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Errors and Corrections of Errors in Levelling

By: Haseeb Jamal / On: Mar 29, 2017 / Notes, Shallow
 
 

Errors in Levelling

Ollowing are some of the errors encountered in Levelling survey

  1. Instrumental Errors
  2. Collimation Error
  3. Error due to Curvature & Refraction
  4. Other Errors

Also See: Levelling Tools and Equipment

1. Instrumental error and Correction

  1. Collimation error
    • Correction: Check before use and equalise sights.
  2. Under sensitive bubble.
  3. Errors in staff graduation
    • Correction: Check
  4. Loose tripod head.
  5. Telescope not parallel to bubble tube
    • Correction: Permanent adjustment.
  6. Telescope not at right angles to the vertical axis
    • Correction: Permanent adjustment

2. Error of Collimation:

Collimation error occurs when the collimation axis is not truly horizontal when the instrument is level. The effect is illustrated in the sketch below, where the collimation axis is tilted with respect to the horizontal by an angle a:

Error of Collimation

3. Curvature & Refraction

For detailed explanation Click here...

Other sources of errors in levelling and their correction:

Type of Levelling Error Correction
1. Incorrect setting-up of instrument.  
2. Movement of staff from position when changing level station.
  • Training the staff men
  • Experienced/Skilled Staff men
3. Staff not held vertically.
  • Hold rod firmly; Use head/body to support it.
4. Parallax: Instrument knocked or moved during backsight-foresight reading
  • Adjust parallax error if any
5. Ground heating causes chaotic refraction of light
  • Shorten the length of shots Shorten the length of shots
  • Keep measurement 2 Keep measurement 2- -3 ft above ground 3 ft above ground
  • Avoid leveling during noon hours
6. Tripod or rod settles between measurements e.g Bubble off center
  • Quick measurements between rods Quick measurements between rods
  • Avoid muddy or thawing ground Avoid muddy or thawing ground
  • Avoid hot asphalt Avoid hot asphalt
  • Don’t exert pressure on turning point
7. Staff not properly extended and locked.  
 

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