Errors may arise from three sources :
1) Instrumental :- 
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 an angle on the circle of a theodolite. Such errors are known as personal errors. 
3) Natural :-  
Error 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 metres at 20°C but its length will change if the field temperature different.
Ordinary errors met with in all classes of survey work may be classified as
a) Mistakes
b) Systematic errors (Cumulative errors) 
c) Accidental errors (Compensating errors)
a) Mistakes :

Mistakes are errors which arise from inattention, inexperience, carelessness and poor judgment or confusion in the mind of the observer. 
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.
b) Systematic Errors (Cumulative Errors) :
A systematic error 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 a 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 too great or too small. 
Their effect is, therefore, cumulative.
All surveying equipments must be designed and used so that whenever possible systematic errors will be automatically eliminated. 
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. 
For example, in ordinary levelling, the levelling instrument must first be adjusted so that the line of sight is as nearly horizontal as possible when bubble is centered. 
Also, the horizontal lengths for back-sight and fore-sight from each instrument position should be kept as nearly equal as possible. 
In precise levelling, every day the actual error of the instrument must be determined by careful peg test, the length of each sight is measured by stadia and a correction to the results is applied.
c) Accidental Errors (Compensating Errors) :
Accidental errors are compensating errors are those which remain after mistakes and systematic errors have been eliminated and are caused by a combination of reasons beyond the ability of the observer to control. 
The tend sometimes in one direcction and sometimes in the other, i.e. they are equally likely to make the apparent result too large or too small. 
An accidental error of a single determination is the difference between 
1) the true value of the quantity, and 
2) a determination that is free from mistakes and systematic errors. 
Accidental errors represent the limit of precision in the determination of a value. They obey the laws of chance and therefore, must be handled according to the mathematical laws of probability.
As stated above, accidental errors are of a compensative nature and tend to balance out in the final results. 
For example, an error of 2 cm in the tape may fluctuate on either side of the amount by reason of small variations in the pull to which it is subjected.

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