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?

<p>Not adjusting the level properly (D)</p> Signup and view all the answers

Which precaution is vital for ensuring a stable instrument setup?

<p>Pressing tripod feet firmly into the ground (A)</p> Signup and view all the answers

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

<p>It is reliable and still in its original position (D)</p> Signup and view all the answers

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

<p>Conduct a 2-Peg test (B)</p> Signup and view all the answers

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

<p>Settlement of level (A)</p> Signup and view all the answers

What is the primary function of a level instrument?

<p>To provide a horizontal line of sight (D)</p> Signup and view all the answers

Which type of level is considered the most popular?

<p>Automatic level (B)</p> Signup and view all the answers

What does Reduced Level (RL) refer to?

<p>The height of a point above or below a datum (A)</p> Signup and view all the answers

What is a Backsight (BS) in levelling terminology?

<p>The first reading after the instrument has been set up (B)</p> Signup and view all the answers

How does curvature affect levelling for longer distances?

<p>The level line and horizontal line diverge (C)</p> Signup and view all the answers

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

<p>BM is permanent, while TBM is temporary (A)</p> Signup and view all the answers

What is the primary characteristic of a level line?

<p>It is located at a constant height above MSL (C)</p> Signup and view all the answers

What does the term Collimation Height refer to?

<p>The height of the line of sight above datum (B)</p> Signup and view all the answers

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

<p>Height of Collimation Method (C)</p> Signup and view all the answers

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

<p>The first point is lower than the second point (D)</p> Signup and view all the answers

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

<p>Summation Check (D)</p> Signup and view all the answers

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

<p>A point where new readings are recorded along the same row (C)</p> Signup and view all the answers

What is indicated by a negative difference in staff reading?

<p>The first point is higher than the second point (D)</p> Signup and view all the answers

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

<p>Follow normal algebra rules for addition and subtraction (C)</p> Signup and view all the answers

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

<p>Backsight (BS) and Foresight (FS) (D)</p> Signup and view all the answers

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

<p>Summation Check (B)</p> Signup and view all the answers

What is the primary purpose of levelling?

<p>To find the height difference of points on the earth's surface (B)</p> Signup and view all the answers

Which of the following best describes a level surface?

<p>A spherical surface tangential to the direction of gravity (A)</p> Signup and view all the answers

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

<p>Any lines to which elevations on the ground are referred (B)</p> Signup and view all the answers

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

<p>A level instrument (B)</p> Signup and view all the answers

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

<p>The average level of the sea over time (C)</p> Signup and view all the answers

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

<p>A straight line tangential to a level line (B)</p> Signup and view all the answers

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

<p>To read elevations at specific ground points (B)</p> Signup and view all the answers

Which aspect differentiates ordinary levelling from precise levelling?

<p>The requirement of highly accurate and refined results (C)</p> Signup and view all the answers

Flashcards

Levelling

Finding the height difference of points on the Earth's surface, often to determine reduced levels.

Level surface

A surface where all points are perpendicular to the direction of gravity.

Level line

A line on a level surface with a constant height above a datum.

Horizontal line

A straight line tangential to a level line.

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Datum line

Any line or surface used as a reference for measuring elevations.

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Mean Sea Level (MSL)

The average level of the sea.

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Levelling staff

A graduated staff used for reading elevations.

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Reduced level

Calculated elevation of a point relative to a datum.

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What is the purpose of a level?

A level is designed to provide a horizontal line of sight, called the line of collimation. It is used for measuring heights or elevations.

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What is a Reduced Level (RL)?

The reduced level (RL) is the elevation of a point above or below a chosen reference point, typically Mean Sea Level (MSL).

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What is a Backsight (BS)?

A backsight is the first reading taken on a leveling staff after setting up the level instrument.

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What is a Foresight (FS)?

A foresight is the last reading taken on the leveling staff before moving the level instrument.

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What is an Intermediate Sight (IS)?

An intermediate sight is a reading taken between a backsight and a foresight.

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What is a Change Point (CP)?

A change point is a point where a staff is held for both a foresight and a backsight reading. It allows the instrument to be moved to a new location.

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What is a Benchmark (BM)?

A benchmark is a permanent fixed point of known elevation above a datum.

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What is a Temporary Benchmark (TBM)?

A temporary benchmark is similar to a benchmark but is likely to be removed or changed.

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What is levelling used for?

Levelling is the process of finding the difference in height between points on the Earth's surface. This is commonly done to determine the reduced levels of points relative to a datum.

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What are BMs and their role in levelling?

BMs (Bench Marks) are established points with known reduced levels. They are used as reference points in levelling surveys to ensure accuracy and control the vertical position of other points.

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Rise in levelling

A rise occurs when the second point in a levelling exercise is higher than the first point, resulting in a positive difference in staff readings.

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Fall in levelling

A fall occurs when the second point in a levelling exercise is lower than the first point, resulting in a negative difference in staff readings.

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What are CPs and their role in levelling?

CPs (Change Points) are temporary points established during a levelling survey to facilitate the transfer of height differences from one section of the survey to another. They allow for the process to be completed in stages, reducing the need for continuous measurements from the starting BM.

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Height of Collimation Method

The Height of Collimation Method is used in levelling when a series of intermediate points are involved during the survey. It involves calculating a constant height of collimation (H.C.) and applying it to each staff reading to determine the reduced levels.

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Inverted staff readings

Inverted staff readings are used when measuring points above the line of sight of the level instrument (e.g., ceilings). The staff is held upside down, and the readings are recorded with a negative sign to maintain consistency.

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Arithmetic check in levelling

The arithmetic check is a process of verifying the accuracy of calculations in levelling. The sum of the backsights should be equal to the sum of the foresights plus the difference in reduced levels.

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Rise & Fall Method

A leveling method that calculates the elevation of each point by adding or subtracting the difference between the backsight reading and the foresight reading.

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Level Datum

A reference point used to determine the relative elevations of other points.

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Closed Level Circuit

A leveling circuit where the last reading is taken on the same benchmark as the first reading, allowing for error checking.

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Open Level Circuit

A leveling circuit where the final reading is taken on a different benchmark, making it harder to check for errors.

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Verification of Level Datum

The process of confirming that a benchmark used as a datum is reliable and has not moved or changed in elevation.

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Sources of Errors in Levelling

Factors that can affect the accuracy of leveling measurements, including instrument errors, handling errors, and equipment displacement.

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Precautions to Minimize Errors

Steps taken to prevent or reduce errors during leveling, such as proper instrument adjustment, handling, and stability.

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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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Related Documents

Singapore Polytechnic Surveying Levelling PDF

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.

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