Errors in surveying may arise from three main sources:
1. Instrumental: Surveying error may arise due to imperfection or faulty adjustment of the instrument with which measurement is being taken. For example, a tape may be too long or an angle measuring instrument may be out of adjustment. Such errors are known as instrumental errors.
2. Personal: Error may also arise due to want of perfection of human sight in observing and of touch in manipulating instruments. For example, an error may be there in taking the level reading or reading and angle on the circle of a theodolite. Such errors are known as personal errors.
3. Natural: Error in surveying may also be due to variations in natural phenomena such as temperature, humidity, gravity, wind, refraction and magnetic declination. If they are not properly observed while taking measurements, the results will be incorrect. For example, a tape may be 20 meters at 200C but its length will change if the field temperature is different.
Types of Surveying Errors
Ordinary errors in surveying met with in all classes of survey work may be classified as:
- Accidental errors
- Systematic or cumulative errors
- Compensating errors
Mistakes: Mistakes are errors which arise from inattention, inexperience, carelessness and poor judgment or confusion in the mind of the observer. They do not follow any mathematical rule (law of probability) and may be large or small, positive or negative. They cannot be measured. However, they can be detected by repeating the whole operation. If a mistake is undetected, it produces a serious effect upon the final result. Hence, every value to be recorded in the field must be checked by some independent field observation. The following are the examples of mistakes:
- Erroneous recording, e.g. writing 69 in place of 96
- Counting 8 for 3
- Forgetting once chain length
- Making mistakes in using a calculator
Accidental Errors: Surveying errors can occur due to unavoidable circumstances like variations in atmospheric conditions which are entirely beyond the control of the observer. Errors in surveying due to imperfection in measuring instruments and even imperfection of eyesight fall in this category. They may be positive and may change sign. They cannot be accounted for.
Systematic or Cumulative Errors: A systematic or cumulative error is an error that, under the same conditions, will always be of the same size and sign. A systematic error always follows some definite mathematical or physical law and correction can be determined and applied. Such errors are of constant character and are regarded as positive or negative according as they make the result great or small. Their effect is, therefore, cumulative. For example, if a tape is P cm short and if it is stretched N times, the total error in the measurement of the length will be P´N cm.
The systematic errors may arise due to (i) variations of temperature, humidity, pressure, current velocity, curvature, refraction, etc. and (ii) faulty setting or improper leveling of any instrument and personal vision of an individual. The following are the examples:
- Faulty alignment of a line
- An instrument is not leveled properly
- An instrument is not adjusted properly
If undetected, systematic errors are very serious. Therefore, (1) all surveying equipment must be designed and used so that, whenever possible, systematic errors will be automatically eliminated, and (2) all systematic errors that cannot be surely eliminated by this means must be evaluated and their relationship to the conditions that cause them must be determined.
Compensating Errors: This type or surveying error tends to occur in both directions, i.e. the error may sometimes tend to be positive and sometimes negative thereby compensating each other. They tend sometimes in on direction and sometimes in the other, i.e. they are equally likely to make the apparent result large or small. The following are a few examples:
- The discrepancy between chain and tape measurements when both are used simultaneously
- Inaccuracy in marking chain lengths on the ground
- Inaccurate centering
- Inaccurate bisection of an object
They obey the laws of chance and therefore, must be handled according to the mathematical laws of probability.