Technical terms used in Levelling

 

Levelling is a branch of surveying used to determine the elevation of points relative to a reference datum. Key terminology includes:

  • Datum: An arbitrary reference surface, such as Mean Sea Level (MSL), used to measure vertical elevations. 
  • Reduced Level (RL): The actual vertical elevation or depth of a specific point above or below the chosen datum. 
  • Benchmark (BM): A permanent or temporary reference point with a definitively known elevation. 

  • Back Sight (BS): The very first staff reading taken on a point of known elevation (e.g., a benchmark) after setting up the instrument. 

  • Fore Sight (FS): The final staff reading taken before moving the surveying instrument, representing a point whose elevation needs to be determined. 
  • Intermediate Sight (IS): Any staff reading taken between the Back Sight and the Fore Sight to measure the Reduced Level of additional points. 
  • Height of Instrument (HI): The calculated elevation of the instrument's line of sight, found by adding the Back Sight reading to the RL of the known point. 
  • Change Point (CP): A temporary, stable point where both a Fore Sight (to measure current elevation) and a later Back Sight (after moving the instrument) are taken. 
  • Level Surface: A curved surface (like a still body of water) that is every perpendicular to the pull of gravity.
For any Clarification go through the video link : https://youtu.be/pzX63BgP9PQ

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