Before building roads, dams, or any other buildings, levelling is one of the most crucial aspects of surveying. It is a subset of surveying that deals with determining a point’s elevation in relation to a datum level above or below the earth. In this article, we will define levelling and mention the several types of levelling used in surveying and their benefits and drawbacks.
See also: Chain surveying technique: Definition, pros and cons
What does levelling mean?
Levelling is figuring out the relative heights or elevations of various spots on the earth’s surface so that they may be shown on a plan or map. The measurements in the vertical plane are primarily involved in the levelling procedure. A levelling method involves measuring the ground level in the vertical plane.
Purpose of levelling
The purpose of levelling is to determine the elevation or height of a point or a series of points relative to a reference datum or benchmark. It is typically done using a levelling instrument and a levelling staff, which are used to measure vertical distances between points. Levelling is an essential surveying technique used in various fields such as construction, engineering, architecture, and land surveying.
- In construction, levelling establishes the correct height and alignment of buildings, roads, and other infrastructure.
- It is also used to ensure that the ground is levelled even before laying the foundation for a building.
- In engineering, levelling is used to determine the slope of a pipeline or drainage system to ensure that it functions correctly.
- In architecture, levelling ensures that floors, ceilings, and walls are level and even.
- In land surveying, levelling is used to measure and map the contours of the land, which is crucial for planning and development purposes.
Levelling: Types
Barometric levelling
The term “barometric levelling” refers to levelling that takes advantage of the fact that the elevation difference between two sites in separate locations is related to the difference in atmospheric pressure at these spots.
It is an old and well-known art to use a barometer device to determine the difference in elevation between two sites on the surface of the Earth.
Atmospheric pressure may be measured with a barometer at any altitude. Seldom is this kind of levelling employed in surveying work. In general, surveillance or exploratory surveys use barometric levelling.
Trigonometric levelling
One of the simplest levelling methods is trigonometric levelling, in which the elevation of the points is calculated from their vertical angles and measured horizontal distances.
Indirect levelling is another name for trigonometric levelling. That is the same as how fundamental trigonometric relations may be used to calculate the length of any side of a triangle.
Direct levelling
It is the most often used levelling technique. Measurements are taken using this approach straight from the levelling equipment. Direct levelling is classified into many forms according to observation points and instrument positions, which are as follows:
Simple levelling
Calculating the difference in elevation between two places is a pretty operation in simple levelling. The instrument is positioned so that both spots are visible between the two.
It should not be too far between the two places because that makes it harder to obtain readings from both points. The simple levelling technique is only appropriate when the points are close to one another and there are no obstacles in their path.
The steps for the simple levelling are as follows:
- Let’s say there are two spots on the earth’s surface. Let’s call them A and B.
- Although the instrument can be set up anywhere, it should be put halfway between the two spots to reduce instrument mistakes.
- After carefully levelling the equipment, focus the telescope on the staff being held vertically at point A.
- Take the readings where the horizontal hair is slicing the staff. Hold the staff now at point B vertically.
- Focus the telescope appropriately on the staff being held vertically at the point. Take readings at point B, carefully ensuring the bubble is in the circle’s centre.
Differential levelling
When it’s necessary to calculate the elevation difference between two places that are too far apart and have a significant height difference, differential levelling is employed in surveying.
Differential levelling is utilised when levelling impediments prevent the points from being seen from one instrument configuration. As a result, it’s essential to set up the instrument in different places and work in several phases.
The differential levelling technique is appropriate when there is a greater distance between the two places.
Reciprocal levelling
When it’s necessary to determine an exact level difference between places far distant from one another, reciprocal levelling is used.
The reciprocal levelling technique is utilised when a river, pond, or other obstruction prevents the level from being set up between the two places.
Curvature and collimation-related errors are eliminated through reciprocal levelling. Moreover, the mistake brought on by the collimation line is eliminated.
In this instance, the instrument is placed on either side of the riverbank or valley bank, and two staff readings are taken by holding the staff in place on both banks.
Fly levelling
Just the rear and foresight readings are taken while levelling a fly; no intermediate sight readings are gathered throughout this process.
The benchmarks are established using fly levelling, which connects the benchmark to the project’s beginning point. Fly levelling is employed when the job location is far from the benchmark. In this instance, the surveyor begins the task by positioning the instrument at an appropriate spot and taking a backsight reading on the benchmark.
Profile levelling
Determining the height of the points at brief, determined intervals along a fixed line, such as the middle of a railway, road, canal, or sewer, is known as profile levelling.
The fixed line may consist of a series of straight lines or a single, simple line. Longitudinal sectioning is another name for it.
Levelling: Benefits
- This technique is typically used to determine the height of the ground surface, the height of buildings or other structures, and the elevation of features such as rivers, lakes, and other bodies of water.
- A critical application of levelling is in constructing the centerline of a road. To ensure that the road is levelled and safe for drivers, surveyors use levelling techniques to determine the height of the ground at various points along the road. It allows them to create a plan for the road that considers elevation changes and ensures that the road surface will be even and smooth.
- Another important use of levelling is in understanding the topography of the soil. By measuring the height of the ground at different points, surveyors can create a contour map that shows the shape of the land. This information is crucial for various purposes, including land-use planning, environmental management, and engineering projects.
- In construction, levelling is extremely useful for developing structures. Builders use levelling to ensure that foundations are level and stable, which is critical for ensuring the safety and durability of the structure. They also use levelling to determine the slope and grade of the land, which is important for planning drainage and preventing erosion.
Levelling: Drawbacks
While levelling can provide many benefits, there are also some potential drawbacks. Here are some of the key ones:
Levelling is a time taking process
Levelling can be time-consuming, particularly for large areas or complex topography. It’s because it involves measuring the height of the ground at multiple points, often using specialised equipment such as a level or total station. The process may also need to be repeated multiple times to ensure accuracy, which can further extend the time required.
Expensive
Levelling can be expensive if it involves a large area or complex topography. It often requires specialised equipment and skilled personnel, which can be costly. Additionally, the cost of levelling may vary depending on the project’s complexity, location, and other factors, such as the need for permits or other regulatory approvals.
Levelling process requires skilled and qualified experts
Levelling is a specialised skill that requires expertise in surveying and engineering. The process typically requires trained professionals with experience in surveying equipment, understanding the principles of levelling, and interpreting the data obtained. Skilled personnel must ensure the levelling process is conducted accurately and efficiently.
Plotting strata on a map or sheet takes time and requires numerous computations
After levelling, the data obtained must be processed and analysed to create a contour map or other output. It can be time-consuming and requires numerous computations, particularly if the surveyed area is complex or large. Creating an accurate map or sheet requires attention to detail and a thorough understanding of the principles of surveying and levelling.
FAQs
Why is levelling important in construction?
Levelling is an important technique for various surveying, construction, and engineering applications. By providing accurate information about the elevation and topography of the land, levelling helps ensure the safety and success of a wide range of projects.
What are some common tools used for levelling in construction?
Some common tools used for levelling in construction include dumpy level, digital level, barometer, and level staff.
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