The sources of error in levelling are as below
1) Instrumental errors: Error due to imperfect adjustment, Sluggish bubble, Faulty
Focusing tube, Erroneous leveling staff.
2) Personal errors: Errors of manipulation, Imperfect sighting ,Errors due to settlement of staff and level stand
3) Errors due to natural sources
1) Instrumental errors:
i) Error due to imperfect adjustment: Line of sight is inclined upward or downwards. Axis of the bubble tube not being perpendicular to the vertical axis, line of collimation not being parallel to the bubble axis. Instrument should be get tested and adjusted before use, equalize the fore sight and back sight distance to minimize this error.
ii) Sluggish bubble: when bubble is sluggish then it does not show correct horizontal
Line of sight. To get good work replace it by good bubble tube.
iii) Faulty focusing tube : Due to this object does not move in horizontal plane during focusing
iv) Erroneous levelling staff: Division on the leveling staff are faulty. Replace it by correct staff.
2) Personal Errors:
i) Errors of manipulation: Careless levelling, Bubble not in center, Resting hands on tripod,
Staff not being held exactly vertical.
ii) Imperfect sighting: Poor focusing, Parallax error not removed.
iii) Error due to settlement of staff and level: set up the instrument, and hold the staff on firm ground, proper selection of C.P. and instrument station location.
3) Errors due to natural causes:
i) Curvature of earth ii)Refraction iii)Effect of wind iv) effect of sun
In ordinary levelling the curvature and refraction error are not consider as they are very less 0.003m for 300m length. Use umbrella to shelter the instrument, use sun shade, avoid levelling work during wind.