Levelling and Its Terminology

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Questions and Answers

What is the primary purpose of using closed level circuits?

To gather data from new benchmarks only

To check for gross misclosure (correct)

To avoid using existing benchmarks

To ensure readings are taken quickly

Which method is NOT used for reducing readings?

Rise & Fall Method

Height of Collimation Method

Benchmark Calculation Method (correct)

Both A and B

Which of the following descriptions best fits an open level circuit?

Starting from a benchmark and finishing at a new mark (correct)

Starting from a benchmark and returning to the same benchmark

Using multiple benchmarks in sequence

Constantly reiterating measurements to verify data

What is a common cause of errors due to handling equipment?

Not adjusting the level properly Signup and view all the answers

Which precaution is vital for ensuring a stable instrument setup?

Pressing tripod feet firmly into the ground Signup and view all the answers

What should be verified before adopting a benchmark (BM) for use as a datum?

It is reliable and still in its original position Signup and view all the answers

What is one way to adjust for instrument errors in leveling?

Conduct a 2-Peg test Signup and view all the answers

What is a common source of errors due to the displacement of equipment?

Settlement of level Signup and view all the answers

What is the primary function of a level instrument?

To provide a horizontal line of sight Signup and view all the answers

Which type of level is considered the most popular?

Automatic level Signup and view all the answers

What does Reduced Level (RL) refer to?

The height of a point above or below a datum Signup and view all the answers

What is a Backsight (BS) in levelling terminology?

The first reading after the instrument has been set up Signup and view all the answers

How does curvature affect levelling for longer distances?

The level line and horizontal line diverge Signup and view all the answers

What is the difference between a Benchmark (BM) and a Temporary Benchmark (TBM)?

BM is permanent, while TBM is temporary Signup and view all the answers

What is the primary characteristic of a level line?

It is located at a constant height above MSL Signup and view all the answers

What does the term Collimation Height refer to?

The height of the line of sight above datum Signup and view all the answers

What method should be used if the levelling exercise has many Intermediate Stations (IS)?

Height of Collimation Method Signup and view all the answers

What does a positive difference in staff reading indicate during levelling?

The first point is lower than the second point Signup and view all the answers

Which check is essential for detecting mistakes in a closed level circuit?

Summation Check Signup and view all the answers

In the context of levelling, what is a Change Point (CP)?

A point where new readings are recorded along the same row Signup and view all the answers

What is indicated by a negative difference in staff reading?

The first point is higher than the second point Signup and view all the answers

What should be done if reduced levels are derived from BS, IS and FS readings?

Follow normal algebra rules for addition and subtraction Signup and view all the answers

What must be recorded at the beginning and end of the levelling process?

Backsight (BS) and Foresight (FS) Signup and view all the answers

Which check is used primarily for identifying arithmetic errors in levelling readings?

Summation Check Signup and view all the answers

What is the primary purpose of levelling?

To find the height difference of points on the earth's surface Signup and view all the answers

Which of the following best describes a level surface?

A spherical surface tangential to the direction of gravity Signup and view all the answers

What is the meaning of 'datum line' in levelling terminology?

Any lines to which elevations on the ground are referred Signup and view all the answers

Which instrument is typically used to obtain a truly horizontal line of sight in levelling?

A level instrument Signup and view all the answers

What does the term 'Mean Sea Level (MSL)' represent in levelling?

The average level of the sea over time Signup and view all the answers

In the context of levelling, what does a 'horizontal line' refer to?

A straight line tangential to a level line Signup and view all the answers

What is the importance of using a graduated staff in levelling?

To read elevations at specific ground points Signup and view all the answers

Which aspect differentiates ordinary levelling from precise levelling?

The requirement of highly accurate and refined results Signup and view all the answers

Study Notes

Levelling

Levelling is a process to find height differences between points on the earth's surface, and to determine the reduced level of a point
It's used in many construction projects like building control and road design
Ordinary levelling uses instruments, graduated staffs, and procedures to determine accurately the height differences
A level instrument is designed to provide a horizontal line of sight (line of collimation)

Terminology

Level surface: A surface perpendicular to the direction of gravity (e.g., a still pond)
Level line: A line on a level surface, with a constant height above a datum
Horizontal line: A line tangential to a level line
Datum line (surface): A line used as a reference for measuring heights
Mean Sea Level (MSL): The average level of the sea, over a period of time (e.g., one year) and used as a reference for heights in Singapore
Reduced level (RL): The height or elevation of a point, above or below a datum

Equipment

Dumpy level: A common levelling instrument that provides a horizontal line of sight
Tilting level: (Description not included)
Automatic level: A popular type of levelling instrument, with automatic compensation features
Digital level: A levelling instrument with digital readout

Levelling Staff

Backsight (BS): The first reading after setting up the instrument
Foresight (FS): The last staff reading before moving the instrument
Intermediate sight (IS): A reading between a BS and a FS
Change point (CP): A point where a staff is held, and BS & FS readings are taken
Benchmark (BM): A point with a known height above a datum, often used as a reference
Temporary benchmark (TBM): A temporary benchmark similar to a BM

Levelling Procedures

Rise: the 1st point is lower than the 2nd point
Fall: the 1st point is higher than the 2nd point
Data is recorded as BS, IS or FS
Reduced Levels are derived from BS, IS and FS
Levelling starts with a BS and ends with a FS in most cases
Rise & Fall Method: Used when several change points (CPs) are involved in a levelling process, easier to use with change points (CPs)
Height of Collimation Method: A method of recording levelling readings, often used when measuring high points

Level Circuits

Closed circuit: Start and end at the same benchmark (BM) - this allows a verification check
Open circuit: Start at a BM and end at a new location, not suited for verification

Sources of Errors & Precautions

Instrument errors: Issues with the line of sight, staff not properly extended
Errors in handling equipment: Instability, bubble misalignment
Errors due to displacement: Ground settling, unstable points
Errors in reading & recording: Inverted staff image
Precautions: Adjusting the levels, maintaining equal distances (BS/IS/FS) and ensuring a stable setup

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Description

This quiz covers the fundamentals of levelling, a critical process used in construction to determine height differences between points on the earth's surface. Key terminology related to levelling, such as level surface, horizontal line, and datum line, will be explored to enhance understanding of this essential surveying technique.