Finding something underground is one thing. Giving a project team information they can actually use is another.

A technician might pick up a signal with an electromagnetic locator. A ground penetrating radar scan might show an anomaly running through a proposed work area. An old drawing might suggest that a utility crosses the site somewhere nearby.

All of those findings matter. But none alone answers the question the contractor or engineer really cares about: What should we do next?

That’s where the quality of a subsurface investigation starts to matter.

Good underground utility locating isn’t just about detecting a line and putting paint on the ground. The information has to make sense in the context of the site. It must be documented clearly enough that someone who wasn’t standing beside the locator can understand it later.

When there’s uncertainty, that uncertainty needs to follow the information instead of disappearing somewhere between the field and the final report.

Detection is only part of the job.

A signal isn’t yet a decision

Picture a crew preparing to trench through an existing commercial property.

During the investigation, an instrument response suggests that something runs across the proposed path.

“Possible utility here” may be an accurate observation, but it leaves plenty unanswered.

Is the response continuous or isolated? Does it line up with a valve, meter, pull box, or other visible feature? Is there anything on the available utility records that supports it? Was the feature detected with one method or several? Is the depth known, estimated, or still uncertain?

The answers can change what happens next.

A suspected abandoned line might be treated differently from an active electrical service. A utility far outside the excavation footprint may have little effect on the work, while another one passing directly through the trench could change the design or sequence.

This is why useful utility locating services should go beyond simply producing detections. They should produce information that supports the decision being made.

Start with what the project needs to know

It’s easy to begin an investigation by talking about equipment. Should we use ground penetrating radar (GPR) scanning? Should we connect an electromagnetic locator? Do we need more extensive utility mapping?

Those are fair questions, but they make more sense after the project team defines what it actually needs to know.

A contractor may simply need to determine whether utilities appear to be present inside a proposed excavation area. A designer might need a clearer horizontal alignment so a new utility can be routed around existing infrastructure. At a tight or high-risk crossing, approximate information might not be enough at all.

That situation may call for stronger field verification of underground utilities, including physical exposure where appropriate.

Those are very different scopes.

A good subsurface utility investigation starts with three basic questions: What information is needed? How accurate does it need to be? What happens if it’s wrong?

Once those questions are clear, the field methods are easier to choose.

The drawing is evidence, not ground truth

Existing drawings are useful. They can also create a dangerous level of confidence when they are treated as a perfect picture of what’s underground.

A utility could have been rerouted during a renovation. A private service might have been added years after the original drawings were produced. An abandoned line may still be in the ground even though it no longer appears on current plans.

Sometimes the consequences of that mismatch are serious.

In November 2023, a crew working at a property in Needham, Massachusetts, struck an active natural gas service. Massachusetts regulators later found that the service record was inaccurate and that the line hadn’t been accurately marked before the incident. The state investigation said the combination of incomplete markouts, inaccurate records, and failures in excavation procedures contributed to the strike and the resulting home explosion.

That example is worth remembering, because the problem wasn’t simply that somebody failed to look at a drawing. Information existed, but part of that information didn’t match the actual field condition.

None of this means records are useless. They remain an important starting point. They simply need to be treated as one source of evidence.

Reliable underground utility locating might involve comparing available plans with visible surface features, electromagnetic responses, ground-penetrating radar data, site history, and other field observations.

When several pieces of evidence agree, confidence usually improves. When they don’t agree, the disagreement itself is information.

Records and field conditions can be far apart

The scale of that problem can be surprising.

A 2025 study of a South Dakota highway reconstruction project compared traditional One Call utility records with subsurface utility engineering investigations performed under ASCE 38 practices.

Only about 32% of the utility footage in the comparison was within two feet between the two datasets. About 21% differed by more than 20 feet. The SUE investigation also documented substantially more utility infrastructure than the traditional records showed.

A 20-ft difference isn’t a minor drafting issue when someone is trying to place a trench, foundation, drainage structure, or other construction feature.

It also shows why the quality question shouldn’t simply be, “Do we have a utility map?”

The better question is, “How much confidence should we place in this map for the decision we’re about to make?”

That’s particularly relevant during private utility locating, where privately owned electrical, water, communication, sewer, irrigation, and other site utilities might not be completely represented through the public one-call process.

Use the right tool for the site

No locating technology works perfectly in every environment.

Electromagnetic locating can be very effective when a conductive utility can be traced, or when a signal can be applied to it.

In other situations, GPR scanning could help identify subsurface features that can’t be traced electromagnetically.

But ground penetrating radar isn’t an underground X-ray. Conditions matter. Soil composition, moisture, target depth, congestion, accessibility, and the materials involved can all affect the response.

Sometimes the result is clear. Sometimes it’s not. That’s normal field work.

The better approach is often to combine information instead of expecting one instrument to settle every question.

An electromagnetic locator could establish a strong trace on one service. Radar might add information about other features nearby. A manhole, riser, valve, or utility box may explain where a line is likely to continue. Existing plans can provide another clue.

A useful subsurface-utility mapping process builds the picture from those pieces of evidence.

Somebody still must interpret the data

Technology doesn’t remove judgment from the process.

A radar unit can show a response. The technician still has to determine what that response might mean.

It might be a utility. It could also be buried debris, an old structure, a change in soil conditions, or something else entirely.

That’s why interpretation matters. Does the response continue in a logical direction? Does it connect with something visible? Can it be supported with electromagnetic locating? Does its apparent route agree with other information on the site?

Sometimes the evidence is strong enough to support a confident interpretation. Sometimes the most accurate description is “probable utility,” “unidentified anomaly,” or “additional verification recommended.”

That may sound less decisive, but it’s more useful than certainty that the evidence can’t support.

A project team can plan around a known uncertainty. It can’t plan around a hidden uncertainty.

Good documentation keeps the field work from getting lost

A technician can spend hours building a clear picture of an area, only for much of that understanding to disappear once the crew leaves.

The locator may know exactly which signal was strong, where two methods agreed, where the records were questionable, and which part of the site couldn’t be investigated.

A week later, someone else may see nothing but paint. That’s where documentation earns its value.

Depending on the project, the record could include photographs, sketches, survey information, utility alignments, field notes, CAD files, or formal underground utility mapping.

It may also document the methods used, records reviewed, inaccessible areas, and conditions that affected the investigation.

Not every job needs a large report. A small marking assignment and a full subsurface utility engineering effort are very different scopes.

The point is to preserve enough of the field story that somebody else can understand what was found and what might still need to be verified.

Good information can change the design before construction

One of the clearest benefits of better subsurface data is that it gives the project team time to make a different decision.

A Federal Highway Administration (FHWA) case study from a Virginia Department of Transportation highway project in Richmond shows what that can look like.

The project team excavated 156 test holes where possible utility conflicts had been identified. The verification showed actual conflicts at 75 locations.

Instead of carrying those conflicts into construction, designers changed the plans. Sixty-one of the potential conflicts were eliminated before the work reached the field.

FHWA reported that the changes avoided more than $731,000 in utility adjustments. The test-hole work cost about $94,000.

The important part of that example isn’t just the savings. The investigation changed a decision.

That’s what useful inspection information is supposed to do.

If the same conflicts had first become obvious after excavation began, the choices available to the project team would have been narrower, more disruptive, and probably more expensive.

Accuracy should match the risk

People often talk about a utility as either “located” or “not located.” Real projects are rarely that simple.

An approximate horizontal path might be adequate during an early feasibility study. During design, the team may need better horizontal information. At a critical crossing with little room for error, the utility might need to be physically exposed and surveyed before work proceeds.

The closer the project gets to a decision where being wrong has serious consequences, the stronger the verification should become.

That risk-based approach is one of the useful ideas behind subsurface utility engineering. Not every foot of a project needs the highest possible level of investigation.

The goal is to identify where uncertainty actually matters and concentrate additional verification there.

There is little benefit in over-investigating everything. There’s also obvious risk in relying on preliminary information where an error could lead to a utility strike, shutdown, redesign, or major delay.

Say what you could not confirm

One of the easiest ways to weaken an inspection report is to make the result sound cleaner than the field conditions really were.

If part of the site couldn’t be accessed, say so. If soil conditions affected radar performance, include that limitation. If a utility was traced for 100 feet and then the signal disappeared beneath a congested area, document the gap. If the evidence isn’t strong enough to support excavation next to a suspected utility, additional verification may be needed.

Those details don’t make the inspection look weak. They make the information more honest.

Quality work isn’t about pretending every answer is certain. It’s about showing where the evidence is strong, where confidence is lower, and what might still need to be resolved.

The final result should help someone make a decision

The value of private utility locating, GPR scanning, electromagnetic locating, and subsurface utility mapping isn’t in the equipment itself. It’s in what the project team can do with the information afterward.

That might mean moving a proposed trench. It may mean changing the route of a new utility. It may lead to physical verification before excavation begins. Or it may simply give the superintendent enough information to plan the work with fewer unknowns.

That’s the real test of a useful subsurface inspection: Not how many lines were painted. Not how many technologies were used. Not how impressive the equipment looked.

The better question is simpler: Does the project team now know enough to make the next decision?

If the answer is yes, the investigation has done more than find something underground. It has turned field data into information people can actually use.