Answer: The tolerance zone represents the approximate location of a marked underground utility facility (e.g., gas pipe), which contains the pipe width plus 18 inches on each side of the markings. A qualified locator marks the facility with yellow paint and/or flags. The locator also marks the type, diameter size, and any other marking requirements.
Excavation within the tolerance zone requires extra care and precaution (hand digging or vacuum excavation methods). Never use powered equipment, such as a backhoe, in the tolerance zone until you verify that locate marks are accurate! Dig safely by visually inspecting the excavation in progress until it’s clear of the existing marked facility. The image below illustrates the tolerance zone.