How To Calculate RL In The Survey

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If you’re involved in land surveying or construction, understanding how to calculate RL in the survey is crucial. 

RL is a fundamental aspect of surveying, as it determines the vertical distance between a point and a benchmark, usually a reference point with a known elevation. 

In this article, we will explore the various methods for calculating RL in the surveys, providing you with a step-by-step guide, and an example calculation, and highlighting the importance of RL. 

We will also discuss common mistakes to avoid during RL calculations, the benefits of accurate RL determination, and provide you with the formula to calculate RL in the survey.

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Step-by-Step Guide to Calculate RL

Here is a step-by-step guide on how to calculate RL in the survey. 

  1. Establish a Benchmark: The first step in calculating RL is to establish a benchmark with a known elevation. This benchmark will serve as the reference point for all your future RL calculations.
  2. Set up the Leveling Instrument: Position the leveling instrument, such as an automatic level or a dumpy level, on a stable tripod. Make sure it is level and properly calibrated.
  3. Take Backsight Reading: Look through the leveling instrument and take a reading on the benchmark’s staff. This reading gives you the height of the instrument’s line of sight above the benchmark.
  4. Move to the Desired Point: Move the leveling instrument to the point for which you want to calculate the RL. Ensure the instrument is stable and level.
  5. Take Fore-sight Reading: Look through the leveling instrument and take a reading on the staff placed at the desired point. This reading gives you the height of the instrument’s line of sight above the desired point.
  6. Calculate the RL: The RL of the desired point can be calculated by adding the back-sight reading to the height of the instrument (HI) and then subtracting the fore-sight reading.

Example Calculation of RL in Survey

Let’s consider a practical example to understand the RL calculation in the survey better:

  1. Backsight Reading (BS): 1.25 meters (height of instrument above the benchmark)
  2. Fore-sight Reading (FS): 2.80 meters (height of instrument above the desired point)
  3. Height of Instrument (HI): BS + 1.25 meters = 2.50 meters
  4. RL of the Desired Point: HI – FS = 2.50 meters – 2.80 meters = -0.30 meters

In this example, the RL of the desired point is -0.30 meters, which means the point is 0.30 meters below the benchmark.

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Understanding the Importance of RL in Surveys

The Reduced Level is an essential parameter in surveying and construction for several reasons:

  • Height Determination: RL helps determine the elevation of various points on the terrain, facilitating the creation of accurate contour maps and topographical surveys.
  • Setting Elevations: Construction projects require precise RL measurements to set elevations for foundations, roads, and other structures.
  • Water Flow Management: In civil engineering projects, RL is crucial for managing water flow and ensuring proper drainage.
  • Datum for GPS: RL serves as a reference datum for GPS and other geospatial technologies.

Common Mistakes to Avoid in RL Calculation

Here are some common mistakes that we do when we are calculating RL in the survey.

  1. Incorrect Benchmark: Using an incorrect benchmark can lead to erroneous RL calculations. Always verify and cross-check the benchmark’s elevation.
  2. Instrument Errors: Failing to properly calibrate the leveling instrument can introduce errors in the readings and subsequent RL calculations.
  3. Reading Mistakes: Carelessness while reading staff measurements can lead to inaccuracies in the RL calculation.
  4. Neglecting Atmospheric Conditions: Changes in atmospheric pressure and temperature can affect the leveling instrument and, consequently, the RL calculation.

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Benefits of Accurate RL Calculation

Accurate RL determination offers several benefits in surveying and construction:

  • Precision in Construction: Accurate RLs enable precise construction, minimizing errors and rework.
  • Data Comparability: Consistent RL data across projects allows for easier data comparison and analysis.
  • Project Integration: Accurate RL data aids in integrating different aspects of a project, leading to better coordination.
  • Safety: Proper RL calculation helps identify potential hazards and ensures safety on construction sites.

Formula to Calculate RL in Survey

The formula to calculate RL in the survey is as follows:

RL = HI – FS

Where: RL = Reduced Level of the desired point HI = Height of Instrument (Backsight Reading + Height of Instrument) FS = Fore-sight Reading


What is the formula for RL in surveying? 

The formula for calculating RL in surveying is RL = HI – FS, where HI is the Height of the Instrument and FS is the Fore-sight Reading.

How do you calculate HI and RL? 

To calculate HI (Height of Instrument), add the Backsight Reading (BS) to the known elevation of the benchmark. To calculate RL, subtract the Fore-sight Reading (FS) from HI.

How do you calculate RL at a dumpy level? 

Calculating RL with a dumpy level follows the same procedure as with any other leveling instrument. It involves taking back-sight and fore-sight readings and applying the RL formula.

What is the RL measurement? 

RL (Reduced Level) is the vertical distance between a point and a benchmark with a known elevation. It is a fundamental parameter in surveying used for various engineering and construction purposes.


In conclusion, calculating RL in the survey is a vital aspect of land surveying and construction projects. 

Understanding the step-by-step process, ensuring accuracy, and avoiding common mistakes is essential for obtaining reliable RL data. 

Accurate RL measurements enable precise construction, data comparability, and project integration, and enhance overall safety. 

So, next time you embark on a surveying endeavor, remember the significance of RL calculations and their impact on the success of your project.

We hope the given guide on “How to calculate RL in the survey” will help you to accurately calculate the RL.