What is Theodolite Surveying? Advantages and Disadvantages of Theodolite Surveying

What is Theodolite Surveying?

Theodolite surveying is the process of using a theodolite to measure horizontal and vertical angles in order to calculate distances and heights. This process is used to survey land and create maps.

Theodolite surveying is a necessary part of civil engineering and construction projects. It is a process of recording data about an area for the purpose of mapping.

Theodolites are used in this process because they are accurate, allow measurements at 360 degrees, have variable magnification settings, and can be used hands-free.

What is Theodolite?

Theodolite is a surveying tool used to determine horizontal and vertical angles using a tiny low telescope that can move within the horizontal and vertical planes.

It is an electronic device that resembles a miniature telescope. It is widely used in the housing industry and for measuring vertical and horizontal angles for scaling functions.

The precision with which these angles can be measured ranges from 5 minutes to 0.1 seconds. It is commonly used in triangulation networks.

Theodolites are used everywhere, from construction sites to major road intersections. It measures angles using age-old pure mathematics principles and aids surveyors in establishing precise locations.

Theodolite Surveying Uses.

Theodolite is used for a variety of purposes, but it is most commonly used for measuring angles and scaling points in constructional works.

Theodolites, for example, are used to determine highway points due to the escalating edges of massive buildings.

Theodolite produces more curved readings, using paradoxical faces and swings or different positions for perfect measuring survey, depending on the nature of the job and the required accuracy.

Theodolite is a highly accurate surveying instrument used primarily for:

• Measuring horizontal and vertical angles.
• Finding points on a line
• Identifying level differences.
• Extending survey lines.
• Variable curves
• Organizing grades
• Tacheometric Examination

How to Use a Theodolite

1. Use a surveyor’s nail or a stake to mark the location of the theodolite. This point serves as the starting point for calculating angles and distances.
2. Mount the tripod. Check that the tripod’s height allows the instrument (the theodolite) to be at eye level. The mounting plate’s centered hole should be over the nail or stake.
3. Using the brackets on the sides of each leg, drive the tripod legs into the ground.
4. Place the theodolite on top of the tripod and secure it with the mounting knob.
5. Determine the distance between the ground and the instrument. This will be used as a point of reference for other stations.
6. Adjust the tripod legs and use the bulls-eye level to level the theodolite. To get it just right, use the leveling knobs to make minor adjustments.
7. Adjust the small sight (the vertical plummet) on the theodolite’s bottom. The vertical plummet enables you to keep the instrument over the nail or stake. Using the bottom knobs, adjust the plummet.
8. Aim the main scope’s crosshairs at the point to be measured. To keep the theodolite aimed on the point, use the locking knobs on the side.

Using the viewing scope on the side of the theodolite, measure the horizontal and vertical angles.

Classification of Theodolites

Theodolites may be classified as;

1. (a) Transit Theodolite. – It is a theodolite that is used to measure horizontal and vertical angles.

(b) Non-Transit Theodolite- It is used for measuring horizontal angles only.

Transit Theodolite vs Non-Transit Theodolites

The main features of a transit theodolite are that it can be used for measuring horizontal and vertical angles, it has a graduated bubble level on the telescope and can be used with a vertical telescope as well.

It may also have an attached telescope level to enable the measurement of vertical angles in separate stations. These features are not present in non-transit theodolites.

The differences are:

1. The transit theodolite is accurate to 0.06 second, while the non-transit model has accuracy of 5 minutes.
2. The transit theodolite can measure horizontal angles only, while it can measure vertical angles as well in the non-transit models.
3. Transit theodolites are more expensive than non- transit models but are more durable and are preferred for wider use.
4. In addition to the adjustment knobs on the vertical plummet, it has adjustable leveling legs.
5. The transit theodolite has three telescopes connected to a graduated bubble level, while in the non-transit models only two telescopes are available and a level is attached to this telescope.
6. The vertical plummet on the transit theodolite can be adjusted without any intervention of turning its screws, while in non-transit models these adjustments require turning of screws.
7. Telescopes are adjustable in the transit theodolite, while in the non-transit models only one is available.
8. The transit model has a screw for focusing, which is not present in the non-transit model
9. The transit model has a threaded arm for attachment of the level and is more durable than the non-transit models
10. Transit Models’ sighting scope can be adjusted and focused on vertical lines. In non-transit models, only a viewing scope is available.
11. The transit model has a tripod with leveling legs, while in non-transit models, it has a tripod without leveling legs.
12. In the transit models, the telescope can be replaced by a vertical level as well in order to measure vertical angles in separate stations. In non- transit models, the level is attached to the telescope and cannot be replaced by a vertical level for measuring vertical angles in separate stations.
13. Vernier Theodolites. – are used for measuring horizontal and vertical angles. It is an instrument that has a vertical telescope attached to it. It also has a vernier scale, which enables the measurement of very small angles.

The graduations on the vernier scale are so close that they cannot be distinguished with bare eyes, although it is possible to do this with the help of a magnifying glass. With the help of these graduations, it is possible to measure angles with precision ranging from 0.1′ to 1 second.

These instruments are more useful for studying the relationship between lengths and angles than the transit models. This is because the no-travers graduations are large enough to be used for measuring very small angles.

Vernier Theodolites can also be fitted with a vertical level with a vernier scale to make it suitable for measuring vertical angles in separate stations.

1. (b) Micrometer Theodolites. – Micrometer Theodolites are also known as vernier theodolite. They are used for measuring horizontal and vertical angles.

These are instruments that have a very important feature, vernier scales. The graduations on a micrometer theodolite’s vernier scale enable the measurement of very small angles, which make them highly useful for measuring distances and lengths with precision ranging from 1 mm to 1 meter.

The vernier scale is always present on micrometer theodolites. Micrometer Theodolites can also be fitted with a vertical level with a vernier scale to make it suitable for measuring vertical angles in separate stations.

The theodolite has a horizontal and vertical telescope, and the vernier scale is attached to this telescope. There are two modes of operation:

1. Manual Mode- It gives an exact reading by mean of a bubble level and then converts it into degrees.
2. Automatic Mode- It gives an approximate reading by mean of the vernier scale, and then converts it into degrees.

History of Theodolite Surveying

Theodolites were used in surveying and astronomy at least since the 16th century when they were introduced.

The word itself is derived from the Greek terms ‘theos’ and ‘dolos’, meaning “god” and “to mislead”, respectively, because of its deceptive power.

The first known theodolite was made by Leonard Digges in 1551. He called it a “Scymitar.” It had a compass that could be set to any angle, and a separate level for cross-leveling.

Toward the end of the 16th century, a Polish mathematician named Tadeusz Kościuszko invented a type of “Dodekamer” theodolite with 12 graduations. Theodolites were used in surveying and astronomy for the next several hundred years.

Theodolites were expensive and were only used by professional surveyors and astronomers. They were accurate to only 1 or 2 minutes of arc, making them unsuitable for civil engineering or construction project.

After the mid-1940s, theodolites became available to the general public. In 1953, the first inexpensive digital model was introduced by Raytheon. This later evolved into computer-based theodolites.

After this, theodolites became more accurate, versatile and affordable. Today, a variety of devices are available to both professional and amateur surveyors. Many variations of these instruments are also manufactured to meet specific needs such as level measurement.

General Instrument Description

Theodolites are used for surveying and leveling, which is the purpose of a level. They are simple instruments that involve the use of only a compass and vernier scale.

The basic parts of a theodolite are as follows:

Leveling Head

It is a part of theodolite that is used for looking at the target or object being measured or observed. It can be adjusted vertically and horizontally to center the object in the telescope. If a level is incorporated in it, it is known as a “Plum-Bob Theodolite”

 T- Frame

It is a strong metal frame that is fixed on the leveling head. It ensures that the reading of the instrument is correct

Vernier Scale

It also has graduations on it in degrees, minutes, and seconds, with 0.1′ to 1′ being shown at 0′ and 0′, respectively

Level Bench or Tripod

It is a wooden platform upon which the horizontal and vertical levels are mounted with two sighting scopes attached. It also has level marks for taking horizontal and vertical readings of an object’s elevation.

Theodolite

Vertical rod with three sighting scopes attached to it. It is an instrument that has a vernier scale, which enables the measurement of very small angles.

Plumb-bob

It is a small weight that is attached to the levelling head. It ensures that the reading of the instrument is correct. Theodolites without a plumb-bob are known as some-body instruments.

Telescope

It is an optical instrument that enlarges an image to view objects clearly.

Lower Circular Plate

It is a thin sheet of metal attached to the leveling head by means of a bolt. It ensures that the reading of the instrument is correct.

Upper Plate

It is a thick, circular plate that fits on the lower one. It is set horizontally with the help of a bolt.

Bubble Level

It is used in leveling to ensure that an angle is taken at right angles to the horizontal level line or vertical plumb bob line. For example, when taking a vertical angle of an object, the bubble in the leveling head should be centered when viewing it through one of the sights.

Advantages of Theodolite Surveying

They include;

1. It is a cheap instrument.
2. It is very portable and can be carried anywhere without making much effort.
3. It is generally easier to use than transit or the level and compass. This makes it ideal for small-scale surveying projects like construction sites, farms, and so on.
4. Its basic parts are easy to understand and can be used with ease by anyone who knows the basics of surveying instruments in general.

Disadvantages of Theodolite Surveying

The major disadvantages are:

1. It is provided with a bubble level, which makes it difficult to use.
2. It gives an approximate reading.
3. It cannot be used when the object to be measured is too small or too far away or when the sight point is fixed in one place.
4. It has to be kept parallel to a horizontal level line for accurate measurements, which means that it must be leveled before taking an angle off of it.
5. It cannot be used for measuring vertical angles in separate stations.
6. It is not as accurate as other surveying instruments such as the transit and the theodolite.

Theodolite Surveying FAQs

What is theodolite surveying?

Theodolite surveying is the process of determining horizontal angles (range angles) and vertical angles (height angles) from a single observation station by measuring the angle of an object in both horizontal and vertical directions.

What are some instruments used to measure altitude?

Some instruments commonly used to measure altitude include Prisms, Reticules, Tangent Arms, and Theodolites.

A prism is used to determine the height or elevation of a distant object by means of refraction.

What are the main advantages of theodolites over other instruments in surveying?

The main advantage of theodolite over others is that it gives an accurate reading to a very fine degree, which is not possible with any other instrument.

It also requires little or no adjustment and can be used for taking horizontal and vertical measurements for an unlimited time.

What are the main disadvantages of theodolites?

The main disadvantage of theodolite is that its accuracy is deterred when there is a change in temperature or during rapid movement.

What are the different types of theodolites?

There are several different types of Theodolites such as surveyors, builders, engineers, and astronomers. The final system used depends on the type of calculation needed.

Is a theodolite suitable for the land survey?

Yes, it is. A theodolite is more accurate than a Swiss Protractor. Surveyors and engineers use it for leveling, triangulation, and finding the elevation of an object from a horizontal plane.

Theodolites are used to take rough measurements.

They are then transferred to large-scale maps or drawings later to be used by surveyors as per their plan.

How do surveyors use a theodolite?

In surveying, a theodolite uses optical plummets (or plumb bobs), a spirit (bubble level), and graduated circles to find vertical and horizontal angles.

An optical plummet ensures that the theodolite is positioned as vertically above the survey point as possible.

What is theodolite surveying in civil engineering?

It is a procedure conducted along with leveling and all other surveying procedures. It involves taking horizontal angles of an object and its elevation.

Theodolites are not used in all surveying works as they are quite heavy to carry around, However, they are used in some cases such as bridge building, rail construction, etc.

In some places where theodolites cannot be used due to certain circumstances, surveyors use Electronic Distance Measurement devices (EDMs) instead.

What are some important factors to be considered when choosing a theodolite?

A few things one should consider while buying a theodolite are its price, size (length and width) and weight, altitude stability, and the accuracy of its readings.

The price is an indication of the quality of the instrument. It must be within your budget.

Size and weight should not be too small as it will make carrying it difficult. It must be lightweight so that the user can easily carry it around.

Accuracy should be within 5%. Keeping these factors in mind, one can easily get a good and accurate theodolite at an affordable price.

Is it difficult to use? What sorts of sights do you have and what is their use?

It is not difficult to use a theodolite as it has sightlines on both sides, which enables one to measure horizontal and vertical angles without any difficulty.

They are usually set on a level bench. The horizontal and vertical levels on the head can measure angles very quickly.

The horizontal level ensures that the instrument is set horizontally in a position facing an object under observation.

While taking angles, the upper and lower-level plates are used to ensure that the instrument is set vertically.

Theodolites have different kinds of sights such as a vertical sight, which enables you to view in both horizontal and vertical directions, a crosshair, and an adjustable eyepiece that enables you to focus on the object you are measuring.

What is the difference between theodolite and total station?

Theodolite uses a single level and the other uses a total station. Total stations use four levels which are the total station base plate, the total station cross-leveling rail, the total station upper-level plate, and the total station lower-level plate.

A cross-leveling rail is used to ensure that an instrument is horizontal to an object. This accuracy increases when an instrument is a level to an object as it can eliminate inaccuracies caused by casting errors.

How do you test theodolite for accuracy?

Theodolite accuracy is tested by using a total station or a protractor and comparing the two instruments on the same object.

Once an instrument is calibrated, these instruments give similar readings when taken in different directions.

Does theodolite measure distance?

It does, it is able to measure distance. When you take an angle with a theodolite, you are actually measuring the distance between two objects by working on a trigonometric law which states that angle is equal to the distance divided by the radius of curvature of a circle and expressed in radians.

How do you know if there’s a difference between horizontal and vertical angles?

There is no difference between these angles as they are simply two different measurements taken from different sides of an object.

What are the conditions for calibrating a theodolite?

A theodolite should be placed on level ground, away from any obstruction, and at an optimum distance from the object under observation. It should be at least half a meter away from any wall or other objects.

Calibration of the instrument is done by using other instruments such as survey grade total stations, leveling rods, and optical plummet, which help in detecting an error or variation in angle reading.