What is Levelling ?
“Levelling” is the process of determining and adjusting the height or elevation of points on the Earth’s surface relative to a common datum, usually mean sea level. It is commonly used in surveying and construction to ensure that structures are built level and accurately reflect the intended design elevation.
Levelling is a surveying technique that determines the relative heights of points on Earth’s surface. It involves establishing a horizontal line of sight and measuring the vertical distance between this line and points of interest.
Table of Contents
Why Do We Level?
Levelling is essential for various reasons:
Determining Elevations
This is crucial for projects like road and dam construction and irrigation systems.
Establishing Gradients
Gradients are vital for designing infrastructure like roads, railways, and drainage systems.
Creating Topographic Maps
These maps show Earth’s surface features, including elevations, which are determined through levelling.
Setting Construction Levels
Levelling ensures structures are built at the correct heights.
How Does Levelling Work?
Levelling relies on the principle that a horizontal line of sight is perpendicular to the direction of gravity at any point on Earth. By establishing this line and measuring vertical distances to different points, we can calculate their relative heights.
Tools of the Trade
The primary tool in levelling is the level. There are two main types:
Dumpy Level: A sturdy instrument with a fixed telescope that requires precise adjustments.
Tilting Level: A modern and convenient level with a tilting telescope that allows for quick adjustments.
Setting Up the Level
Before using a dumpy level, it must be adjusted to ensure accurate readings. This involves:
Leveling the Instrument: Using leveling screws and a bubble tube to make the instrument horizontal.
Focusing the Eyepiece: Adjusting the eyepiece for a clear view of the crosshairs.
Focusing the Objective Lens: Adjusting the objective lens for a clear image of the staff (a graduated pole used for measurements).
Methods of Levelling
There are two primary methods:
Height of Instrument (H.I.) Method
Calculate the height of the instrument’s line of sight above a reference point.
Determine the elevation of other points by subtracting staff readings from the H.I.
Rise and Fall Method
Calculate the difference in elevation between two points by subtracting lower readings from higher ones.
A rise indicates an upward slope, while a fall indicates a downward slope.
Recording and Calculating Levels
Levelling data is recorded in a level book, which typically includes columns for:
- Station: The point where the staff reading is taken.
- BS (Backsight): The reading on a point of known or assumed elevation.
- IS (Intermediate Sight): Readings on points between the backsight and foresight.
- FS (Foresight): The reading on a point of unknown elevation.
- H.I. (Height of Instrument): The height of the instrument’s line of sight.
- RL (Reduced Level): The elevation of the point relative to a reference point.
- Rise: The difference in elevation between two points, calculated as BS – IS or IS – FS.
- Fall: The difference in elevation between two points, calculated as IS – BS or FS – IS.
Both the H.I. and Rise and Fall methods can be used to calculate elevations. The choice of method depends on personal preference and the specific needs of the project.
Frequently Asked Questions (FAQ)
How are elevations calculated using the Rise and Fall method?
The Rise and Fall method involves calculating the difference in elevation between consecutive points by subtracting lower readings from higher readings.
How are elevations calculated using the H.I. method?
The H.I. method involves calculating the height of the instrument’s line of sight and then subtracting staff readings to determine elevations.
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