1.15.7. Different types of errors

These are of several types:

(i) Error in permanent adjustment of level: For any major surveying work, the 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 centre before each reading and balancing of sights are to be maintained.

(ii) Staff defective and/or of non-standard quality: If the graduation in staff are not standardized that may cause error in reading. In an ordinary leveling, the error may be negligible but in case of precise leveling, the defective graduations in staff may cause significant error.

(iii) Error due to defective level tube: The bubble of the level tube may be in the center, otherwise it may cause error. Sometimes improper curvature of the bubble tube can also cause error.

(iv) Error due to defective tripod: The tripod should be set up on a stable and firm ground. The tripod stand should be strong and stable otherwise it takes considerable time to make the instrument level. The nuts provided at the joints of the legs to the tripod head should be well-tightened before mounting the instrument.

Instrumental errors refer to inaccuracies caused by the tools and equipment used in the surveying process. Each of the types listed plays a critical role in ensuring precise measurements. For instance, if the level instrument is not properly adjusted or maintained, it may give incorrect readings. Likewise, if the staff used for taking measurements is not calibrated correctly, it can lead to significant deviations in the final elevation calculations.

Imagine you're using a ruler to measure something but it has a manufacturer mistake where the first inch is actually one and a half inches. Every measurement you take thereafter will be off by that error. This is similar to the staff being defective. If your tools are not reliable, your entire building design could be based on incorrect measurements, which could lead to structural problems.

These include;

(i) Due to imperfect 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.

(ii) Error 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 minimised by keeping the small sight distance.

(iii) Error due to staff held incorrectly: 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 taking help of a bubble level tube.

(iv) Error in reading the staff: These errors occur if staff is read upward, instead of downward, read against the top or bottom hair instead of the central hair, mistakes in reading the further graduations wrongly.

(v) Error in recording: The common errors are entering a wrong reading (with digits interchanged or mistaking the numerical value of a reading), recording in wrong column, e.g., BS as IS, omitting an entry, entering the inverted staff reading without a minus sign etc.

(vi) Error in computing: Errors may occur by adding the foresight reading instead of subtracting it and or subtracting a backsight reading instead of adding.

Personal errors arise from the human element in surveying. These can happen during the setup of the instrument or during the process of taking measurements. For instance, if the person recording data miswrites a reading or miscalculates, it directly affects the accuracy of the results. Additionally, if the person using the level does not hold the staff vertically, the measurement will be higher than it should be, which is a common personal error.

Think about a time you were taking notes during a lecture but missed a few words because you were distracted. Your notes wouldn't accurately reflect what was said, similar to how errors in measurement can occur if attention isn't paid to detail during the leveling process. Just like clear note-taking leads to better understanding, careful measurement practices lead to more accurate surveying results.

These may include;

(i) Error due to Earth’s curvature: In case of small sight distance, error due to the curvature is negligible, but if the sight distances are large, the error should be accounted for. However, the error can be minimized through balancing of sight or reciprocal observations. The combined error due to curvature and refraction (e_comb) is thus given by:

e_comb (m) = 0.0675 D^2

where D is the distance in km. It is finally subtractive in nature as the combined effect provides increase in staff reading.

(ii) Error due to wind: Strong wind disturbs the leveling of an instrument and verticality of staff. Thus, the levelling work should not be performed in strong winds.

(iii) Error due to sun: Due to bright sunshine on the objective, sometimes the staff reading cannot be read properly. To avoid such error, it is recommended to cover objective lens with a shed.

(iv) Error 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. The instrument therefore is placed under the shade or survey umbrella is used.

Natural errors are environmental factors that can disrupt measurements. For instance, the Earth's curvature can cause inaccuracies when measuring long distances, especially over hills or valleys. Similarly, strong winds can sway the leveling instrument or the staff, leading to improper readings. Sun glare can obscure visibility, making it difficult to accurately read measurements. To mitigate these issues, surveyors often take precautions such as seeking sheltered locations or working under appropriate weather conditions.

Consider trying to hit a target with a dart while someone is blowing wind in your face—your aim (like your leveling) would be thrown off. Similarly, if it's a bright sunny day and you can't see the target well, you're more likely to miss. Surveyors have to account for these external factors just like dart players need to be mindful of their surroundings to ensure they're hitting the mark accurately.

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