What Happens During a Utility Locate?

A utility locate is a field process used to identify the approximate path of buried infrastructure. The goal is to provide clear markings and documentation so excavation can proceed safely and predictably. Whether the locate is performed through the 811 process or by a private locating provider, the core process follows the same general sequence, though the depth of investigation and documentation can vary significantly.

Step 1: Understanding the Scope

Locating starts with context: ticket details, site access, known utility types, and any available records or constraints. A good locate is not "one size fits all."

Before a locator arrives on site, they should review available information — the locate ticket or work order, any site plans or as-builts provided, aerial imagery, and notes about known conditions. For private locates, this may include a pre-site phone call with the client to clarify exactly what needs to be identified and where the excavation or investigation area is.

What the Client Should Do to Prepare

Site preparation makes a meaningful difference in locate quality. Before a locator arrives, clients can help by:

• Clearly marking or communicating the excavation area or area of interest
• Ensuring access to the site, including gates, locked areas, and utility rooms
• Providing any available site plans, as-builts, or utility records
• Identifying known access points; meters, cleanouts, valve boxes, junction boxes
• Clearing snow, debris, or vehicles from the work area when possible
• Having a site contact available to answer questions about the property

The more information a locator has going in, the more thorough and accurate the results will be.

Step 2: Locating Methods in Detail

Most utility locating relies on electromagnetic locating (EML) technology. The principle is straightforward: a transmitter places an electrical signal on a target utility, and a receiver traces that signal from the surface. But the method of applying that signal makes a significant difference in accuracy and reliability.

Direct Connect

Direct connection is the gold standard in electromagnetic locating. The locator physically connects the transmitter to the target utility, typically by attaching a lead to a tracer wire, exposed conductor, valve, or accessible metallic component. A ground stake completes the circuit.

This method produces the strongest, cleanest signal because the current flows directly on the target utility rather than being induced from the surface. It provides the best accuracy for both horizontal path and depth estimation. Direct connect is preferred whenever an access point is available.

Induction

When no access point is available for direct connection, a locator can use induction. The transmitter is placed on the ground surface and radiates a signal downward. Any metallic conductor in the area will pick up the induced signal, which the receiver can then trace.

Induction is useful but comes with trade-offs. Because the signal is not targeted to a specific utility, multiple conductors in the area may respond simultaneously. This can make it harder to isolate individual utilities in congested areas. Signal bleed between adjacent lines is a common challenge. Experienced locators manage this by adjusting frequency, gain, and position, but induction inherently carries more uncertainty than direct connect.

Passive Locating

Passive locating does not use a transmitter at all. Instead, the receiver detects signals that are already present on buried conductors, primarily power frequency (50/60 Hz) from energized electrical lines and radio frequency signals coupled onto metallic utilities from nearby broadcast sources.

Passive mode is a useful sweep technique. It can quickly identify the presence of energized conductors in an area. However, it cannot distinguish between different utilities carrying similar signals, and it does not work on de-energized or non-metallic lines. It is a starting point, not a complete locate method on its own.

Ground Penetrating Radar (GPR)

GPR is sometimes used as a supplemental tool, particularly for non-metallic utilities (PVC, HDPE, concrete, clay) that cannot carry an electromagnetic signal. GPR works by sending radar pulses into the ground and interpreting the reflections from subsurface interfaces.

GPR is effective in certain soil conditions but is affected by soil moisture, clay content, and depth. It requires trained interpretation; the raw data shows anomalies, not labeled utility lines. In the right conditions and in experienced hands, it is a valuable complement to EML. In poor conditions, it may provide limited useful information.

Step 3: Marking and APWA Color Codes

Once utilities are traced, they are marked on the surface using paint, flags, or both. Professional locating follows the APWA (American Public Works Association) Uniform Color Code, which assigns specific colors to utility types:

• Red: Electric power lines, cables, conduit, and lighting
• Yellow: Gas, oil, steam, petroleum, and other hazardous materials
• Orange: Communications, cable TV, alarm, signal lines, and fiber
• Blue: Potable water
• Purple: Reclaimed water, irrigation, and slurry lines
• Green: Sanitary sewer and storm drain
• White: Proposed excavation area (placed by the excavator, not the locator)
• Pink: Temporary survey markings

Markings should be clear, consistent, and tied to the actual signal path. Where visibility is limited or conditions are complex, documentation and communication become just as important as paint and flags.

Depth Readings and Their Limitations

Most electromagnetic receivers can estimate the depth of a detected utility. This information is valuable for planning excavation, but it comes with important caveats:

• Depth readings are estimates, not guarantees. Accuracy depends on signal quality, soil conditions, and whether the receiver is properly positioned over the target.
• Depth accuracy degrades with increasing depth. A reading at 2 feet is generally more reliable than a reading at 8 feet.
• Signal distortion from adjacent utilities, rebar, or metallic structures can affect depth accuracy.
• Depth readings on induced signals (vs. direct connect) are less reliable because the current path may not be fully on the target utility.

Experienced locators note depth readings as approximate and will communicate confidence levels. In high-risk situations, potholing (hydro-excavation) is the only way to confirm exact depth and position.

Documentation and Deliverables After a Locate

For 811 locates, documentation typically consists of closing out the ticket with notes about what was marked, any limitations, and completion status. For private locates, documentation can be significantly more detailed:

• Written summary of findings, methods used, and limitations
• Annotated site maps showing utility paths and approximate depths
• Photo documentation of markings and site conditions
• GNSS-captured data for GIS integration when required
• Recommendations for additional investigation if needed

When a Locate Might Need Follow-Up

Not every locate produces a complete picture. Follow-up investigation may be needed when:

• Utilities could not be traced due to poor continuity or lack of access points
• Site conditions (snow, standing water, heavy traffic) prevented full coverage
• Non-metallic utilities are suspected but could not be confirmed with EML
• Depth verification is required and potholing is recommended
• Additional areas are added to the project scope after the initial locate

A professional locator will clearly communicate what was and was not located, and will recommend next steps when uncertainty remains. That transparency is what separates a quality locate from a checkbox exercise.

What a Locate Does and Doesn't Guarantee

Locates identify an approximate path based on detectable signals. Not all utilities are traceable, and not all sites have complete records. When the risk is high or uncertainty is unacceptable, additional verification may be required, including private locating, hydro-excavation, or coordinated verification with utility owners.

The difference between a routine locate and a professional one is simple: taking the time to verify what is real on site instead of assuming. If you are evaluating whether your project needs more than standard 811 markings, read about when to hire a private locator.