Technical terms used in levelling

 

Basic Terminologies Used In Leveling

  • Vertical Line : A line that follows the local direction of gravity as indicated by using the plumb Line.
  • Horizontal Line : A line in a horizontal plane. In Surveying , it is a line perpendicular to the local vertical Surface.
  • Level Surface : A Curved surface that at every point is perpendicular to the local plumb line. The plumb line means the direction in which the gravity acts. Level Surfaces are approximately spheroidal in the shape. The common example of a level surface is a body of the still water.
  • Level Line : In a level Surface, it is a curved line .
  • Datum : It is a set of reference point on the surface level by using it measurements of depth and elevation are taken. Geoid is a particular level surface that serves as a datum for all elevations and astronomical observations.
  • Mean Sea Level (MSL) : It was previously Used as a datum line for the measurements.
  • Vertical Datum : It is used as a reference points for the elevation and it is the surface that is arbitrarily assigned an elevation of zero. This level surface is also known as reference datum since using this vertical datum many heights have been related to this surface.
  • Elevation / Reduced level : It is the vertical distance of a point above or below the Datum Line. For example : If the elevation of a point X is 792.8 Ft , So X is a point 792.8 Ft above the reference datum line. The elevation of a point is also called its height above the datum and orthometric height.
  • Benchmark (BM) : A relatively permanent object, natural or artificial having a marked point whose elevation above or below a reference datum is known. Common Examples are metals disk set in a concrete.
  • Back Sight (BS) : It is the first reading taken on a point of Elevation .
  • Fore Sight (FS) : It is the last reading taken on a point before shifting the instrument. It is usually the point whose elevation is to be measured.
  • Line Of Sight / Line of Collimation : It is the line passing through the optical center of the object lens and its intersection with the cross hair and continuation towards the eyepiece and entering the thick eye piece.
To know more about this concept in video  click the below link :https://youtu.be/pzX63BgP9PQ

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