TYPES OF ERRORS

This mistake occurs on the part of survey personnel due to lack of experience or carelessness.
For example: If a surveyor reads the tape reading as 29.5 m instead of 30 m, then it is a mistake or the gross error.

Mistakes, if not detected, can lead to erroneous results thereby making the whole survey as faulty. Adequate check measurements are thus made to detect this type of error.

These errors are called as systematic because they always follow a definite pattern or a mathematical/ physical law. These errors are of same magnitude and sign.
For example: Measuring a length with a steel tape and error involved due to temperature. This is a systematic error because it follows the physical law of expansion of solids on increasing the temperature.
This type of error makes the result either too large or too small.
The systematic error can be computed and suitable corrections applied.

This type of error occurs due to human limitation in reading an observation.
For example: While measuring an angle from a protractor (say 30.6°), then it is quite possible that the observer may read 30.5° or 30.7° due to inability of human eye to judge the exact division.
A good thing about accidental errors is that when a large number of observations are made, then they use to cancel out because there is equal probability of the error to be positive or negative. Thus this type of error is also called as compensating error.
But compensating effect of accidental errors is not full proof and there always remains some accidental errors. This error cannot be eliminated altogether from the observations whatever precautions are taken but magnitude of this error is generally very small.
Smaller the random error, more precise is the measurement. Thus random/accidental errors limit the level of precision while taking an observation.
Accidental errors occur purely as a matter of chance and thus theory of probability is used to account for these types of errors.

Note:

The theory of errors deals with accidental/random errors only with the presumption that all the systematic and gross errors have been eliminated from the measured values.
Accuracy denotes the closeness of a measurement to its true value. If the measured value is very close to its true value, it is very accurate. It is degree of perfection achieved in measurement.
Precision of a measurement denotes its closeness to another measurement of the same quantity. If the quantity is measured several times and the values obtained are very close to one another, the precision is high. However, it does not necessarily mean that accuracy is high, because the values though close to one another may not be close to true value.