 20 degrees.

Vertical Circle

This part can be found on the telescope. It moves along with the telescope. Its function is for measuring the vertical angle.

T-frame

Also known as the vernier frame, the T-frame consists of two arms, one horizontal and one vertical. The horizontal arm is for measuring vertical angles while the vertical arm is for locking the telescope at the desired level.

Plumb Bob

A plumb bob is used to center the theodolite exactly over a station point.

Tripod

It is a stand on which the instrument is mounted. The legs of the tripod may be framed or solid, with solid legs being the most common. At the legs lower end there are pointed steel shoes, which allow the user to press the tripod firmly into the ground.

How to Use Theodolite

  1. First, start by marking the point at which the instrument will be set with a stake or a surveyor’s nail. This point will be the basis for measuring distances and angles.
  2. Set up the tripod. Adjust the tripod’s height so that it allows the instrument to be eye-level. The centered hole of the mounting plate should be placed over the stake or nail.
  3. Use the brackets on the tripod legs to drive the legs into the ground.
  4. Mount the instrument on the tripod. Use the mounting know to screw it in place.
  5. Measure the height between the theodolite and the ground. Take note of the height as it will be used as a reference to other stations.
  6. Adjust the tripod legs so that the instrument is level. Use the bulls-eye level.
  7. Adjust the vertical plummet or small sight on the bottom of the instrument. This part is what allows you to make sure that the instrument stays over the stake or nail. Use the knobs on the bottom to adjust the plummet.
  8. Next, use the main scope and aim its crosshairs at the point you want to measure. To keep the instrument aimed at the point, use the locking knobs on the side.
  9. Record the vertical and horizontal angles using the viewing scope on the instrument’s side.

A theodolite is a precision instrument often used for surveying applications. The precision instrument, however, can also be used for specialized purposes. For example, building and infrastructure construction and even meteorology and rocket launch technology. It has many adjustable parts.

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