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Glossary

Utility Locating

What is Utility Locating?

Utility locating is the process of finding buried gas, electric, water, sewer, and communication lines from records, one-call (811) marks, electromagnetic locators, and ground-penetrating radar, then marking their approximate path on the ground.

Utility locating draws on several sources to estimate where underground lines run: record drawings, the marks utility owners place in response to a one-call (811) ticket, electromagnetic locators that trace a signal along a conductive line, and ground-penetrating radar that images changes in the soil. The estimated path is painted and flagged on the surface using the uniform color code.

Each source has blind spots. Records can be outdated or missing private lines, electromagnetic locators cannot follow a plastic pipe unless it carries a tracer wire, and radar fades in clay and wet soil. Every method gives an approximate horizontal position, and most give little or no reliable depth. Engineers grade how far this information can be trusted using the ASCE 38 quality levels.

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Guides that cover Utility Locating

Related terms

811Call Before You DigPotholingDaylighting

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Utility Locating: common questions

What is Utility Locating?+

Utility locating is the process of finding buried gas, electric, water, sewer, and communication lines from records, one-call (811) marks, electromagnetic locators, and ground-penetrating radar, then marking their approximate path on the ground.

What is the difference between active and passive utility locating?+

Active locating uses a transmitter to put a known signal onto a line, either by connecting directly to an exposed part of it, clamping around it, or inducing the signal from above ground, and then traces that signal with a receiver. Passive locating skips the transmitter and listens for signals already on buried conductors, such as current on power lines or radio energy picked up by long metal lines. A direct connection or clamp puts the signal on one specific line, while induction can spread to nearby conductors; passive locating is a quick sweep for lines nobody knew were there.

How do locators find plastic or other non-metallic pipe?+

A non-metallic pipe carries no signal of its own, so locators rely on what was installed with it or can be put inside it. Tracer wire laid along the pipe lets a standard electromagnetic locator follow it. A sonde, a small battery transmitter pushed or floated through the pipe, can be traced from the surface. Without either, crews fall back on ground-penetrating radar and records, so an untraced plastic line is worth exposing before any digging close to it.

Can ground-penetrating radar find every buried utility?+

No. Ground-penetrating radar performance drops in clay, saturated soil, and at greater depths, and it can miss small or nonmetallic lines. Because every method has blind spots, crews confirm the actual position and depth by exposing the line with hydro excavation or air excavation.