If you understand what has caused the systematic error, you can remove it from each shot with simple math. For example, if the compass has a five degree clockwise bias, you simply subtract five degrees from each azimuth. Systematic errors are usually dealt with by calibrating instruments on a survey course, or by adding correction factors to the survey data. Blunders are usually caused by human errors. Blunders are mistakes in the processing of taking, reading, transcribing or recording survey data.
Blunders are the most difficult errors to deal with because they are inconsistent.
For example, if you read the wrong end of the compass needle, the reading will be off by 180 degrees. If you transpose the ones and tens digits on tape measure, the reading could be off by anything from 0 to 90 feet. Blunders are extremely common in cave surveys. By extrapolating I have concluded that in surveys like these, there is at least one blunder in every 40 shots. Paul Burger did careful study of blunders during of a recent resurvey of the commerical section of Cave of the Winds.