Levelling

Introduction

Levelling is an essential technique used in civil engineering to determine the relative heights or elevations of different points on the ground. It plays a crucial role in various construction and surveying projects. This topic will cover the fundamentals of levelling, different types of levels and levelling staff, measurements and recording in levelling, curvature and refraction correction, reciprocal levelling, sensitivity of level, real-world applications, and the advantages and disadvantages of levelling.

I. Introduction

A. Importance of Levelling in Civil Engineering

Levelling is of utmost importance in civil engineering as it helps in establishing accurate elevations for various construction projects. It ensures that the structures are built on a level plane and provides a reference for determining the slope and gradient of the land.

B. Fundamentals of Levelling

Levelling is based on the principle of establishing a horizontal line of sight using a levelling instrument and measuring the vertical distance between the instrument and the points of interest.

II. Types of Levels and Levelling Staff

A. Definition and Purpose of Levels

Levels are optical instruments used to establish a horizontal line of sight. They consist of a telescope mounted on a tripod and are equipped with a spirit level to ensure accuracy.

B. Different Types of Levels

There are several types of levels commonly used in levelling:

1. Dumpy Level

The dumpy level is a simple and commonly used level. It consists of a telescope mounted on a tripod and is manually adjusted to establish a horizontal line of sight.

1. Tilting Level

The tilting level is similar to the dumpy level but has an additional feature that allows the telescope to be tilted to different angles. This makes it suitable for measuring slopes and gradients.

1. Automatic Level

The automatic level is a more advanced level that uses a compensator to automatically level the telescope. It is faster and more accurate than manual levels.

1. Digital Level

The digital level is a modern level that uses electronic sensors to measure the height difference between points. It provides precise and instant readings.

C. Levelling Staff

A levelling staff is a graduated rod used to measure the vertical distance between the level and the points of interest. It is usually made of wood or aluminum and is marked with graduations in meters or feet.

1. Definition and Purpose

The levelling staff is used to measure the vertical distance or height difference between the level and the points being surveyed. It is placed at the points of interest, and the readings are taken using the level.

2. Types of Levelling Staff

There are different types of levelling staffs available:

a. Self-reading Staff

The self-reading staff has a movable target that can be read directly through the telescope of the level. It eliminates the need for manual reading and reduces errors.

b. Target Staff

The target staff has a fixed target that is viewed through the telescope. The readings are taken by aligning the target with the crosshairs of the telescope.

c. Folding Staff

The folding staff is a compact and portable staff that can be folded for easy transportation. It is commonly used in field surveys.

III. Measurements and Recording in Levelling

A. Levelling Instrument Setup

To perform levelling, the level and the levelling staff need to be set up correctly.

1. Setting up the Level

The level is set up on a stable tripod, and the bubble in the spirit level is adjusted to center. This ensures that the line of sight is horizontal.

2. Setting up the Levelling Staff

The levelling staff is placed vertically at the points of interest, and the staff readings are taken using the level.

B. Taking Levelling Measurements

To measure the height difference between points, the staff readings are taken using the level.

1. Booking the Staff Readings

The staff readings are recorded in a field book. The readings include the staff reading, the instrument height, and the distance between the level and the staff.

2. Calculation of Reduced Levels

The reduced levels are calculated by subtracting the staff reading from the instrument height. This gives the height difference between the level and the points being surveyed.

C. Recording Levelling Data

The levelling data is recorded in a field book and can also be sketched to visualize the levelling line.

1. Field Book Entries

The field book entries include the date, the point number, the staff reading, the instrument height, the distance, and the reduced level.

2. Sketching the Levelling Line

The levelling line can be sketched to visualize the height difference between points. It helps in understanding the topography of the surveyed area.

IV. Curvature and Refraction Correction

A. Curvature Correction

Curvature correction is necessary in levelling to account for the curvature of the Earth.

1. Definition and Explanation

The curvature of the Earth causes the line of sight to deviate from a straight line. This deviation needs to be corrected to obtain accurate levelling measurements.

2. Formula for Curvature Correction

The formula for curvature correction is given by:

$$C = \frac{{d^2}}{{2R}}$$

Where:

• C is the curvature correction
• d is the distance between the level and the point being surveyed
• R is the radius of the Earth

B. Refraction Correction

Refraction correction is necessary in levelling to account for the bending of light as it passes through the Earth's atmosphere.

1. Definition and Explanation

Refraction causes the line of sight to deviate from a straight line. This deviation needs to be corrected to obtain accurate levelling measurements.

2. Formula for Refraction Correction

The formula for refraction correction is given by:

$$R = \frac{{0.13h}}{{d}}$$

Where:

• R is the refraction correction
• h is the height of the instrument
• d is the distance between the level and the point being surveyed

C. Combined Curvature and Refraction Correction

The combined curvature and refraction correction is obtained by adding the curvature correction and the refraction correction.

V. Reciprocal Levelling

A. Definition and Purpose of Reciprocal Levelling

Reciprocal levelling is a technique used to check the accuracy of levelling measurements by taking readings in both the forward and backward directions.

B. Procedure for Reciprocal Levelling

To perform reciprocal levelling, the following steps are followed:

1. Setting up the Level

The level is set up at a point, and the instrument height is recorded.

2. Taking Backsights and Foresights

Backsights are taken by sighting a point of known elevation, and foresights are taken by sighting a point whose elevation is to be determined.

3. Calculation of Reduced Levels

The reduced levels are calculated by subtracting the backsight reading from the foresight reading and adding the instrument height.

VI. Sensitivity of Level

A. Definition and Explanation of Sensitivity

The sensitivity of a level refers to its ability to detect small changes in elevation. It is a measure of the smallest difference in height that can be observed using the level.

B. Factors Affecting Sensitivity

The sensitivity of a level is affected by various factors:

• The design and construction of the level
• The quality of the optics
• The precision of the leveling staff
• The skill and experience of the operator

C. Importance of Sensitivity in Levelling

A sensitive level is essential for obtaining accurate and precise levelling measurements. It allows for the detection of small changes in elevation, which is crucial in construction and surveying projects.

VII. Real-World Applications and Examples

A. Use of Levelling in Construction Projects

Levelling is extensively used in construction projects to establish accurate elevations for foundations, floors, and other structural elements. It ensures that the structures are built on a level plane and provides a reference for determining the slope and gradient of the land.

B. Levelling for Road and Railway Alignment

Levelling is crucial in road and railway alignment to ensure a smooth and safe passage for vehicles and trains. It helps in determining the required cut and fill for the road or railway track.

C. Levelling for Determining Ground Profiles

Levelling is used to determine the ground profiles for various purposes, such as designing drainage systems, planning landscaping projects, and assessing the suitability of land for construction.

VIII. Advantages and Disadvantages of Levelling

A. Advantages

Levelling offers several advantages in civil engineering and surveying:

1. Accurate Determination of Elevations

Levelling provides accurate and precise measurements of elevations, ensuring that structures are built on a level plane.

1. Essential for Construction and Surveying

Levelling is an essential technique in construction and surveying projects. It provides a reference for determining the slope and gradient of the land.

B. Disadvantages

Levelling has some limitations and disadvantages:

1. Time-consuming Process

Levelling can be a time-consuming process, especially for large-scale projects. It requires careful setup and measurement, which can take a significant amount of time.

1. Affected by Environmental Factors

Levelling measurements can be affected by environmental factors such as temperature, humidity, and atmospheric conditions. These factors can introduce errors and affect the accuracy of the measurements.

This concludes the topic on levelling, covering the fundamentals of levelling, different types of levels and levelling staff, measurements and recording in levelling, curvature and refraction correction, reciprocal levelling, sensitivity of level, real-world applications, and the advantages and disadvantages of levelling.

Summary

Levelling is an essential technique used in civil engineering to determine the relative heights or elevations of different points on the ground. It plays a crucial role in various construction and surveying projects. This topic covers the fundamentals of levelling, different types of levels and levelling staff, measurements and recording in levelling, curvature and refraction correction, reciprocal levelling, sensitivity of level, real-world applications, and the advantages and disadvantages of levelling.

Analogy

Levelling can be compared to using a ruler to measure the height difference between two points on a piece of paper. The ruler represents the levelling staff, and the measurement markings on the ruler represent the graduations on the staff. By placing the ruler vertically between the two points and reading the measurement, we can determine the height difference. Levelling works in a similar way, but instead of a ruler, we use a levelling instrument and a levelling staff to measure the vertical distance between points on the ground.