2. levelling

Levelling, a key surveying technique, is used to determine the vertical distance between points, establishing their relative elevation. This process is crucial for various applications, including creating contour maps, longitudinal sections, and estimating earthwork volumes for construction projects. Levelling instruments and staffs are used to measure the height of points, typically relative to a known benchmark. 
 
Elaboration:
  • Purpose:
    Levelling primarily aims to determine the difference in elevation or height between points on the Earth's surface. It's essential for tasks like creating contour maps (showing lines of equal elevation), longitudinal sections (profiles along a line), and calculating volumes of earthwork for excavations and embankments in engineering projects. 
     
  • Process:
    Levelling involves using a levelling instrument (e.g., a dumpy level, tilting level, or automatic level) and a levelling staff (a graduated rod). The instrument is set up and leveled, and readings are taken on the staff placed at different points. 
     
  • Key Concepts:
    • Reduced Level (RL): The elevation of a point above a reference datum (e.g., Mean Sea Level__)). 
       
    • Bench Mark (BM): A permanently established point with a known elevation, used as a reference for levelling. 
       
    • Backsight (BS): A reading taken on the staff at a point with a known elevation (e.g., a bench mark). 
       
    • Foresight (FS): A reading taken on the staff at a point where the elevation is to be determined. 
       
    • Height of Instrument (HI): The height of the level instrument's line of sight above the datum. 
       
  • Methods:
    Levelling can be performed using different methods, including direct levelling (where the instrument is directly set up between the points), trigonometric levelling (where vertical angles and distances are used), and barometric levelling (using a barometer to estimate elevation differences). 
     
  • Instruments:
    Common levelling instruments include dumpy levels, tilting levels, automatic levels (also called auto-levels), and digital levels. They are used in conjunction with a levelling staff, tripod, and other accessories.