Utility Locating Glossary
Plain-English definitions of 41 locating terms: instruments, standards, methods, and the lingo every project owner should recognize before a crew shows up with a locator and a can of paint.
811 (One-Call System)
811 is the national number for requesting underground utility marks before excavation in the United States. Each state runs its own 811 program; a ticket notifies the member utilities, and each marks the buried lines it owns on the dig site.
Abandoned Line
A buried utility no longer in service and usually no longer maintained on any map. Abandoned lines are one of the hardest categories to locate because they carry no current, often have no accessible connection point, and belong to nobody who will come mark them.
Acoustic Leak Detection
Pinpointing a leak on a pressurised line by listening to the sound it makes escaping. Ground microphones, listening sticks, and correlators are used to narrow the loss to a point on the surface so the repair excavation can be sized to the break.
Active Locating
Tracing a buried line by deliberately applying a known signal to it with a transmitter, then following that signal with a receiver. Active locating is the standard method because the operator controls the frequency and knows which line is being traced.
APWA Color Code
The American Public Works Association uniform color code for marking underground utilities: red for electric, yellow for gas and oil, orange for communications, blue for potable water, green for sewer and drain, purple for reclaimed water and irrigation, pink for temporary survey marks, and white for proposed excavation.
As-Built
A drawing recording where infrastructure was actually installed, as opposed to where it was designed to go. As-builts are the starting point for records research, and their unreliability is the reason field locating exists.
ASCE 38
The American Society of Civil Engineers standard that defines how existing subsurface utility data is collected, classified, and shown on drawings. Its central contribution is the four quality levels — QL-D through QL-A — that state how a given utility position was determined.
Attenuation
The loss of signal strength as it travels through the ground. Attenuation is the main limit on ground penetrating radar: in high-conductivity soils the radar pulse weakens so quickly that usable depth can fall to a foot or two.
Bleedover
When a signal applied to one buried line couples onto neighbouring metal and appears to come from it too. Bleedover produces ghost traces and distorted depth readings, and it is the most common source of error in congested corridors.
Concrete Scanning
Ground penetrating radar run at higher frequency against a slab, deck, or wall to map rebar, embedded conduit, radiant tubing, and post-tension tendons before anyone cores, cuts, or drills.
Daylighting
Exposing a buried utility to open air so its position, depth, and material can be seen and measured directly. Used interchangeably with potholing when the hole is small and made for verification rather than for excavation.
Depth of Cover
The vertical distance from the ground surface to the top of a buried utility. Depth estimated by an electromagnetic locator or read off a radar record is an interpretation; depth measured at an open test hole is a measurement.
Direct Connection
Applying a locating signal by physically clipping the transmitter to the target line at a meter, valve, riser, pedestal, or panel. It is the cleanest way to energize a line because the signal goes where the operator put it.
Electromagnetic Locator
The paired transmitter and receiver that is the workhorse of utility locating. The transmitter puts a signal on a conductive line and the receiver traces the electromagnetic field radiating from it.
Frost Depth
The maximum depth at which ground freezes in winter. Frost changes both what a locator can do and how long a locate lasts: frozen ground and snow cover interfere with radar, and marks on frozen or snow-covered surfaces do not hold.
Ground Penetrating Radar (GPR)
An instrument that images the subsurface from reflected radio pulses. GPR finds what carries no signal — plastic pipe, clay tile, concrete, tanks, and voids — and at higher frequencies scans slabs for rebar, conduit, and post-tension tendons.
Induction
Applying a locating signal to a buried line without touching it, by setting the transmitter on the ground above the line and letting the field couple onto it. Used when no access point exists.
Inductive Clamp
A hinged coil placed around an exposed cable or pipe to couple the transmitter signal onto it without an electrical connection. The usual choice for energized cable and for gas service where direct connection is not permitted.
Leak Noise Correlator
An instrument that pinpoints a leak by comparing the sound arriving at two contact points on the same pipe and calculating where it originated from the difference in arrival time.
Line Marking
Putting the traced position of a utility onto the ground surface in APWA color-coded paint and flags, normally with a field sketch recording offsets to permanent features so the marks can be re-established later.
One-Call
The general term for the system that notifies utility owners about planned excavation. In the United States it is 811; in Canada it is a province-by-province network including Alberta One-Call, BC 1 Call, Info-Excavation in Quebec, and Ontario One Call.
Passive Locating
Detecting a buried line from a signal already present on it rather than one the operator applied — typically 50/60 Hz power frequency, or radio energy that has coupled onto long conductors.
Positive Response
The status each notified utility posts back against a one-call ticket, saying whether it has marked its lines, has no facilities in the dig area, or needs more time. Digging before every member has responded is digging without knowing what was missed.
Post-Tension Tendon
A high-strength steel cable tensioned after concrete has cured, used to reinforce slabs and decks. Cutting one releases stored energy violently and turns a routine penetration into a structural repair, which is why post-tensioned concrete is scanned before any core or saw cut.
Potholing
Opening a small, precise hole to expose a buried utility and measure its true position, depth, and material. Vacuum potholing uses compressed air or water to break the soil and an industrial vacuum to lift it out, so the line is exposed without being touched.
Private Utility Locating
Finding and marking the buried lines a one-call ticket does not cover — everything on the property owner's side of the meter, plus lines no utility owns or maps at all.
Quality Level A (QL-A)
The highest ASCE 38 quality level. The utility has been physically exposed — normally by a vacuum test hole — and its position, depth, material, and size measured and surveyed directly.
Quality Level B (QL-B)
The ASCE 38 level for utilities found by field geophysics — electromagnetic locating and ground penetrating radar — and surveyed. QL-B gives a reliable horizontal position and an estimated depth.
Quality Level C (QL-C)
The ASCE 38 level for utility information derived from surveying visible surface features — manholes, valve boxes, meters, pedestals — and correlating them with records.
Quality Level D (QL-D)
The lowest ASCE 38 quality level: utility information taken from existing records and verbal recollection, with no field verification of any kind.
Rebar
Steel reinforcing bar cast into concrete. Rebar is both a scanning target — you need to know where it is before coring or drilling — and an obstacle, because a dense mat reflects radar and makes it harder to see what lies beneath it.
Sonde
A small battery-powered transmitter on a push rod or camera head, fed through a non-metallic line and tracked from the surface. It is how a plastic water service, a sewer lateral, or an empty conduit gets located when there is nothing conductive to put a signal on.
SUE (Subsurface Utility Engineering)
The engineering practice of collecting, classifying, and depicting existing subsurface utility data on a project drawing, with each segment labelled by ASCE 38 quality level so designers know how far to trust it.
Sweep
A systematic pass across a site with a locator and radar to find everything present, rather than tracing one known line. A sweep is how undocumented and abandoned utilities get found at all.
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.
Tone
The signal a locating transmitter puts on a line, named for the audible note the receiver produces when it picks it up. "Putting a tone on the line" is the everyday phrase for energizing a target so it can be traced.
Tracer Wire
An insulated copper wire buried alongside a non-metallic pipe or duct so it can be located electromagnetically later. When it is present, continuous, and accessible, plastic pipe traces as easily as metal; when it is broken or was never installed, it usually does not trace at all.
Utility Locating
Detecting and marking buried utilities using electromagnetic locators, ground penetrating radar, sondes, and records. The output is normally surface marks in the APWA color code, a field sketch, and — where the work calls for it — coordinates.
Utility Mapping
Recording a locate as coordinates tied to survey control and delivering it as CAD, GIS, or a point file, so the result survives the weather that erases the paint.
Utility Strike
Damage to a buried utility caused by excavation. Consequences range from a nicked cable jacket to a ruptured gas main with evacuation and injuries, and liability generally follows whether the one-call ticket was filed and the marks respected.
White Lining
Outlining the proposed excavation area in white paint or flags before submitting a one-call ticket, so responding utilities know exactly where to sweep instead of marking an entire frontage.
