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Errors and Corrections of Errors in Levelling

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

Errors in Levelling

Following 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
    • Collimation error occurs when the line of sight of the instrument is not parallel to the horizontal axis or the line of collimation. This error can lead to inaccurate measurements. The correction for collimation error involves checking the alignment of the instrument before its use and making adjustments to equalize the sights.
    • Correction: Check before use and equalize sights.
  2. Under sensitive bubble
    • The bubble is an important component of a leveling instrument, such as a level or a theodolite, used to ensure the instrument is leveled correctly. An under-sensitive bubble means that the bubble does not respond adequately to slight variations in the instrument's position. This error can be corrected by adjusting the instrument's leveling screws or using a more sensitive bubble.
  3. Errors in staff graduation
    • Staff graduation refers to the markings on a leveling staff, which are used for height measurements. Errors in staff graduation can occur due to inaccuracies in the markings, such as improper spacing or misalignment. To correct this error, the staff should be carefully checked against a known reference or recalibrated if necessary.
    • Correction: Check
  4. Loose tripod head
    • The tripod head is the part of the tripod that holds the instrument. If the tripod head is loose, it can result in instability and movement of the instrument, leading to errors in measurements. The correction for a loose tripod head involves tightening the screws or clamps that secure the instrument to the tripod.
  5. The telescope is not parallel to the bubble tube
    • This error occurs when the telescope of the instrument is not parallel to the bubble tube, affecting the accuracy of measurements. The correction for this error typically involves making a permanent adjustment to align the telescope and the bubble tube properly.
    • Correction: Permanent adjustment.
  6. The Telescope is not at right angles to the vertical axis
    • If the telescope is not perpendicular to the vertical axis, it can introduce angular errors in measurements. The correction for this error also involves making a permanent adjustment to ensure the telescope is 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 a detailed explanation Click here...

Other sources of errors in leveling and their correction:

Type of Levelling Error Correction
1. Incorrect setting-up of the instrument.  
2. Movement of staff from the position when changing level station.
  • Training the staff men
  • Experienced/Skilled Staff men
3. Staff not held vertically.
  • Hold the rod firmly; Use your 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 the turning point
7. Staff not properly extended and locked.  
 

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