Whenever a surveying instrument is used to make measurements, it is necessary to consider the quality with which it can perform such an operation. As mentioned, this quality depends on several factors, the most important of which are instrument imperfections, natural environmental influences and operational errors. While most of these errors are irrelevant to typical civil engineering works, some need to be investigated and corrected to ensure that the quality of the measurement results is within the limits specified by the manufacturer or by calibration procedures.