A field error where the instrument operator sights the wrong control point, enters the wrong point number in the data collector, or forgets to set the angle to zero when establishing the initial reference line. This mistake rotates the entire coordinate system for that setup, causing all subsequent foresight measurements to be plotted in the wrong location.
Heard on the job
We've to re-shoot the whole retaining wall because the new guy blew a backsight and rotated the job by thirty degrees.
Insider tip
You can usually catch a blown backsight immediately if you check into a third known control point before taking side shots.