Sources of Error in Levelling PDF

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Sources of Error in Levelling 1) Tripod condition – tripod head and shoe bolts/screws may have become loose thus making the tripod unsteady; ensure all bolts are fastened tightly so that there is no movement of the tripod once the legs have been set-up by firmly pressing the tripod shoes into the ground. Also ensure that the tripod clamps on the legs are working satisfactorily.

2) Instrument not levelled correctly – ensure instrument is levelled correctly using the circular bubble; electronic instruments give warning when not levelled correctly. 3) Collimation error – evident when the line of collimation is not horizontal; this will result in error when measuring staff readings and thus final elevations. This error will be evident if the backsight and foresight lengths are not of an equal distance. To eradicate collimation error, backsight and foresight lengths must be at an equal distance. It is preferable to perform a “two peg test” to determine whether the instrument is in adjustment, i.e. that the line of collimation is horizontal or not. If it is found that the instrument (level) is out of adjustment then the instrument must be adjusted to ensure that the line of collimation is horizontal. 4) Compensator not functioning – in automatic levels the function of a compensator is to deviate a horizontal ray of light at the optical centre of the object lens through the centre of the cross hairs. The compensator mechanism ensures that the line of sight is horizontal even when the telescope may be slightly tilted. To check that the compensator is working move one of the levelling footscrews slightly off level. If the reading to the levelling staff remains the same then the compensator is functioning correctly. If this is not the case then gently tapping the instrument will release the compensator; some instruments have a dedicated lever or button which “moves” the compensator. This action will move the cross hairs after which they will return to the horizontal line of sight.

5) Levelling staff defects – levelling staves can become worn over time or they may be damaged resulting in the base of the staff not being at zero measurement. New and well maintained staves will have a zero measurement, i.e. the base of the staff measures zero (0.000 metres). If a levelling staff does not have a zero error and is the only levelling staff being used then height differences will not be affected. However, if two staves are used that have differing “zero” base measurements then errors will be introduced. Ensure that both staves have a coincident zero measurement or the same constant error.

6) Levelling staff not fully extended – levelling staves that are of the extendable type must be fully extended. If a section of the staff is not fully extended then measurement errors will be introduced. 7) Levelling staff not held vertical – the levelling staff must be held vertical to obtain the true vertical difference from the ground to the plane of collimation; failing this, incorrect measurements will be taken. To overcome this problem, a circular bubble mounted in a level bracket (that has been calibrated) must be used. Alternatively, the staff can be gently rocked back and fore in the direction of the instrument and the minimum measurement recorded. 8) Incorrect measurements recorded – if the levelling staff is held near to, but not on the benchmark, levelling plate or point to be levelled, then the face of the levelling staff must be turned away from the instrument to avoid measurements being recorded by mistake. 9) Unstable ground – may cause tripod legs and levelling plates (at the staff) to sink (or rise) slightly into the ground. This may occur on soft surfaces such as sand, grass, marsh or even tarred surfaces that are subjected to hot weather conditions. The impact of such movement at the instrument will be that the height of collimation will have changed and the instrument may no longer be level. The impact of such movement at the levelling staff will be that the staff elevation reading relative to the height of collimation will have changed. To overcome these problems it is advisable to ensure that the tripod feet or levelling plate are pushed well into the ground and that measurements are

recorded in quick succession to reduce the time that the tripod and/or levelling plate may have to sink into the ground. Ideally, it is preferable to set-up the tripod and have change points on stable ground. 10) Movement/disturbance of instrument and tripod – ensure personnel in the vicinity of the tripod do not touch or disturb the instrument and/or tripod. The surveyor must handle the instrument carefully and not straddle the tripod legs which may cause accidental movement of the tripod and thus the instrument. If the instrument and tripod are disturbed and the instrument is no longer level then the procedure of re -levelling the instrument and re-measuring must be performed.

11) Parallax – is evident when eyepiece and focusing screws are not adjusted correctly for use. Parallax can be detected by moving the eye about the eyepiece, if the staff appears to move about the eyepiece in relation to the cross hairs then parallax is present; reading accurate measurements will be difficult under these conditions. The cross hairs must be focused and focusing of the image using the focusing screw must be done correctly.

12) Measurement reading and booking errors – the transposing of numbers can easily be done and will result in errors of reduced elevations. What is booked (written) in the fieldbook may not be what was sighted and read from the levelling staff, i.e. the measurement of 2.234m may be entered in the fieldbook as 2.324m. This blunder will result in a 90mm error in one reading! It is important to enter the reading immediately into the fieldbook and then re-read the levelling staff as a check and check the entry in the fieldbook. When using digital levels this problem is eradicated because measurements are automatically recorded if using the data-logger.

13) Reduced elevation calculation errors – ensure all checks are performed for both “Height of Collimation” and “Rise and Fall” methods for the reducing of elevations. All levelling exercises must start and finish on a known point (with known elevation) or be closed-loop or double-run circuits with a point of known elevation within the circuit. Do not delay in calculating reduced elevations; it is

important to know of any errors as soon as possible so that they can be rectified either by re-calculating or re-levelling (if necessary). Where levelling exercises have multiple pages of readings, each page should be checked separately. 14) Weather conditions – do not level in strong winds or when there is excessive shimmer, especially for precise levelling. In strong winds, the levelling staff will be difficult to hold steady and readings to the levelling staff will difficult because of movement of the levelling staff. Additionally, strong winds will cause the instrument to “move” or “shudder”; this is more evident when using an automatic level because of induced movement of the compensator. With excessive shimmer, the image of the levelling staff will be blurred and thus make it difficult to read. It is uncomfortable for the surveyor to work in poor weather conditions and concentration will be tested! 15) Vibration – when levelling (and surveying in general) next to or in the vicinity of heavy machinery, e.g. conveyors, mills, generators etc., vibration may cause the instrument to vibrate. The vibration of the instrument will make it difficult to read measurements and may cause damage to the instrument.

16) Moving of the levelling plates and instrument at the wrong time – when reading backsights and foresights it is extremely important that neither staff set-up is moved until the measurement has been taken, entered into the fieldbook and then checked. If a levelling plate at a change point is moved before a clear signal to move has been conveyed then it will be necessary to re-level that leg of levelling. The same applies for moving the instrument before both staves have been read. This is usually caused by a lack of concentration especially after a long day in the field! This particular source of error in levelling highlights the importance of installing temporary bench marks along the levelling route to mitigate the risk of having to start the levelling run from the start point. 17) Care and maintenance of equipment – if any piece of equipment used for levelling is in poor condition then this may be reflected in the levelling results and certainly reflects on the ability of the surveyor. Instruments must be kept clean and in working order and must be sent for servicing at regular intervals to

ensure that they are suitable for use.

Tripods and staves must be well

maintained. Transportation of the levelling equipment can be the most likely cause of damage therefore it is important that all equipment is stowed securely and safely in their respective carrying cases, covers, boxes etc. Staff bases and levelling plates must be kept clean, especially the top of the levelling plate as loose dirt may influence reduced levels, i.e. dirt may be removed between measurements (from foresight to backsight measurements) thus effecting final reduced elevations.

18) Misidentification or use of wrong benchmark/datum – it is extremely important that the correct benchmark or peg with the correct corresponding elevation is used.

All benchmark or peg elevations must be linked to the mine or

construction site or project benchmark elevation/height, i.e. the elevation datum. Using an elevation that is not linked to the benchmark (elevation datum) will result in incorrect elevations being derived from that levelling....