What is a Theodolite?

A theodolite is an optical instrument that can measure angles between any predetermined points along a horizontal and vertical plane. It has been traditionally used for land surveying, but it has also been used for the construction of buildings and infrastructure as well as for other more specialized applications (such as meteorology and rocket launching).

It has a mounted telescope that can be rotated both horizontally and vertically, and it has angular readouts that indicate the orientation of the telescope. They can also be used to relate the first sighted point through the telescope to the detection of other points from the same position. These angles can be measured with an incredible degree of accuracy (down to microradians or seconds of arc). And from these readings, a plan can be drawn or objects can be placed in accordance with an existing design. Modern theodolites have evolved into what are known as “total stations,” which allow you to measure distances and angles electronically and can be stored into a computer for further processing.

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How a Theodolite Works

A theodolite combines the functions of an optical plummet (also called a plumb bob) and a spirit level by incorporating a set of graduated circles that can be used to determine horizontal and vertical angles, which are needed for surveying. The optical plummet makes sure the instrument is placed close to the exact vertical above the survey point, while the built-in spirit level makes sure the device is level to the horizon. The graduated circles (one vertical and one horizontal) allow you to measure angles, so you can survey specific points.

Where Theodolites Can Be Used

Theodolites are primarily used for surveying, but they can also be used in the following applications:

Navigation.
Meteorology.
Laying out building corners and lines.
Measuring layout angles and straight lines.
Aligning wood frame walls.
Forming panels.
Plumbing columns or building corners.

Theodolites can have many advantages compared to other types of leveling instruments. Some of them include but may not be limited to:

Improved accuracy.
An internal optical system that can be magnified.
The ability to take electronic readings.
Horizontal circles can be instantly zeroed in or set to a different value.
Horizontal circle readings can be taken to the left or right of zero.
Reduces the need for repeat readings.

The internal optical device allows you to take more accurate readings than many other instruments. And because you don’t have to take as many repeat readings, you can get what you need more quickly.

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Types of Theodolites

There are two types of theodolites: digital and non-digital. While the non-digital theodolite is rarely used anymore, a digital theodolite has a telescope that’s mounted to a base and a screen that displays the measurements of horizontal and vertical angles. Performing this type of theodolite survey is convenient because the digital readouts can replace the graduated circles (which will allow you to take more accurate measurements).

Parts of a Theodolite

Like many other leveling instruments, a theodolite has a mounted telescope that’s attached to a base. The telescope has a sight located on the top of it, which you can use to align your target. It has a focusing knob that can be used to make the object clearer, and the telescope has an eyepiece that you can look through to find your desired target. An objective lens is located on the other end of the telescope, which can be used to find an object. And with the help of the mirrors located inside, you can magnify it if you need to.

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How to Use a Theodolite

To perform a theodolite survey of a specific object, you can take the following steps:

Mark the point where the digital theodolite will be set up with a surveyor’s nail or stake. This will be the basis for which you'll measure angles and distances.

Set up the tripod, while making sure its height allows you to set up the instrument at eye level. The centered hole of the mounting plates should be over the nail or stake.

Drive the tripod’s legs into the ground by using the brackets on the sides of each leg.

Mount the theodolite to the top of the tripod by screwing it in place with the mounting knob.

Measure the height between the instrument and the ground, which will be used as a reference to other stations.

Level the theodolite by adjusting the tripod legs, while looking at the bulls-eye level. You can make slight tunings with the leveling knobs to get it exactly where you need it to be.

Adjust the small sight (called a “vertical plummet) located on the bottom of the theodolite by using the knobs on the bottom. This will make sure the instrument is kept over the nail or stake.

Aim the crosshairs on the main scope at the point you want to measure and use the locking knobs on the side of the instrument to keep it in place.

Record the horizontal and vertical angles by looking through the viewing scope located on its side.

If you’re looking for a place where you can find a quality theodolite you can use for surveying, be sure to take a look at what we have at Engineer Supply.