Locating an underground utility line is not an exact science. Colored markings and flags indicate the approximate location of a buried line. The tolerance zone—a pre-defined horizontal distance extending from the outer edge of underground facility marks, to the depth of your proposed excavation—acts as a warning to proceed with caution while digging in this area.
If possible, dig outside the tolerance zone. The exact distance is 18″ on either side of the marks (underground utility) plus the diameter of the line or pipe itself as marked by the owner to the depth of the excavation.
If you must excavate within the tolerance zone, dig next to the marks, not on top of them. Hand dig, pot hole or vacuum excavate with extreme caution. If damage does occur, click here for the steps to take.