Levelling is a fundamental branch of surveying used to determine the relative heights of points on, above, or below the ground surface. The core principle involves establishing a horizontal line of sight and measuring vertical distances from this reference line. However, achieving accurate results requires awareness of various sources of error. Just as there are common errors in chaining that affect linear measurements, levelling operations can be compromised by instrumental faults, environmental conditions, and human mistakes. Understanding these error types is essential for producing reliable survey data.
Instrumental Errors in Levelling
Instrumental errors arise from defects or problems associated with the levelling equipment itself. These are among the most frequently encountered error categories and can be minimised through proper inspection and adjustment. Similar to how errors in plane table surveying stem from equipment imperfections, levelling instruments present their own set of potential faults.
a. Imperfect Adjustment
The most common instrumental error is imperfect adjustment of the levelling instrument. When the instrument is properly adjusted, the line of collimation lies exactly parallel to the bubble axis, making it perfectly horizontal when the bubble is centred. If adjustment is incorrect, the line of collimation becomes inclined even with a centred bubble. This error can be eliminated by:
- Properly testing and adjusting the instrument before each use
- Equalising backsight and foresight distances to cancel the inclination effect
b. Defective Level Tube
A sluggish bubble tube may remain centred even when the bubble axis is not horizontal, while an over-sensitive tube makes it difficult to level the instrument properly. Both conditions introduce systematic errors. The level tube must be checked beforehand to ensure it has the required suitable sensitiveness.
c. Incorrect Staff Graduations
When the levelling staff is incorrectly graduated, every reading taken with it will contain a uniform error. New staff graduations must be verified using an invar tape to confirm their accuracy before field use.
d. Shaky Tripod
A tripod with loose joints makes the instrument unstable, significantly increasing the probability of erroneous readings. All tripod joints must be inspected and tightened properly before beginning any levelling work.
e. Telescope Alignment Issues
Two alignment problems can occur: the telescope may not be parallel to the bubble tube, or it may not be at a right angle to the vertical axis. Both conditions introduce systematic errors that can only be corrected through permanent adjustment of the instrument.
Collimation Error and Its Elimination
The collimation error, also known as the error of collimation, occurs when the axis of collimation is not truly horizontal even though the instrument is levelled. This means the line of sight is tilted relative to the horizontal by a certain angle. It is a particularly significant concern in reciprocal levelling operations. Understanding various levelling surveying methods helps surveyors choose the right technique to minimise such errors.
The most effective way to eliminate collimation error is by balancing the backsight and foresight distances. When these distances are equal, the angular error in the line of sight produces equal and opposite effects on both readings, cancelling out in the height difference calculation. Field procedure should therefore ensure that the instrument is set up approximately midway between the staff positions for each setup.
| Error Type | Primary Cause | Prevention Method |
|---|---|---|
| Collimation Error | Line of sight not horizontal when levelled | Equalise backsight and foresight distances |
| Imperfect Adjustment | Bubble axis not parallel to collimation line | Test and adjust before use; balance sight distances |
| Defective Bubble Tube | Sluggish or over-sensitive bubble movement | Check tube sensitivity before fieldwork |
| Staff Graduation Error | Incorrect markings on levelling staff | Verify with invar tape before first use |
| Shaky Tripod | Loose joints in tripod assembly | Tighten all joints; test stability |
Errors Due to Settlement of Level and Staff
When levelling is carried out on soft ground, the instrument or the staff may settle during the observation process, introducing systematic errors into the readings. These settlement errors are distinct from common sources of errors in structural design and drawings because they occur in the field measurement phase rather than during office calculations.
Settlement of the Level
When the instrument is set up on soft ground, the level may settle between taking the backsight and the foresight readings. This settlement causes the foresight reading to be smaller than its true value, leading to an overestimation of the height difference. Prevention measures include:
- Setting up the instrument on firm, hard ground wherever possible
- Pressing tripod legs firmly into the ground
- Using wooden stakes to provide a stable base
- Taking the foresight reading immediately after the backsight
Settlement of the Staff
Staff settlement typically occurs at change points on soft ground. Unlike level settlement, staff settlement makes the backsight reading larger than its true value. To prevent this, soft ground should be avoided as a change point location. Steel footplates or metal pegs can be driven into the ground to serve as temporary change points that resist settlement.
Manipulation and Human Errors
Human errors in levelling arise from incorrect manipulation of the equipment. These mistakes are preventable through proper training and adherence to field procedures. Applying modern surveying techniques for levelling can help reduce the frequency of such errors through improved equipment design and automation.
The following are the main types of manipulation errors encountered in levelling work:
- Improper levelling of the instrument – Careless or incomplete levelling introduces errors into all subsequent readings. The surveyor must ensure the bubble is centred before every observation.
- Non-centralisation of the bubble – Taking a reading when the bubble is off-centre is one of the most common levelling mistakes. The bubble position must be checked immediately before each reading using the levelling screws.
- Inefficient removal of parallax – Parallax occurs when the eyepiece and object-glass are not properly focused. This causes apparent movement between the crosshairs and the staff image. Proper focusing before each series of readings eliminates this error.
- Non-vertical staff holding – When the staff is not held vertically, the reading will be larger than the true value. A spirit level attached to the staff or a pendulum plumb bob should be used to ensure vertical alignment.
Natural Sources of Error in Levelling
Environmental and natural factors can introduce errors that are beyond the surveyor’s direct control but can be accounted for through corrections and careful fieldwork planning. These natural errors are comparable to types of design and detailing errors in construction and their prevention, where understanding the source is the first step toward mitigation.
a. Curvature and Refraction
The curvature of the earth causes distant objects to appear slightly lower than they actually are, while atmospheric refraction bends light rays downward, making objects appear slightly higher. For short sight lengths, these two effects partially cancel each other. For a 200-metre sight, the combined error is only about 0.003 metres and is typically considered negligible. However, for longer sight distances, corrections for both curvature and refraction must be applied to obtain accurate results.
b. Wind
Wind causes vibration of the instrument, making it difficult to obtain steady readings. Under windy conditions, long sight distances should be avoided, and the instrument should be shielded where possible. If wind speeds are excessive, fieldwork should be postponed.
c. Temperature Effects
Uneven heating of the instrument by sunlight can cause differential expansion of metal parts, leading to a tilted line of sight. Shading the instrument with an umbrella and avoiding measurements during the hottest part of the day help reduce this source of error.
Best Practices for Minimising Levelling Errors
Errors cannot be eliminated entirely, but they can be reduced to acceptable levels through systematic field practices and proper equipment maintenance. The following checklist summarises the key steps every surveyor should follow:
- Inspect and test all equipment before beginning fieldwork
- Verify staff graduations using a reference tape
- Set up the instrument on firm, stable ground
- Equalise backsight and foresight distances at each setup
- Check bubble centring before every staff reading
- Focus the eyepiece and object-glass properly to eliminate parallax
- Ensure the staff is held vertically using a spirit level
- Avoid soft ground for change points; use steel footplates
- Take foresight readings immediately after backsight readings
- Apply curvature and refraction corrections for long sights
- Shield the instrument from direct sunlight and wind
Understanding the specific tolerances allowed for different survey tasks is also important. For foundation work, for instance, the allowable limits follow well-established standards such as the tolerances of errors in pile foundation construction, which provide benchmarks for acceptable measurement deviations.
By recognising the five main categories of levelling errors (instrumental, collimation, settlement, manipulation, and natural), surveyors can take proactive steps to minimise their impact. Proper training, careful fieldwork, and regular equipment maintenance remain the most effective defences against inaccurate levelling results.