# What is Levelling in Surveying?

Levelling is a branch of surveying whose goal is to establish, verify, or measure the height of specified points in relation to a datum.

It is commonly used in geodesy and mapping to measure geodetic height, as well as in construction to quantify height disparities between building objects. It is sometimes referred to as spirit levelling and differential levelling.

## Types of Levelling in Surveying

They are:

• Direct Leveling (Spirit Leveling)
• Barometric Leveling
• Trigonometric Leveling

### Direct Levelling

It is the most often utilized leveling procedure. Measurements are taken straight from the leveling tool in this manner. Direct leveling is classified into the following types based on observation points and instrument positions:

#### Simple Leveling

It is a simple and straightforward method of leveling in which the leveling instrument is placed between the points where elevation is to be determined.

Leveling rods are put at certain locations and seen with a leveling device. It is only performed when the spots are closer to each other and there are no obstructions between them.

#### Differential Leveling

When the distance between two places is greater, differential leveling is used.  During this process, a number of interstation are identified, and an instrument is moved to each station to measure the height of interstation locations.

Finally, the difference between the initial two positions is calculated. Differential leveling is the process of determining the difference in elevation between two or more sites without consideration for their alignment. It is used when:

• Two points are separated by a great distance.
• The height difference between the two points is significant, and
• Some impediment stands between them.

#### Reciprocal Leveling

Reciprocal leveling is used when it is impossible to locate the leveling instrument between the inter-visible spots.

This situation arises in the case of ponds or rivers, for example. In the event of reciprocal leveling, the instrument is positioned closer to the first station and sighted towards the second station.

For such long views, the faults in reading the staff, the curvature of the earth, and the instrument’s faulty corrections become apparent. To reduce these inaccuracies, special approaches such as reciprocal leveling should be utilized.

#### Fly Leveling

When the benchmark is a long distance from the work station, fly leveling is used. In such a circumstance, a temporary bench mark is set up at the workstation based on the original benchmark. Even though it is not extremely exact, it is used to determine approximate levels.

Fly leveling is also used when the original benchmark is not accessible.

#### Precise Leveling

Precise leveling is comparable to differential leveling, however the precision is higher in this situation. A serious observation process is used to obtain great precision. It is possible to obtain a precision of 1 mm every 1 km.

### Trigonometric Leveling

The procedure of leveling in which the elevation of a point or the difference between points is measured from the observable horizontal distances and vertical angles in the field is known as trigonometric leveling.

Trigonometric relations are utilized in this method to find the elevation of a point given an angle and a horizontal distance, hence the name trigonometric leveling. It is also known as indirect leveling.

### Barometric Levelling

A barometer is a device used to measure the atmosphere at any altitude. So, in this method of leveling, atmospheric pressure is measured at two distinct sites and the vertical difference between the two points is calculated. It is a rough estimate and is only used infrequently.

It is a modified form of trigonometric leveling in which the Tacheometer method is employed to determine the elevation of a spot. In this scenario, the line of sight is angled from horizontal. It is more accurate and ideal for surveying in mountainous terrains.

## Levelling Instrument

Leveling Instrument Devices are grouped into three types based on their accuracy:

1. Accuracy: the settlement in which the bubble is highly sensitive as well as high magnification power and employs this sort of job and Geodetic Survey businesses that require high precision.
2. Precision medium: This category is less accurate than the first and has dominated the use of this kind in the majority of engineering projects.
3. Low-precision devices: and design this type of hardware specifically for the purposes of approximate settlement, such as in building projects Ltd. and short-distance settlement.

the line of sight by the amount that the telescope is out of level.

These main instruments used are:

Dumpy levels – These are more basic levels that are frequently used in construction work. The telescope is rigidly attached to a single bubble, and the assembly is adjusted either by a screwed ball-joint or by foot screws that are adjusted in one direction first, then at 90°.

Tilting levels – are equipped with a circular bubble for preliminary approximate leveling and a main bubble that is attached to the telescope.

• For each observation (not setup), the main bubble is viewed through an eyepiece and the telescope is tilted by a fine screw to bring the two ends of the bubble into coincidence.

Automatic levels – This more modern type of level is now widely used. It has a compensator made up of three prisms. The two outer ones are connected to the telescope’s barrel.

• The middle prism is suspended by fine wiring and responds to gravity. The instrument is first roughly leveled with a circular bubble; the compensator then deviates the line of sight by the amount that the telescope is out of level.

Digital level- Staff read automatically at the digital level.

Water level- Useful in cramped quarters.

Laser Level- can be used in any conditions.

## Errors in Leveling

Leveling errors are classified as follows:

### Instrumental Errors

• Collimation Errors. Correction: Check and equalize sights before use.
• Sensitive bubble.

### Collimation Error

Collimation error arises when the collimation axis is not truly horizontal when the instrument is level.

### Error due to curvature and refraction

Distance causes the earth to appear to “slip away.” Because the earth has a curved form, the level surface seen through the telescope will deviate from the horizontal plane as the line of sight moves closer to the horizon.

Curvature has the following effects:

• Rod reading is excessively high.
• Error increases exponentially with distance.

### Other Errors

• Incorrect instrument set-up
• Staff movement from position during changing level station
• The staff is not held vertically.
• Parallax: When an instrument is knocked or moved during a backsight-foresight reading, this is referred to as parallax.
• The tripod or rod settles between measurements, for example Off-center bubble

## Importance of levelling in surveying

Leveling is a significant part of land surveying and is used in a variety of situations. It is used when building roads, towns or cities and it also has a role in the construction of airports and railways.

Leveling instruments, such as the theodolite and electronic distance meters are very important in these scenarios.

Leveling can also be used for the construction of private housing, roads and buildings.

Leveling is used in engineering surveys and mapping. It is especially important in the construction of dams, bridges and large facilities where it is important to know the elevation of points on the ground surface with accuracy.

Leveling is also used to determine soil type and drainage conditions.

It is used by geologists and archaeologists in mapping. It is also used to measure the level of groundwater at different locations.

The leveling process alone is capable of determining the height of a point on the ground surface.

The National Survey is used by surveyors to determine the height and slope of any point in the Earth’s surface.

Leveling is used to resolve the height differences between two points.

It is a very important feature in surveying and it can be used to help you find the bottom of a hole or accurately determine the elevation of a hillside or any other type of terrain with respect to sea level.

## Benefits of Levelling in surveying

Accuracy: The level of accuracy and the correct reading of the instrument are very important in surveying and levelling can assist you in meeting these requirements.

As the line of sight is the most important requirement, levelling is highly accurate.

Ease: Levelling can be done quickly and easily at any time without having to wait for a surveyor to do it.

Closely: It can be used to determine the elevation of any point.

Precision: It makes it possible to measure accurately the angles between the ground and other things such as the rod, line of sight, or object being measured.

Convenience: The measurement of an angle can be made with great accuracy without any problems or difficulties.

## Challenges of Levelling in Surveying

When doing a leveling survey, there are certain challenges to overcome. Some of these challenges are listed below:

1. The obstruction of the line of sight may be from
2. a) physical obstacles such as a hill or tree
3. b) clouds or fog.
4. It is not easy to determine the height of some low areas such as ravines, valleys, and streams.
5. The instrument used for levelling may not be suitable for the task at hand.
6. Lack of experience with the instrument can lead to inaccurate readings.
7. It is hard to get good alignment when using a tripod and rod
8. It is difficult to level when there are rough or uneven surfaces where one has to use a tripod, rod or staff that is smaller than the other objects being measured or read from.
9. Instrument’s response speed may be slow and cannot enable backing up of readings very fast
10. Costs of instruments may be high to use and maintain

## Levelling in surveying FAQs

### What is Levelling in Surveying?

Levelling is a branch of surveying whose goal is to establish, verify, or measure the height of specified points in relation to a datum.

### What are methods of levelling?

The following are some of the most important leveling procedures used in surveying: Barometric leveling is a term used to describe the process of adjusting the pressure Trigonometric leveling, trigonometric levelling and Spirit Levelling.

### What is the aim of levelling?

Leveling Goals, the primary goal of levelling in surveying is to determine the heights of specified points with respect to a specific datum. To establish points at specified or various elevations with respect to a given or imagined datum.

### What is a leveling instrument?

Leveling instruments are used to calculate the relative heights of various spots on the earth’s surface. A level is essentially a telescope with a bubble tube attached so that the axis of the bubble tube and the telescope’s line of collimation are parallel to each other.

### What are the principles of levelling?

The levelling principle is to obtain a horizontal line of sight with respect to which vertical distances between places above and below this line of sight are calculated.

### What causes the errors in levelling?

The levelling errors may be due to

1. Inaccurate instruments
2. Lack of experience or training with the instrument
3. Obstruction of line of sight by buildings, trees and clouds.
4. Lack of proper site preparation for levelling.
5. Sight drift due to temperature changes, wind and vibrations from vehicles etc.
6. A small bubble in the telescope (as compared to the height of the point being measured).  g) Incorrect technique in aligning the tripod on a single point.

### How many chief types of levels are used in levelling?

There are four major types of levels in use. They are the Dumpy, Wye, Reversible, and Tilting levels. A leveling staff is a straight rectangular rod with graduations beginning at the staff’s foot.

### What is BS and FS in surveying?

1. h) Back sight: (B.S.) – The first sight taken on a leveling staff held at a known elevation. B.S. permits the surveyor to acquire HI +sight, i.e., Height of Instrument or line of sight. I Fore Sight: (F.S.) – The last staff reading received from a level setting. It is also known as minus sight.

### What is Profile levelling?

The procedure of leveling along a defined line to ascertain the elevations of the ground surface along the line is known as profile levelling. Longitudinal sectioning is another term for profile leveling.

Levelling plays a major role in the science of surveying. It can be said that levelling is the most widely used and most important part of the entire field.

This is true because levelling has a number of advantages.

1. It helps to figure the location of a fixed point on the ground, by identifying its elevation and by setting at a known elevation.
2. it’s very helpful in determining the slope of a hill or terrain and thereby helps in finding out accurate distances between points.
3. it helps in determining distance between two points on the Earth’s surface.
4. it is useful in locating sunken objects on or under water, etc.

There are also disadvantages of levelling.

1. It is not easy to tell the exact height of a point by just observing an object.
2. It requires a large amount of time, effort and patience to set up the instrument, at a known elevation and to take readings from the baseline.
3. It may be difficult due to neglecting setting up instrument where there are trees and buildings in between it and other points which have to be levelled against.
4. It suffers from a small bubble in the telescope that could be as much as 1/6 of the height of the point being measured.
5. Costly to use and maintain which is one of the main drawbacks of levelling in surveying.

### What is collimation line?

Collimation line: The line that connects the intersection of the crosshairs to the optical center of the objective and its continuation. Line of sight is another name for it. The intersection of the cross hairs and the optical center of the objective lens is defined as the line of sight.