Errors in Surveying Measurement

 

Natural Errors

These are caused due to variations in nature i.e., variations in wind, temperature, humidity, refraction, gravity, and magnetic field of the earth.

Instrumental Errors

These result from imperfection in the construction or adjustment of surveying instruments, and movement of their individual parts.

 

Personal Errors

These arise from limitations of the human senses of sight, touch, and hearing.

 

Types of Errors

Errors are traditionally classified into three types.

 

Gross Error

Gross errors, also known as blunders or mistakes, are results from

• Carelessness on the part of the observer in taking or recording reading
• Faults in equipment
• Adoption of the wrong technique
• Misinterpretation

The blunders or mistakes result in large errors and thus can easily be detected by comparing them with other types of errors (generally small in value). The maximum permissible error in an observation is ± 3.29s (where s is the standard deviation of sample distribution) and is used to separate mistakes or blunders from the random errors. If any error deviates from the mean by more than the maximum permissible error, it is considered a gross error and the measurement is rejected.

After mistakes have been detected and eliminated from the measurements, the remaining errors are usually classified either as systematic or random errors depending on the characteristics of the errors.

Systematic errors

It occurs according to a system. These errors follow a definite pattern. Thus, if an experiment is repeated, under the same conditions, the same pattern of systematic errors reoccurs. These errors are dependent on the observer, the instrument used, and the physical environment of the experiment. Any change in one or more of the elements of the system will cause a change in the character of the systematic error. Depending on the value and sign of errors in successive observation, systematic errors are divided into two types.

• Cumulative Error
• Compensating Error

Systematic errors are dealt with mathematically using functional relationships or models.

Random Error

After mistakes are eliminated and systematic errors are corrected, a survey measurement is associated with random error only. This error is small and is equally liable to be plus or minus thus partly compensating in nature. Random errors are unpredictable and they cannot be evaluated or quantified exactly.