Errors in Levelling

  Errors in Levelling 

Instrumental errors: Error in permanent adjustment of level: For any major surveying work, instrument needs to be tested and if required, gets to be adjusted. For small works, bubble of the level tube should be brought to the center before each reading and balancing of sights are to be maintained. Staff defective and/or of non-standard quality: The graduation in staff may lack standard distance and thus may cause error in reading. In an ordinary leveling, the error may be negligible but in the case of precise leveling, the graduations are to be standardized with invar tape. Error due to defective level tube: The bubble of the level tube may remain central even though the bubble axis is not horizontal due to its sluggishness or it may take considerable time to occupy central position, if it is very sensitive. Also, there may be irregularity in the curvature of the tube causing delirious effect. Error due to defective tripod: The tripod stand should be strong and stable otherwise it causes setting of the instrument unstable and considerable time is required to make it level. The nuts provided at the joints of the legs to the tripod head should be well-tightened before mounting the instrument. The tripod should be set up on a stable, firm ground.

 Personal errors: Due to imperfection in temporary adjustment of the instrument These errors are caused due to careless setting up of the level, improper leveling of the instrument, lack in focus of eyepiece or/and objective and error in sighting of the staff. Careless set-up of the instrument: If the instrument is not set up firmly, it gets disturbed easily. If the ground is not firm, it may have settled down and on hard ground, it may get slipped.  Imperfect leveling of the instrument: Due to improper leveling of the instrument, bubble does not remain at the center when the sights are taken resulting error in reading. To avoid the error, the bubble should be brought to the center before each reading.  Imperfect focusing. If either the eye-piece or the objective or both are not properly focused, parallax and thus error in the staff readings occur. Due to movement of eyes if there is any apparent change in the staff reading the eye-piece and objective need proper focusing.

Errors in sighting: This occurs when the horizontal cross-hair does not exactly coincide with the staff graduation or it is difficult to see the exact coincidence of the cross hairs and the staff graduations. The error can be minimized by keeping the sight distance small. 

Error due to staff held Non-vertical. If the staff is not held vertical, the staff reading obtained is greater than the correct reading. To reduce the error, the staff should be held exactly vertical or the staff man should be asked to waive the staff towards the instrument and then away from the instrument and the lowest reading should be taken. 

Errors in reading the staff: These errors occur if staff is read upward, instead of downwards, read against the top or bottom hair instead of the central hair, mistakes in reading the decimal part and reading the whole meter wrongly. Errors in recording: The common errors are entering a wrong reading (with digits interchanged or mistaking the numerical value of a reading called by the level man), recording in wrong column, e.g., B.S. as I.S., omitting an entry, entering the inverted staff reading without a minus sign etc. 

Errors in computing: adding the fore sight reading instead of subtracting it and or subtracting a back sight reading instead of adding

Natural errors:

Error due to curvature: In case of small sight distance error due to the curvature are negligible, but if the sight distances are large, the error should be estimated and accounted for, as discussed below. However, the error can be minimized through balancing of sight or reciprocal observation.

 Errors due to wind: Strong wind disturbs leveling of an instrument and verticality of staff. Thus, it is advisable to suspend the work in this condition.

 Errors due to sun: Due to bright sunshine on the objective, staff reading cannot be taken properly. To avoid such error, it is recommended to maintain a shed to the objective. 

Errors due to temperature: Temperature of the atmosphere disturbs setting of parts of instrument as well as causes fluctuation in the refraction of the intervening medium. These lead to error in staff reading. Disturbance caused to instrument may be minimized by placing the instrument under shed.