Like all measurements, downhole directional surveys are subject to error. The validity of error model of wellbore positional accuracy is highly dependent on the application of rigorous quality control (QC) procedures. Failure to apply necessary operational procedures causes the reliability of generated survey data to be called into question. Survey errors can be significant with respect to the positional objectives for a well. Directional survey data that does not conform to the associated model predictions presents a risk in terms of lost production, damage to infrastructure and loss of life.