Unit III: Levelling (10 hrs)
3.1 Levelling- Basic Definitions and Importance
-Levelling is a fundamental surveying technique used to measure and establish the elevation or height difference between various points on the Earth's surface. It is an essential component of surveying and construction, and it serves several critical purposes. Here are some basic definitions and the importance of levelling in surveying and construction:
**1. Levelling Definitions:**
- **Benchmark**: A benchmark is a point with a known or assumed elevation used as a reference for levelling. It is a fixed point from which other elevations are determined.
- **Height of Instrument (HI)**: This is the elevation of the line of sight (typically the level instrument) above a benchmark or a reference point.
- **Reduced Level (RL)**: The reduced level is the elevation of a point above or below a reference point or benchmark. It is calculated by adding the height of the instrument to the staff reading (back sight or fore sight).
- **Backsight (BS)**: The backsight is the first staff reading taken on a benchmark or reference point to establish the starting point for levelling.
- **Foresight (FS)**: The foresight is the staff reading taken on the point to which the elevation is being determined.
**2. Importance of Levelling:**
- **Establishing Elevations**: Levelling allows surveyors and engineers to determine and establish the elevations of various points on the ground. This information is crucial for designing and constructing infrastructure such as roads, bridges, buildings, and pipelines. It ensures that structures are built at the correct elevations to meet design specifications and regulatory requirements.
- **Ensuring Level Surfaces**: Levelling is used to create level surfaces, ensuring that floors, foundations, and other structural elements are horizontal. This is critical for the stability and functionality of structures.
- **Determining Slope and Gradients**: Levelling is used to determine slopes and gradients on the ground. This information is vital for designing drainage systems, road alignments, and other features where the slope needs to be controlled.
- **Monitoring Subsidence or Uplift**: Levelling is also used to monitor changes in elevation over time, which can indicate subsidence or uplift of the land. This is particularly important in areas prone to geological activity or land settlement.
- **Quality Control**: In construction, levelling is a crucial part of quality control. It ensures that construction work is performed accurately and according to design specifications.
- **Reference for Mapping**: Levelling provides reference points for creating topographic maps and contour lines. These maps are used for land planning, geology, and a variety of other applications.
- **Precision and Accuracy**: Levelling techniques are highly precise and accurate, ensuring that elevations are determined with a high degree of reliability.
In summary, levelling is a fundamental surveying technique that plays a crucial role in construction, engineering, and land management. It provides the necessary data for establishing elevations, ensuring level surfaces, determining slopes, and monitoring changes in the land's elevation over time. Levelling is essential for quality control and maintaining accuracy in various applications.
3.2 Types of Levelling- Spirit, Trigonometrical and Barometric
-In addition to spirit leveling and trigonometric leveling, there is another method known as barometric leveling. Here are the three types of leveling methods: spirit leveling, trigonometric leveling, and barometric leveling:
**1. Spirit Leveling (Differential Leveling):**
Spirit leveling, also called differential leveling, is a traditional and widely used method for measuring the difference in elevation between different points on the Earth's surface. It involves using a leveling instrument (spirit level) and a graduated leveling staff to obtain precise elevation data. Key features of spirit leveling include:
- **High Accuracy**: Spirit leveling provides highly accurate results and is suitable for most applications where precise elevation information is required.
- **Direct Measurement**: It directly measures height differences by reading the leveling staff.
- **Use of Benchmark Points**: Benchmarks with known elevations are crucial reference points in spirit leveling.
- **Common Applications**: Spirit leveling is commonly used in construction, civil engineering, floodplain mapping, and other projects requiring accurate elevation data.
**2. Trigonometric Leveling (Height by Trigonometry):**
Trigonometric leveling, also known as height by trigonometry or indirect leveling, is a method used when direct leveling is impractical due to obstacles, rough terrain, or long distances. It involves measuring angles and distances to calculate elevation differences. Key features of trigonometric leveling include:
- **Indirect Measurement**: Elevation differences are calculated indirectly using angles and distances.
- **Less Accuracy**: Trigonometric leveling is generally less accurate than spirit leveling due to potential errors in angular and distance measurements.
- **Use of Theodolites**: Theodolites or total stations are often used to measure horizontal and vertical angles.
- **Use Cases**: Trigonometric leveling is applied in geodetic surveys, large-scale mapping, and when direct leveling is not feasible due to obstacles or challenging terrain.
**3. Barometric Leveling:**
Barometric leveling is a less common method of determining elevation differences using atmospheric pressure measurements. Key features of barometric leveling include:
- **Principle**: Barometric leveling is based on the principle that atmospheric pressure decreases with increasing elevation. Surveyors use barometers to measure atmospheric pressure at different points and then calculate the elevation differences.
- **Lower Precision**: It is generally less precise than spirit leveling and trigonometric leveling due to the sensitivity of atmospheric pressure to weather changes.
- **Limited Use**: Barometric leveling is not widely used and is typically reserved for situations where other methods are impractical.
- **Weather Sensitivity**: Changes in atmospheric pressure due to weather can introduce errors, making barometric leveling less reliable.
- **Applications**: Barometric leveling is used in specific cases where other leveling methods are not feasible, such as in remote areas or for preliminary elevation estimations.
In summary, spirit leveling is the most accurate and direct method for measuring elevation differences, trigonometric leveling is used when obstacles or challenging terrain make direct leveling impractical, and barometric leveling relies on atmospheric pressure measurements and is used in select situations where other methods cannot be applied. The choice of leveling method depends on the project's requirements, accuracy needs, and environmental constraints.
3.3 Spirit Levelling- Basic Definitions and Types
-Spirit leveling is a fundamental surveying method for measuring the difference in elevation between various points on the Earth's surface using a leveling instrument called a spirit level or level. Here are some basic definitions and types related to spirit leveling:
**Basic Definitions in Spirit Leveling:**
1. **Benchmark (BM)**: A benchmark is a reference point with a known or assumed elevation. It serves as a starting point for leveling surveys, and all other elevations are determined relative to benchmarks.
2. **Height of Instrument (HI)**: The height of instrument (HI) is the vertical distance from the reference point (usually a benchmark) to the horizontal crosshair of the leveling instrument. It is crucial in determining reduced levels.
3. **Backsight (BS)**: The backsight is the first reading taken with the leveling instrument. It is usually taken on a benchmark or a reference point to establish the starting point for the leveling survey.
4. **Foresight (FS)**: The foresight is the reading taken on the point to which the elevation is being determined. It is used to calculate the reduced level of that point.
5. **Reduced Level (RL)**: The reduced level is the elevation of a point above or below a benchmark. It is calculated by adding the height of instrument (HI) to the staff reading (backsight or foresight).
6. **Intermediate Sights**: In spirit leveling, several intermediate sights may be taken between the backsight and foresight to establish a series of elevations.
7. **Line of Sight**: The line of sight is an imaginary horizontal line along which the surveyor sights while taking readings. It should be as level as possible to ensure accuracy.
**Types of Spirit Leveling:**
1. **Single-Section Leveling**: In single-section leveling, a series of sights are taken along a straight line, starting with a backsight on a benchmark and ending with a foresight. The elevation differences are calculated based on these sights. This method is suitable for relatively short surveys.
2. **Double-Section Leveling**: Double-section leveling is an extension of single-section leveling. Two lines are established, and sights are taken in both directions, from the benchmark (BS1) to a turning point (TP) and then from the turning point (BS2) to the foresight (FS). This method is used to minimize errors due to curvature and refraction of light.
3. **Fly Leveling**: Fly leveling is a technique used when there is a significant difference in elevation between the starting benchmark and the endpoint. In fly leveling, a series of benchmarks are established at intermediate points to break the leveling survey into manageable sections.
4. **Profile Leveling**: Profile leveling is used to determine the elevation along a specific path, such as a road or pipeline alignment. It involves taking readings at regular intervals to create an elevation profile of the route.
5. **Reciprocal Leveling**: Reciprocal leveling is a technique used to check the accuracy of the initial leveling results. It involves taking sights in both directions (forward and backward) between two benchmarks to detect errors or inconsistencies.
6. **Check Leveling**: Check leveling is a quality control method where leveling is repeated at certain points to verify the accuracy of the initial survey. Any discrepancies found during check leveling can be addressed.
Spirit leveling is a precise method for determining elevations and is commonly used in construction, civil engineering, and land surveying. The choice of the leveling method and the number of benchmark points used will depend on the specific requirements of the survey and the desired level of accuracy.
3.4 Methods of Reducing Levels and their Booking – HI and Rise & Fall Method, Arithmetic Checks
-In surveying, after collecting raw elevation data through leveling, these data need to be reduced and processed to obtain accurate reduced levels. The two common methods for reducing levels are the "Height of Instrument (HI)" method and the "Rise and Fall" method. Arithmetic checks are employed to ensure the accuracy of the reduced levels. Here's an overview of these methods:
**1. Height of Instrument (HI) Method:**
The Height of Instrument (HI) method is a straightforward approach for reducing levels. It involves calculating the elevation of each surveyed point by adding the height of the instrument (HI) to the backsight (BS) and subtracting the foresight (FS). Here's how it works:
- **Elevation of Point (EP) = HI + BS - FS**
- EP is the reduced level of the surveyed point.
- HI is the height of the instrument (elevation of the instrument above a benchmark).
- BS is the backsight reading on a benchmark.
- FS is the foresight reading on the point whose elevation is being determined.
**2. Rise and Fall Method:**
The Rise and Fall method is a systematic way of reducing levels that is particularly useful for longer leveling surveys or when intermediate sights have been taken. It helps in identifying and correcting systematic errors by ensuring that the algebraic sum of the rises and falls equals zero for a closed-loop leveling circuit. Here's how it works:
- **Sum of Rises = Sum of Falls**
Rise is the elevation difference when moving from a lower point to a higher point, and fall is the elevation difference when moving from a higher point to a lower point. To calculate the reduced level of a point, you start with a benchmark's elevation (known as Benchmark Reduced Level or BRL) and apply the rise or fall as you move along the leveling route. The rise and fall values are obtained by taking the difference between each backsight (BS) and foresight (FS) reading.
**Arithmetic Checks:**
Arithmetic checks are performed to ensure the accuracy of the leveling calculations. Here are some common checks:
- **Check on Intermediate Points**: If you have taken intermediate sights between the backsight and foresight, the algebraic sum of the rises and falls should equal zero, as mentioned above in the Rise and Fall method.
- **Check on Benchmark**: If the last point in a leveling circuit is a benchmark, the calculated reduced level should match the known benchmark elevation (Benchmark Reduced Level or BRL).
- **Check on Closing Error**: For a closed-loop leveling circuit (a circuit that starts and ends on the same benchmark), the closing error should be minimal, ideally zero. A significant closing error indicates potential mistakes or inconsistencies in the leveling survey.
- **Consistency Check**: Compare the calculated reduced levels with the known elevations (BRLs) of benchmarks and known points. Any significant deviations may indicate errors in the survey.
- **Check on Instrument Height (HI)**: Ensure that the height of the instrument (HI) remains constant throughout the survey. Any changes in HI can introduce errors into the calculations.
These arithmetic checks help identify and correct errors, ensuring that the reduced levels are accurate and reliable. Levelling is a precise operation, and these checks are essential to maintain the quality of the survey data.
3.5 Two peg test
-The "Two-Peg Test" is a simple but important procedure used in surveying to check the accuracy of a theodolite or total station instrument, specifically to verify if the instrument is correctly aligned and the horizontal and vertical circles are properly calibrated. This test ensures that the instrument is in good working condition and capable of providing accurate measurements. Here's how the Two-Peg Test works:
**Equipment Required:**
1. Theodolite or Total Station: The instrument you want to test.
2. Two Surveying Pegs: These are typically wooden or metal stakes with a known and accurately measured horizontal distance between them.
**Procedure:**
1. Set Up the Instrument: Place the theodolite or total station on a stable tripod at a location with a clear line of sight to both surveying pegs.
2. Measure the Horizontal Distance: Use the instrument to measure the horizontal distance between the two pegs. This distance is known as the "Peg Interval" and should be accurately known in advance.
3. Record the Angle Measurements: Measure and record the horizontal (azimuth) angle and the vertical (zenith) angle between the two pegs using the instrument.
4. Perform the Calculations: Calculate the expected horizontal and vertical angles based on the known Peg Interval and the instrument's distance measurement capabilities.
5. Compare the Measured and Calculated Angles: Compare the measured angles from the instrument to the calculated angles based on the known Peg Interval. The measured and calculated angles should be nearly identical. Any significant deviation indicates an error in the instrument's alignment or calibration.
**Interpreting the Results:**
- If the measured and calculated angles closely match, it indicates that the instrument is correctly aligned and calibrated.
- If there is a significant discrepancy between the measured and calculated angles, it suggests that the instrument may be misaligned, or there could be issues with the horizontal and vertical circles. In such cases, the instrument may require adjustments or servicing.
The Two-Peg Test is a valuable quality control tool in surveying because it allows surveyors to quickly verify the accuracy of their instruments in the field. By ensuring that the instrument is functioning correctly, surveyors can have confidence in the accuracy of their measurements, which is crucial for the success of surveying projects.
3.6 Effect of Curvature and Refraction
-The effects of Earth's curvature and atmospheric refraction are important considerations in surveying and other applications that involve long-distance measurements. These phenomena can introduce errors and deviations in measurements and need to be accounted for to ensure accuracy. Here's an overview of the effects of curvature and refraction:
**1. Earth's Curvature:**
Earth is not flat but rather an oblate spheroid, meaning it is roughly spherical in shape but slightly flattened at the poles and bulging at the equator. As a result, the surface of the Earth curves away from a straight line over long distances, which has implications for surveying and measurements:
- **Line of Sight**: When making long-distance measurements, the line of sight between two points appears to curve, which means that the elevation of the target point is slightly lower than expected due to the Earth's curvature.
- **Reduction in Elevation**: As surveyors extend their measurements over greater distances, the cumulative effect of the Earth's curvature can result in an apparent reduction in elevation between two points. This reduction is significant for surveys covering hundreds of kilometers.
- **Compensation**: Surveyors typically use leveling techniques (e.g., Rise and Fall method) to correct for the curvature effect. Additionally, geodetic datums and coordinate systems are designed to account for the curvature in order to accurately represent the Earth's surface.
**2. Atmospheric Refraction:**
Atmospheric refraction is the bending of light rays as they pass through the Earth's atmosphere, caused by variations in air density. This bending can have the following effects:
- **Elevation Lifting**: Atmospheric refraction can make distant objects appear slightly higher than they actually are. This is known as the "dip correction" or "dip of the horizon." It occurs because the light rays curve upward through the atmosphere.
- **Inversion Layers**: Refraction is influenced by atmospheric conditions, especially temperature and pressure variations. In situations with temperature inversions, where the temperature increases with height, the bending of light can be more pronounced.
- **Surveying Implications**: Refraction can introduce errors in measurements, particularly in long-distance sightlines. Surveyors must account for refraction to obtain accurate results, especially in geodetic surveys and precise astronomical observations.
- **Correction Formulas**: Several correction formulas exist to compensate for atmospheric refraction effects. These formulas consider factors like temperature, pressure, and the vertical angle of observation. The most common correction is the "astronomical refraction correction."
In summary, Earth's curvature and atmospheric refraction are two natural phenomena that can affect surveying and long-distance measurements. Surveyors and scientists take these effects into consideration and apply correction methods to ensure the accuracy of their measurements. Understanding the principles of these phenomena is crucial for precision in geodetic and land surveying, as well as in fields like astronomy and navigation.
3.7 Reciprocal Levelling
-Reciprocal leveling is a surveying technique used to verify the accuracy of leveling measurements by taking readings in both the forward and backward directions between two benchmark points. This method helps identify and correct systematic errors and ensures the reliability of the elevation data obtained from leveling surveys. Here's an explanation of reciprocal leveling:
**Key Components of Reciprocal Leveling:**
1. **Two Benchmark Points (BM1 and BM2):** To perform reciprocal leveling, two benchmark points are required. BM1 is the starting benchmark, and BM2 is the endpoint. The goal is to check if the elevations determined from BM1 to BM2 (forward leveling) match the elevations determined from BM2 to BM1 (backward leveling).
2. **Surveyor's Level or Automatic Level:** A leveling instrument is used to measure the height differences between the benchmark points. This instrument should be properly calibrated and in good working condition.
**Procedure:**
1. **Setup at BM1:** Place the surveyor's level at BM1 and take a backsight reading (BS1) on BM1. The backsight reading records the elevation of BM1 relative to the instrument's line of sight.
2. **Measure Intermediate Points:** If necessary, take intermediate sights (IS1, IS2, etc.) to other benchmark points or survey locations between BM1 and BM2. Record the intermediate readings.
3. **Foresight on BM2:** Move the leveling instrument to BM2 and take a foresight reading (FS2) on BM2. The foresight reading records the elevation of BM2 relative to the instrument's line of sight.
4. **Calculate Forward Leveling:**
- Calculate the height difference (Rise or Fall) from BM1 to BM2 using the difference between BS1 and FS2.
5. **Move Back to BM1:** Return to BM1 with the leveling instrument.
6. **Setup at BM2:** Place the leveling instrument at BM2 and take a backsight reading (BS2) on BM2.
7. **Calculate Backward Leveling:**
- Calculate the height difference (Rise or Fall) from BM2 to BM1 using the difference between BS2 and FS1 (FS1 is the foresight reading taken at BM1 during the forward leveling).
**Interpreting the Results:**
- In ideal conditions, the calculated height difference in the forward leveling (from BM1 to BM2) should match the calculated height difference in the backward leveling (from BM2 to BM1).
- If there is a significant difference between the two height differences, it indicates errors in the leveling process, calibration issues with the instrument, or potential systematic errors. These discrepancies need to be investigated and corrected.
- Reciprocal leveling is an important tool for quality control in surveying. It helps ensure the accuracy and reliability of elevation data, particularly in long leveling traverses.
- Reciprocal leveling is often performed at the beginning and end of a leveling survey to check for closure errors in a closed-loop leveling circuit. A closure error suggests that there may be systematic errors in the measurements.
In summary, reciprocal leveling is a method used to verify the accuracy of leveling measurements by taking readings in both the forward and backward directions between benchmark points. This technique helps surveyors identify and correct errors in the leveling process and ensures the consistency of elevation data.
3.8 Permissible Error in Levelling
- Permissible error in leveling refers to the acceptable level of accuracy or tolerance that is considered reasonable and allowable in leveling surveys. Surveying projects often have defined standards for acceptable errors to ensure that the data collected meets the project's requirements and standards. The permissible error can vary depending on the specific project, the level of precision required, and the applicable surveying standards. Here are some factors to consider when determining the permissible error in leveling:
1. **Project Requirements:** The permissible error in leveling should align with the specific requirements and goals of the surveying project. For example, construction and engineering projects may have different tolerance levels compared to geodetic or topographic mapping projects.
2. **Surveying Standards:** National or international surveying organizations often provide guidelines and standards that specify permissible errors for various types of surveys. Surveyors should adhere to these standards when determining acceptable error limits.
3. **Instrument Accuracy:** The accuracy of the leveling instrument being used plays a significant role in determining permissible errors. High-precision instruments may have stricter error limits than less accurate equipment.
4. **Survey Methodology:** The survey method employed, such as single-section leveling, double-section leveling, or trigonometric leveling, can impact the permissible error. Different methods have varying degrees of accuracy.
5. **Terrain and Survey Conditions:** The topography and conditions of the survey area can also influence the permissible error. For example, surveys in rugged terrain may have more relaxed error limits compared to surveys in flat, open areas.
6. **Length of Sightlines:** Longer sightlines in leveling surveys may require tighter permissible error limits because small errors can accumulate over distance.
7. **Quality Control:** Surveyors often implement quality control measures, such as reciprocal leveling, to check for errors and ensure that measurements fall within permissible limits.
8. **Legal and Regulatory Requirements:** Some surveying projects, particularly those related to property boundaries or legal disputes, may have legally defined error tolerances that must be adhered to.
9. **Client or Stakeholder Expectations:** Client or stakeholder expectations can also influence the acceptable error limits. Clients with specific requirements may set stricter tolerance levels.
10. **Cost Considerations:** Balancing the permissible error with project costs is important. Achieving higher precision often requires more time, resources, and potentially more advanced equipment.
To determine the permissible error in leveling, surveyors should consider the factors mentioned above and define the acceptable error limits in consultation with project stakeholders. These error limits should be documented in the surveying plan and adhered to throughout the surveying process to ensure that the collected data meets the required standards and precision levels.
3.9 Sources of Error in Levelling
- In leveling surveys, like any measurement process, there are various sources of error that can affect the accuracy and reliability of the collected data. Understanding these sources of error is crucial for surveyors, as it allows for error mitigation and quality control. Here are some common sources of error in leveling:
1. **Instrument Errors:**
- **Collimation Error**: This occurs when the line of sight through the instrument is not truly horizontal or vertical. Proper instrument adjustment is essential to minimize collimation errors.
- **Index Error**: It's the error caused by an incorrect setting of the horizontal or vertical circles' zero point.
- **Reading Errors**: Errors can occur when reading the staff, which may result from parallax or misinterpretation of markings.
2. **Environmental Conditions:**
- **Temperature Variations**: Changes in temperature can cause the instrument to expand or contract, leading to inaccurate readings. Thermal gradients can also affect the line of sight.
- **Air Pressure and Density**: Variations in atmospheric pressure and air density can affect the bending of light and cause refraction errors.
- **Wind and Vibrations**: Wind and ground vibrations can disturb the stability of the instrument and introduce errors.
3. **Staff Errors:**
- **Instrument Height (HI) Errors**: Errors in determining the height of the instrument can lead to inaccuracies in the leveling survey.
- **Graduations and Markings**: Imperfections in the staff's graduation marks or the presence of dirt or wear can result in reading errors.
4. **Natural Phenomena:**
- **Earth's Curvature**: Over long distances, the curvature of the Earth can cause a discrepancy between the expected and observed elevations.
- **Atmospheric Refraction**: Refraction of light as it passes through the atmosphere can lead to inaccuracies in sightline angles and distances.
5. **Personnel Errors:**
- **Reading Errors**: Surveyors might make mistakes when recording or interpreting readings from the leveling instrument.
- **Setup Errors**: Errors during instrument setup, including improper leveling or alignment, can affect the survey results.
6. **Gravity**: Variations in local gravitational force can lead to errors in leveling surveys.
7. **Topographical Features:**
- **Slope Effects**: Surveying on slopes can introduce errors, as leveling measurements are typically made along horizontal lines.
- **Obstacles**: Obstacles such as trees, buildings, or terrain features can obstruct the line of sight and introduce errors.
8. **Systematic Errors:**
- **Systematic errors** can arise from errors that consistently affect measurements, often due to a flaw in the instrument, a calibration issue, or a systematic error in the surveying process. Identifying and correcting systematic errors is important for ensuring survey accuracy.
9. **Uncertainty in Benchmarks:**
- Errors in the elevations of benchmarks or reference points can propagate through the leveling survey.
10. **Human Factors:**
- Fatigue, inattention, or lack of training can lead to human errors during the survey process.
To minimize and correct these errors, surveyors often employ a combination of techniques, including reciprocal leveling, quality control checks, calibration, careful instrument handling, and adherence to surveying standards and best practices. Rigorous data analysis and adjustment methods are also used to correct systematic errors and improve the overall accuracy of the leveling survey.
3.10 Trigonometrical Levelling- Problems of Height and Distances
-Trigonometric leveling, also known as height by trigonometry, is a surveying technique used to determine elevations and height differences between points by measuring angles and distances. While it offers advantages for covering long distances and rough terrains, it is not without its challenges. Here are some common problems and considerations associated with trigonometric leveling:
**1. Inaccuracies in Angle Measurements:**
- Angle measurements need to be highly precise for trigonometric leveling. Even small errors in measuring horizontal and vertical angles can lead to significant inaccuracies in determining elevation differences.
**2. Atmospheric Conditions:**
- Variations in atmospheric pressure, temperature, and humidity can affect the bending of light and, consequently, the accuracy of angle measurements. Surveyors need to account for refraction effects when conducting trigonometric leveling.
**3. Line of Sight:**
- Ensuring an unobstructed line of sight between the instrument and the target point is crucial. Obstacles like trees, buildings, and uneven terrain can hinder the surveyor's ability to measure angles accurately.
**4. Distance Measurements:**
- Accurate distance measurements are vital for trigonometric leveling. Errors in distance measurements can lead to significant errors in height calculations. Surveyors should use precise methods for distance measurement, such as EDM (Electronic Distance Measurement) instruments.
**5. Calibration and Maintenance:**
- Theodolites or total stations used for angle measurements need to be regularly calibrated and maintained to ensure their accuracy. Any discrepancies in instrument calibration can introduce errors into the survey.
**6. Earth's Curvature:**
- Over long distances, the curvature of the Earth can affect the accuracy of trigonometric leveling. Surveyors need to account for the Earth's curvature and make corrections to height measurements when working over extended ranges.
**7. Terrestrial Refraction:**
- Terrestrial refraction, the bending of light as it passes through the Earth's atmosphere, can cause discrepancies in observed angles and distances, particularly for long sightlines. Correction formulas must be applied to account for refraction effects.
**8. Geodetic Datums:**
- Accurate trigonometric leveling requires that surveyors use consistent geodetic datums and coordinate systems. Mismatched datums or coordinate systems can result in height discrepancies.
**9. Survey Control Points:**
- The accuracy of trigonometric leveling is dependent on the quality and accuracy of control points used for reference. Errors in the reference points will propagate through the survey.
**10. Complex Topography:**
- Surveying in mountainous or rugged terrain can be challenging for trigonometric leveling, as sightlines may be obstructed, and the terrain may introduce irregularities in elevation data.
**11. Survey Planning:**
- Proper planning and site reconnaissance are essential for successful trigonometric leveling. Identifying suitable instrument stations and planning for control point placement are crucial for accurate results.
To overcome these challenges, surveyors often combine trigonometric leveling with other leveling methods, such as direct leveling, and employ rigorous quality control procedures. Additionally, advances in surveying technology, including GNSS (Global Navigation Satellite Systems) and laser scanning, have provided tools for improving the accuracy of elevation data collection and addressing some of the challenges associated with trigonometric leveling.
3.11 Practical Cases in Levelling
-Levelling, both spirit leveling and trigonometric leveling, is commonly used in various practical applications. Here are some practical cases in which leveling is employed:
1. **Construction and Civil Engineering:**
- Leveling is extensively used in construction projects to establish reference points and determine elevations for site grading, foundation construction, and ensuring that structures are level and plumb.
2. **Road Design and Pavement Construction:**
- Levelling is crucial for road design to ensure proper drainage and safety. It is used to establish the desired road profiles and to check the elevations of various road components.
3. **Floodplain Mapping:**
- In floodplain mapping, leveling is used to determine the elevations of land and water surfaces to predict and manage flood risks.
4. **Pipeline Installation:**
- Leveling is used to lay pipelines at a consistent grade to ensure the smooth flow of fluids from one point to another.
5. **Land Development:**
- In land development projects, leveling is employed to design and prepare land for residential, commercial, or industrial use, ensuring proper drainage and grading.
6. **Land Surveying:**
- Land surveyors use leveling to establish property boundaries, prepare topographic maps, and determine the elevations of various features on the land.
7. **Geodetic Surveying:**
- In geodetic surveying, leveling is used for mapping large areas, establishing a geodetic control network, and measuring the Earth's shape and size.
8. **Airport Runway Construction:**
- Precise leveling is critical for airport runway construction to ensure that runways have the proper slopes and grades for takeoff and landing.
9. **Mining and Quarrying:**
- In mining and quarrying operations, leveling is used to establish benches and gradients to safely extract and transport minerals and materials.
10. **Monitoring Subsidence:**
- In areas prone to subsidence, leveling is used to monitor ground settlement and uplift, helping to detect potential issues with buildings and infrastructure.
11. **Utilities Installation:**
- Leveling is crucial for the installation of utilities such as water supply lines, sewage systems, and underground electrical conduits.
12. **Environmental Studies:**
- Leveling is used in environmental studies to assess the elevation and topography of ecosystems, wetlands, and areas undergoing ecological research.
13. **Archaeological Excavations:**
- Leveling is employed in archaeological excavations to establish the elevation and depth of historical artifacts and structures.
14. Monitoring Sea Level Changes:
- To monitor sea level changes, leveling measurements are used along coastlines to track relative changes in sea level over time.
15. Structural Monitoring:
- In structural engineering, leveling can be used for monitoring the settlement and deflection of structures such as bridges and buildings.
16. Railroad Construction:
- Leveling is essential for designing and constructing railroads to ensure that tracks are level and have the appropriate gradients.
17. Navigation:
- Levelling is used for nautical navigation to establish tidal datums and determine the clearance under bridges or the depth of water at different points in a waterway.
These practical cases demonstrate the versatility and importance of leveling in a wide range of applications, where accurate elevation data is critical for the successful planning and execution of projects. Leveling not only ensures the safety and efficiency of construction and engineering projects but also contributes to land management, environmental monitoring, and various scientific studies.
3.12 Instruction on Field Works
-Instruction on fieldwork in surveying is essential for ensuring that surveyors understand the tasks, methods, and procedures involved in collecting accurate and reliable data. Fieldwork instructions provide surveyors with the necessary guidance to carry out surveys effectively and safely. Here's a general outline of key instructions for fieldwork in surveying:
1. Safety Precautions:
- Start with a safety briefing to address potential hazards in the field, such as uneven terrain, traffic, extreme weather conditions, and wildlife.
- Emphasize the use of personal protective equipment (PPE), including helmets, safety vests, and appropriate footwear.
- Instruct surveyors on the proper handling and safe use of surveying instruments to avoid accidents and damage to equipment.
**2. Equipment Preparation:**
- Ensure that surveying instruments, such as levels, theodolites, total stations, and GNSS receivers, are in good working condition and properly calibrated.
- Provide instructions on instrument setup, including tripod assembly, leveling, and secure mounting of the instrument.
- Train surveyors on the use of various measurement tools, including leveling rods, stadia rods, prisms, and targets.
**3. Survey Control Points:**
- Explain the importance of establishing accurate control points with known coordinates or elevations, and provide instructions on how to set up and maintain control points in the field.
- Train surveyors on techniques for referencing and recording control point information in the field notes.
**4. Field Procedures:**
- Describe the survey method to be employed, whether it's spirit leveling, trigonometric leveling, GNSS surveying, or a combination of methods.
- Instruct surveyors on how to perform backsights, foresights, intermediate sights, and instrument checks, depending on the chosen survey method.
- Provide guidance on measuring distances accurately, including the setup of distance measurement equipment (e.g., EDM, total station, GNSS), and measuring distances according to the chosen method.
- Emphasize the importance of level lines and maintaining a straight line of sight between instrument and target points.
**5. Data Collection:**
- Explain the procedures for recording field data, including elevation readings, angles, distances, and additional survey-specific information.
- Instruct surveyors on how to document any unusual or noteworthy observations, changes in instrument setup, or other relevant details in their field notes.
**6. Quality Control:**
- Stress the need for quality control measures, such as reciprocal leveling, checking for systematic errors, and implementing consistency checks.
- Train surveyors on how to perform error checks and rectify errors in the field, if possible.
**7. Adverse Conditions:**
- Provide guidelines on how to handle adverse weather conditions (e.g., rain, strong wind, extreme temperatures) and their potential impact on measurements.
8. Navigation and Safety:
- Instruct surveyors on how to navigate to survey locations safely, using maps, GPS, or other navigation tools.
- Discuss emergency procedures in case of accidents, equipment failures, or other unforeseen circumstances.
9. Field Notes and Documentation:
- Emphasize the importance of accurate and detailed field notes, including sketches, annotated diagrams, and photographs of the survey area.
- Provide guidance on how to maintain the organization of field notes for later data processing and reporting.
10. Environmental Considerations:
- Discuss the need to minimize environmental impact during fieldwork, especially in sensitive areas.
11. Legal and Ethical Guidelines:
- Ensure that surveyors are aware of any legal requirements, permits, or regulations that apply to the surveying project.
12. Health and Hygiene:
- Remind surveyors to carry essentials like water, food, and first aid supplies, especially in remote or challenging terrain.
13. Debriefing:
- Conclude the fieldwork with a debriefing session to discuss any issues, questions, or observations from the survey.
- Provide a forum for surveyors to share feedback on the fieldwork process and suggest improvements for future surveys.
Clear and comprehensive fieldwork instructions are essential to ensure that the surveying team performs fieldwork efficiently, accurately, and safely. Tailor the instructions to the specific project, survey method, and conditions to achieve the best results. Regular training and ongoing communication between the field team and project managers contribute to the success of surveying projects.