Levelling is the process of determining the relative heights or elevations of points on the Earth’s surface, whether on the ground or below it. This method is a critical part of surveying, focusing on vertical measurements rather than horizontal ones. It plays a vital role in the planning and execution of various engineering projects, including railways, highways, canals, dams, and water supply systems. Accurate levelling is essential because the success of these projects depends on having precise elevation data for design and construction.
When engineers undertake a project, they need detailed information about the differences in elevation along the proposed routes or alignments. This data allows them to plan the layout, drainage, and structural features accurately. Levelling provides the framework of height measurements across the entire area involved in the project, ensuring smooth execution and minimizing errors.
Types of Levels
The instrument used to carry out levelling is called a level. Depending on the way these levels are constructed, they can be categorized into different types, which will be discussed in the following sections. This classification helps to choose the right type of level for specific tasks based on precision and ease of use.
1.The Dumpy Level
The Dumpy Level, originally designed by Gravatt, is one of the most commonly used instruments in levelling operations. Its defining feature is the telescope, which is rigidly fixed to its support. This means the telescope cannot rotate about its longitudinal axis, nor can it be removed from its base. This fixed structure ensures stability and precision, making it ideal for taking several observations from a single setup.
The name “dumpy” comes from the instrument’s relatively short and thick appearance compared to other levelling instruments, such as the Wye Level. The levelling head of the dumpy level typically consists of two parallel plates, held together by either three or four foot screws. The upper plate, known as the tribrach, is connected to the telescope, while the lower plate, or trivet, is mounted on the tripod, providing a stable base for the instrument.
Key Advantages of the Dumpy Level
- Simple Construction: The dumpy level has few moving parts, which makes it easy to use and maintain.
- Long-lasting Adjustments: Once the instrument is adjusted, these settings remain stable over a long period, reducing the need for frequent recalibration.
- Reliable for Multiple Observations: Its fixed design is particularly beneficial when several measurements need to be taken from the same location, as the instrument maintains its accuracy throughout the process.
2.The Wye Level
The Wye Level (or Y-Level) is another commonly used levelling instrument, similar to the dumpy level but with a key difference: the telescope in the wye level is supported by two Y-shaped uprights. These supports allow the telescope to be easily removed and rotated around its longitudinal axis. Unlike the dumpy level, where the telescope is rigidly fixed, the wye level’s telescope can be detached by releasing two clamping collars on top of the Y-supports.
This design offers the advantage of allowing adjustments to be tested quickly and easily. Since the telescope is not permanently fixed, it can be rotated, removed, or turned end-to-end for rapid checks. However, the wye level has more movable and exposed parts, which can lead to frictional wear over time. This makes it less durable compared to the dumpy level.
Key Features of the Wye Level
- Telescope in Y-Supports: The telescope is held in Y-shaped supports, which can be loosened or tightened to rotate or remove the telescope as needed.
- Rapid Adjustment Checks: The ability to quickly test adjustments is a significant advantage, making it useful for projects requiring frequent recalibration.
- More Movable Parts: While it offers flexibility, the wye level’s loose parts are prone to wear, meaning adjustments must be checked more frequently than with the dumpy level.
Advantages of the Wye Level
- Quick Testing of Adjustments: The flexible design allows for rapid checking and testing of settings, saving time when precision is essential.
- Rotation of the Telescope: The telescope can be rotated and removed, making it easier to conduct various checks without disturbing the overall setup.
Disadvantages of Wye Level
Frictional Wear: Due to the presence of many movable parts, the wye level is more prone to wear and tear, requiring frequent inspection and maintenance.
3.The Reversible Level
The Reversible Level combines the key features of both the dumpy and wye levels, allowing users to benefit from the strengths of each. The design includes a telescope similar to that of a wye level, supported by two rigid sockets. The telescope can be inserted into these sockets from either end and fastened securely with screws, giving it the ability to be reversed. When the telescope is fastened, the reversible level functions like a dumpy level, offering stability for precise levelling tasks.
This ability to reverse the telescope provides an advantage in eliminating collimation error, which is the misalignment of the line of sight. By taking readings with the telescope reversed (often referred to as “bubble left” and “bubble right” readings), any small errors in alignment can be detected and corrected.
Additionally, the reversible level allows for easier permanent adjustments compared to other levelling instruments. It offers both the flexibility of the wye level and the stability of the dumpy level.
Key Features of the Reversible Level
- Reversible Telescope: The telescope can be removed and reinserted in the opposite direction, helping to check for errors in alignment and making it easier to adjust the instrument.
- Combination of Features: It combines the rigid setup of the dumpy level with the flexible testing capabilities of the wye level.
- Collimation Error Elimination: By allowing the telescope to be reversed, collimation errors can be identified and minimized, improving accuracy
- Easy Permanent Adjustments: Adjustments can be made more easily compared to other levels, making it a versatile choice for engineers.
4.The Tilting Level
The Tilting Level is a specialized levelling instrument designed for precision work. Its key feature is a telescope attached to a level tube, which can be tilted slightly (up to 4 degrees) in a vertical plane using a tilting screw. This feature enables the surveyor to fine-tune the centering of the bubble for each observation without the need for precise initial levelling.
Unlike the dumpy or wye level, where the line of collimation (the line of sight through the telescope) must be perpendicular to the vertical axis, the tilting level does not require this alignment. Instead, the surveyor can achieve accurate leveling by roughly setting up the instrument and then using the tilting screw to make the final adjustments before taking each reading. This saves time and increases efficiency, especially when only a few observations are needed
Key Features of the Tilting Level:
- Tilting Screw: The telescope can be tilted within a few degrees to accurately center the bubble before each reading.
- Quick Levelling: The instrument requires only rough levelling, as the tilting screw allows for fine adjustments to the line of sight.
- Circular Bubble: A circular bubble is used to approximately level the instrument with the help of foot screws. The main bubble tube is then centered using the tilting screw.
Advantages of the Tilting Level:
- Precise Adjustment: The tilting screw allows for exact centering of the bubble, improving the accuracy of each reading.
- Time-Saving: The need for only rough levelling reduces setup time, as the final adjustments are made using the tilting screw.
- Efficiency: Ideal for situations where only a few observations need to be taken from a single setup, such as in fly levelling.
The Tilting Level is especially useful for high-precision work, providing flexibility and efficiency through its tilting mechanism, while allowing for quick and accurate adjustments.
5.Automatic or Self-Leveling Level
The Automatic Level, also known as the Self-Leveling Level, simplifies the levelling process by utilizing a stabilizer or compensator that automatically adjusts the line of sight to be horizontal. While its design is similar to the dumpy level, with the telescope fixed to the tribrach, it incorporates a compensator mechanism to provide more efficient and accurate levelling.
Key Features of the Automatic Level:
- Compensator Mechanism: The compensator is a prismatic device suspended on fine non-magnetic wires within the telescope. This system automatically adjusts the line of sight by responding to gravity, ensuring it remains horizontal even if the instrument is not perfectly level.
- Circular Spirit Bubble: A circular spirit bubble is used for rough levelling. The instrument needs to be leveled within 15–30 minutes of arc for the compensator to function properly. This is typically done using three foot screws and a small circular level (often called a “pond bubble”).
- Gravity-Based Operation: The compensator uses the force of gravity to stabilize the line of sight. As the instrument is roughly leveled, the compensator shifts the optical system, bringing the horizontal cross-hair to the correct alignment.
How the Compensator Works:
- The compensator consists of two fixed prisms placed in the optical path between the eyepiece and the objective. An inverted pendulum, suspended by four non-magnetic wires, moves the compensating prism.
- When the instrument is approximately level, the compensator adjusts, ensuring the line of sight remains horizontal. The pendulum swings until its center of gravity aligns with the wire supports, automatically adjusting the compensating prism to provide an accurate horizontal view.
- The instrument is tested by slightly adjusting one of the foot screws. If the reading to the staff remains constant, the compensator is functioning properly. Some automatic levels have a button to check the compensator’s function, allowing the horizontal hair to move and return to its original position.
Advantages of the Automatic Level:
- Speed and Efficiency: The automatic compensator eliminates the need for precise manual leveling with a bubble, making it faster to use.
- Error Reduction: There is no risk of errors due to improper bubble adjustment, as the compensator automatically maintains the horizontal line of sight.
- Ease of Use: The instrument provides an erect image, making it user-friendly and straightforward to operate.
Limitations:
- Sensitivity to Wind and Vibrations: The compensator mechanism can be affected by strong winds or vibrations, which may interfere with its proper functioning.
The automatic level is highly efficient for large-scale levelling tasks and ensures accurate results without requiring meticulous manual adjustments. However, it is less suited for use in environments with significant wind or vibration, where its accuracy might be compromised.
6.Digital Level
The Digital Level represents a modern advancement in levelling technology, combining traditional optical techniques with electronic image processing to enhance precision and reduce human error. It operates in much the same way as a conventional optical level, but with the added capability of electronically measuring and recording observations.
Key Features of the Digital Level:
Electronic Image Processing: The digital level reads from a specially designed bar-coded staff using electronic image processing. A detector diode array captures the signal pattern from the staff’s binary bar code, and a built-in correlation procedure translates this into the vertical staff reading and the horizontal distance from the instrument to the staff.
Automatic Recording: Once the reading is obtained, the digital level stores the data automatically on its recording module. This eliminates potential errors associated with manual staff reading, data recording, and calculations.
Ease of Use: The digital level is set up in the same way as a traditional level. After attaching it to a tripod and leveling it using the foot screws and circular bubble, the operator sights the bar-coded staff, adjusts the focus, and presses a measuring key. Within 4 seconds, the instrument displays the staff reading on the screen.
Bar-Coded Staff: The staff used with digital levels has a binary bar code on one side and conventional graduations on the other. It is typically made from synthetic materials strengthened with glass fibers. The bar code comprises 2000 elements, with each element measuring 2 mm in width, allowing for precise electronic measurements.
Display and Range: The digital display provides staff readings with selectable precision (either 1 mm or 0.1 mm). In addition, it can display the horizontal distance to the staff with a precision of 10 mm. The range of the digital level, under ideal conditions, is up to 100 meters, though performance may diminish under poor lighting.
Power Supply: The digital level is powered by a small internal battery capable of running for a full day of levelling work before needing recharging.
Advantages of the Digital Level:
Error Reduction: By automating the measurement process, the digital level removes common sources of error, such as misreading the staff and recording incorrect values in the fieldbook.
Efficiency: With its automated readings and data storage, the digital level speeds up the levelling process, potentially reducing fieldwork time by up to 50%.
Accuracy: The digital level offers consistent precision and reliability, reducing the chances of calculation errors through built-in programs for height computation and other tasks.
Fatigue-Free Operation: Since the staff reading is taken electronically, the observer does not have to visually interpret the staff, reducing strain and fatigue.
Online Data Transfer: The data collected can be transferred to a computer for further processing and analysis, making it easier to generate longitudinal and cross-sections in a short time.
Dual Functionality: In situations where digital measurement is not feasible, the digital level can be used as a conventional level, providing versatility in the field.
Limitations and Considerations:
External Factors: The performance of the digital level can be affected by environmental conditions such as wind, traffic vibrations, heat shimmer, and insufficient lighting, which may impact the compensator or the reading accuracy.
Power Dependency: As the instrument relies on a battery, it is important to ensure it is charged for the duration of fieldwork.
Overall, the Digital Level offers significant advantages in terms of speed, accuracy, and error reduction, making it an excellent choice for modern surveying tasks. Its ability to store and transfer data electronically further enhances productivity and accuracy in levelling operations.
7. Hand Level
