Tolerance Zone

The buffer on either side of a marked utility within which mechanical excavation is prohibited and only careful hand or vacuum digging is allowed. Its width is set by state or provincial law and differs between jurisdictions.

The tolerance zone exists because a mark is an approximation, not a survey line. Locating carries real uncertainty — coupling, bleedover, and soil conditions all move a trace — so the law builds a margin around it and requires non-destructive methods inside that margin. Digging mechanically inside the tolerance zone is both the most common cause of preventable strikes and, in most jurisdictions, a violation in its own right. Check the width that applies where you are digging; it is not the same everywhere.