Errors In Leveling
As explained earlier, in levelling, it is possible to make blunders, systematic errors, and accidental errors.
Proper note keeping and systematic Fieldwork will eliminate the first two, while multiple readings can reduce the third to a minimum.
Blunders in levelling may occur due to,
• Using the wrong point for a benchmark
• Reading rod incorrectly
• Reading on the stadia crosshair instead of the middle crosshair
• Reading wrong numbers
Systematic errors occur when the instrument is out of adjustment; for example when the line of sight is not horizontal when the bubble is at the centre of its run.
When a survey starts from a point and loops back to the same point, the accidental errors in reading, sighting, and atmospheric conditions are proportional to the number of setups and/or distances between benchmarks.
Checking of levels with known R.L.
Errors Can Also Be Classified As:
• Error Due to Impact Adjustment
• Error Due to Sluggish Bubble
• Error in the Movement of the Objective Slide
• Rod Not Standard Length
• Error Due to Defective Joint
• Earth’s Curvature
• Atmospheric Refraction
• Variations in Temperature
• Settlement of Tripod or Turning Points
• Wind Vibrations
• Mistakes in Manipulation
• Mistake in Rod Handling
• Mistake in Reading the Rod
• Errors in Sighting
• Mistake in Recording
Instrument error refers to the error of a measuring instrument or the difference between the actual value and the value indicated by the instrument.
There can be errors of various types, and the overall error is the sum of the individual errors. Its types are as follows.
#1. Error Due to Impact Adjustment
- The essential adjustment of a level is the line of sight shall be parallel to the axis of the bubble tube.
- If the instrument isn’t in this, adjustment, the line of sight will be inclined upwards or downwards when the bubble is centered, and the rod readings will be incorrect.
- The error in the rod reading will be proportional to the distance and could be eliminated by balancing the backsight and foresight distances.
- The error is likely to be cumulative, particularly in going up or down a steep hill, where all backsights are longer or shorter than all foresight unless care is taken to run a zigzag line.
#2. Error Due to Sluggish Bubble
- When the bubble is sluggish, it will come to rest in the wrong position, though it could creep back to the correct position while the sight is being taken.
- Such a bubble is a constant source of delay and aggravation.
- However, the error might be partially avoided by observing the bubble after the target has been sighted. The error is compensating.
#3. Error in the Movement of the Objective Slide
- In the case of external focusing instruments, if the objective slide is slightly worn ow, it may not move in a truly horizontal direction.
- From the short sights, the objective slide is moved out nearly its entire length, and the error is, thus, more.
- Because of this reason, extremely short sights must be avoided. The error is compensating and may be eliminated by balancing backsight and foresight since in that case, the focus isn’t changed, and hence, the slide isn’t moved.
#4. Rod Not Standard Length
- Incorrect lengths of divisions on a rod cause errors similar to those resulting from the incorrect marking on a tape.
- The error is systematic and can be directly proportional to this difference in elevation.
- When the rod is too long, the correction is added to a measured difference in elevation; when the rod is too short, then the correction is subtracted.
- Uniform wearing of the shoe in the bottom of the rod makes H.I. values incorrect, but the effect is cancelled when included in both back and foresight readings.
- For accurate leveling, the rod graduation Should be tested and compared with any standard tape.
Also, read: What Is EDM in Surveying | Type | Errors
- #5. Error Due to Defective Joint
- The joint of the extendable rods might be worn out by setting the rod down ‘on the run’ and from other sources.
- The failure to test the rod at frequent intervals may result in a large cumulative error.
- Natural Errors
- The types of Error Due to Natural Causes are as follows
- #1. Earth’s Curvature
- The effect of curvature is to increase the rod readings.
- If the distances are small that the error is negligible, but for greater distances, once the back and foresight aren’t balanced, then a systematic error of considerable magnitude is produced.
- #2. Atmospheric Refraction
- Atmospheric Refraction
- Because of refraction, the ray of light bends downwards in the form of the curve with its concavity towards the earth surface, thus decreasing the staff readings.
- Since the atmospheric refraction often changes rapidly and greatly in short distance, it’s impossible to eliminate entirely the effect of refraction through the backlight, and foresight distances are balanced.
- It’s particularly uncertain once the line of sight passes close to the ground.
- Errors due to refraction tend to be compensating over a long period of time but maybe cumulative on a full day’s run.
- Also, read: Total Station in Surveying | Operations | Advantage & Disadvantage | Types
- #3. Variations in Temperature
- The effect of variation in temperature on the adjustment of this instrument isn’t of much consequence in levelling of aching precision, but it might produce an appreciable error in precise work.
- The adjustment of this instrument is temporarily disturbed by unequal heating, as well as the consequent warping and distortion.
- The heating of the level vial will cause the liquid to expand and bubble to shorten.
- If one end of the vial is warmed more than the other, the bubble will move towards the heated end, and appreciable errors will be produced.
- In precise leveling, it’s quite possible that errors from a change of length of levelling rod from variations in- temperature might exceed the errors arising from the levelling itself.
- Heatwaves near the ground surface or adjacent to heated objects make the rod appear to wave and prevent accurate sighting.
- The beating effect is practically eliminated by shielding the instrument from the rays of the sun.
- The error is usually accidental, but under certain conditions, it might become systematic.
- #4. Settlement of Tripod or Turning Points
- If the tripod settles at the interval that elapses between taking a backsight and the following foresight, the observed foresight will be too small and the elevation of this turning point will be too great.
- Similarly, if a turning point settles from the interval that elapses between taking the foresight and the following backsight at the next set up, the observed backsight will be too great and H.I. calculated will be too great.
- Thus, whether the tripod settles or the turning point settles, the error is always systematic and the resulting elevation will always be too high.
- #5. Wind Vibrations
- High wind shakes the instrument and thus disturbs the bubble and the rod.
- Precise leveling work should never be done in high wind.
- Also, read: What Is Transit Theodolite | Theodolite Parts
- Personal Errors
- The errors which are introduced due to fault of the observer is known as Personal error, for example, carelessness in taking observation etc. Its types are as follows.
- Type of Personal errors
- Mistakes in Manipulation
- Mistake in Rod Handling
- Mistake in Reading the Rod
- Errors in Sighting
- #1. Mistakes in Manipulation
- These include mistakes in setting up the level, imperfect focusing of the eye-piece and of objective, errors in centring the bubble and failure to watch it after each sight, and errors because of resting the hands-on tripods or telescope.
- From the long sights, the error due to the bubble not being’ centred in the time of sighting is more important.
- Habit ought to be developed of checking the bubble before and after each sight.
- Parallax caused by an improper focusing results in incorrect rod readings it produces an accidental error and can be eliminated by carefully focusing.
- #2. Mistake in Rod Handling
- If the rod isn’t in plumb, the reading taken will be too great. The error varies directly with the magnitude of the rod reading and directly as the square of the inclination.
- In running a line of levels uphill, backsight readings are likely to be increased more than foresight from this source along with the evaluation of a bench -Mark on top will be too great.
- Similarly, the elevation of a benchmark at the bottom, while leveling downhill, will be too small.
- Therefore, a positive systematic error results. Over level ground, the resultant error is accidental since the backsights are about equal to the foresight.
- The error can be minimized by carefully plumbing the rod either by eye estimation or from using, a rod level, a special attachment devised for plumbing the rod or by waving the level rod slowly towards or away in the level thereby taking the minimum rod reading.
- Vertical cross-hair may be used to plumb the rod at the direction transverse to the line of sight.
- Also, read: Principle of Plane Table Surveying Methods |Equipment | Error | Advantage | Limitation
- #3. Mistake in Reading the Rod
- The common mistakes in reading the rod are :
- Reading upwards, instead of downwards.
- Reading downwards, instead of upwards when the staff is inverted.
- Reading the wrong meter mark when the staff, is near the level and only one-meter mark is visible through the telescope.
- To omit a zero or even two zeros from a reading. For example, 1.28 instead of 1.028 or 1,06 instead of 1.006.
- Reading against stadia hair.
- Concentrating more attention on the decimal part of the reading and noting the whole meter reading wrongly.
- #4. Errors in Sighting
- The error is caused when’ it is difficult to tell when the crosshair coincides with the center of the target in a target rod and to determine the exact reading that the cross-hair appears to cover in the case of this self-reading rod.
- This is an accidental error the magnitude of which depends upon the coarseness of this cross-hair, the type of rod, the form of target., atmospheric conditions, length of sight and the observer.
- #5. Mistake in Recording
- The common mistakes are:
- Entering the reading with digits interchanged i.e., 1.242 Stead of 1.422.
- Entering backsights and foresight in the wrong column.
- Mistaking the numerical value of reading called out by the level man.
- Omitting the entry.
- Entering the wrong remark- against a reading.
- Adding a foresight instead of subtracting it and/or subtracting a backsight reading instead of adding it.
- Ordinary arithmetical mistakes.