Levelling in surveying is one of the most important techniques for determining the relative heights of points on the earth’s surface. It is essential for creating accurate maps, designing roads, railways, pipelines, and drainage systems. The main purpose of levelling is to find the elevation of different points and establish them with respect to a common reference point, usually called the benchmark. In construction and civil engineering projects, accurate levelling ensures proper drainage, stable foundation design, and precise execution of plans.
What is Levelling in Surveying?
Levelling in surveying is the process of measuring the height of points relative to a datum to determine elevation differences. The datum is usually mean sea level (MSL) or another fixed reference point. The process involves using instruments such as the dumpy level, auto level, or digital level to sight a graduated staff placed on the points being measured. Levelling is a fundamental part of surveying because even minor elevation errors can cause major problems in construction and infrastructure projects.
Objectives of Levelling in Surveying
The main objectives of levelling include determining the elevation of points for design and construction, establishing points of known height called benchmarks, creating contour maps, ensuring proper slope for roads, drainage, and irrigation channels, and transferring elevations from one location to another. In short, levelling helps engineers make informed decisions about land development and structural stability.
Principles of Levelling in Surveying
The principle of levelling is based on the concept that a horizontal line of sight from the levelling instrument is at the same height throughout its length. By measuring the vertical distances between this line of sight and the ground, the relative heights of different points can be determined. Levelling requires the instrument to be properly levelled so that its line of sight is truly horizontal.
Equipment Used in Levelling
Levelling requires specialized equipment to ensure accuracy and efficiency. The most common instruments and accessories include the dumpy level, which is a traditional optical instrument with a telescope fixed to its support; the auto level, which is self-levelling and reduces human error; the digital level, which uses electronic measurement and data storage; levelling staff or rod, which is graduated and placed vertically on points being measured; tripod stand, which provides stable support for the instrument; plumb bob for centering; and measuring tape for short distances.
Types of Levelling in Surveying
Levelling can be classified into several types depending on the method and purpose.
Direct Levelling
Direct levelling is the most common method and involves directly reading staff measurements through a levelling instrument. It includes simple levelling between two points, differential levelling for points at different elevations, and fly levelling to connect distant benchmarks.
Indirect Levelling
Indirect levelling, also called trigonometrical levelling, uses angular measurements from a theodolite and distances to calculate elevations using trigonometric formulas. This is useful for long distances or inaccessible points.
Reciprocal Levelling
Reciprocal levelling is used when two points are separated by an obstacle such as a river or valley. Readings are taken from both sides to eliminate errors caused by instrument misalignment or refraction.
Profile Levelling
Profile levelling is carried out along a specific line, such as a proposed road or railway, to obtain elevations at regular intervals. This helps in designing the gradient and alignment.
Cross-Section Levelling
Cross-section levelling is used to determine the elevations of points perpendicular to the main line of a project, often used in road or canal construction to understand the terrain on both sides.
Precise Levelling
Precise levelling uses high-quality instruments and strict procedures to achieve very high accuracy, often for scientific or large-scale engineering purposes.
Barometric Levelling
Barometric levelling uses air pressure measurements to estimate elevation differences. It is less accurate and mainly used for reconnaissance surveys in rough terrain.
Procedure for Levelling in Surveying
The levelling process involves several systematic steps: setting up the tripod and mounting the levelling instrument, adjusting the instrument so that its line of sight is perfectly horizontal, placing the levelling staff vertically on a known benchmark, taking the first reading called backsight (BS), moving the staff to a new point and taking a foresight (FS) reading, and calculating the difference between readings to find elevation changes. This process is repeated until all required points are covered, and the data is tabulated for further calculation.
Basic Levelling Table Example:
| Point | BS (m) | IS (m) | FS (m) | Reduced Level (RL) | Remarks |
|---|---|---|---|---|---|
| BM | 1.245 | 100.000 | Benchmark | ||
| A | 1.865 | 98.135 | Point A | ||
| B | 2.345 | 97.655 | Point B |
Importance of Levelling in Surveying
Levelling plays a crucial role in ensuring accuracy in construction and land development. It helps in creating proper drainage systems by ensuring water flows in the intended direction, avoiding flooding or waterlogging. It ensures foundations are laid at the correct depth and slope, contributing to structural stability. Levelling data is used for earthwork calculation, helping in cost estimation and resource management. It aids in road and railway design by determining the optimal gradient for safety and efficiency. Accurate levelling prevents costly construction errors and ensures compliance with design specifications.
Common Sources of Error in Levelling
Even with careful work, errors can occur in levelling due to instrumental imperfections like misadjusted line of sight, natural factors like temperature changes, wind, and refraction, and human errors such as improper staff holding or incorrect recording. To minimize these errors, regular instrument calibration, proper procedure, and reciprocal observations are recommended.
Applications of Levelling in Surveying
Levelling has a wide range of applications in civil engineering, including construction site preparation, designing and laying pipelines, creating topographic maps, determining flood levels, monitoring settlement of structures, railway track laying, airport runway design, and agricultural irrigation planning.
FAQs on Levelling in Surveying
What is the purpose of levelling in surveying?
The main purpose is to determine the elevation of points and ensure proper slope and alignment for construction projects.
Which instrument is most commonly used for levelling?
The dumpy level and auto level are the most widely used instruments for accuracy and reliability.
What is a benchmark in levelling?
A benchmark is a fixed reference point with a known elevation used as the starting point for levelling measurements.
How is levelling different from contouring?
Levelling measures elevations at specific points, while contouring connects points of equal elevation to create contour maps.
Can GPS replace traditional levelling methods?
GPS can provide elevation data, but traditional levelling is still more accurate for detailed engineering projects.
Conclusion
Levelling in surveying is a vital process for accurate measurement of elevation differences, which directly impacts the success and safety of construction and infrastructure projects. With various types of levelling methods available, surveyors can choose the most suitable one depending on terrain, distance, and required accuracy. By following proper procedures, using the right equipment, and minimizing errors, levelling ensures projects are executed precisely as designed.
