A set of utility drawings from permit records or an as-built file is often the first thing a project team reaches for before excavation or trenching. Those records have real value, but they represent a snapshot of what was recorded at one point in time, not a verified picture of what is underground today. Understanding that gap matters well before the excavator arrives on a commercial site in Florida or Georgia.
What Utility Records Actually Tell You
Existing utility records are the foundation of any subsurface investigation. They can show the general routing of buried infrastructure, flag congested corridors and give a project team a starting sense of what services cross or run through a work area. The Federal Highway Administration describes this level of information as Quality Level D, the most basic tier of utility data available.
At Quality Level D, information comes solely from existing records or verbal recollections. The FHWA’s guidance notes plainly that these are typically unreliable sources and are useful primarily for project planning and route selection activities, not for design or excavation decisions.
That limitation is not a criticism of the records themselves. It reflects the reality that drawings are often made from field notes that were never fully verified, that underground infrastructure is routinely added, modified or abandoned without updated documentation, and that paper records transferred across ownership changes or municipal systems accumulate errors over time.
What Records Frequently Miss
Several categories of buried infrastructure are commonly absent from or inaccurate in utility drawings:
- Abandoned lines. Decommissioned pipes and conduits are often left in place underground and removed from active records without being physically marked or documented as abandoned in place.
- Private laterals and site utilities. Irrigation feeds, site lighting conduits, private sewer laterals and private water services past the meter are installed by property owners and contractors. They frequently never appear in municipal or utility-company records.
- Relocated lines. Infrastructure that was moved during a prior renovation or road project may appear on drawings in its original location long after it was physically relocated.
- Depth discrepancies. Even when a line’s horizontal route is drawn correctly, the recorded depth may not reflect actual installation depth, which can vary due to grading changes, settlement or original field decisions that differed from the design drawing.
- Unmarked crossings. Where utilities from different owners cross at different depths, one or more of those crossings may be entirely absent from any single records set.
Correlating visible surface features such as manholes, valve boxes and cleanout covers with existing records adds a layer of confidence, but it still does not confirm what is underground between those visible access points. The FHWA refers to this as Quality Level C and notes that at this stage it is not unusual to find that many underground utilities have been either omitted or erroneously plotted.
Surface Geophysics: Moving from Records to the Ground
Closing the gap between a drawing and actual subsurface conditions requires field investigation. Two methods do most of that work on commercial sites.
Electromagnetic Tracing
Electromagnetic tracing applies a signal to a conductive line and tracks that signal at the surface. It is effective on metallic pipes, tracer wire installed alongside non-metallic lines, and energized or de-energized electrical conductors. The method establishes horizontal position along the traced route and can indicate depth at any measurement point along the path.
Electromagnetic tracing cannot trace a line that has no conductivity and no accompanying tracer wire. PVC water mains, clay sewer pipe and HDPE gas pipe without a locatable wire are not directly traceable by this method.
Ground Penetrating Radar
Ground penetrating radar transmits electromagnetic pulses into the ground and reads the reflections that return from buried objects and layer boundaries. Unlike electromagnetic tracing, ground penetrating radar is not limited to conductive targets. It can detect PVC pipe, concrete structures, clay tile, voids, and abandoned lines that carry no signal and have no tracer wire.
GPR results are affected by soil conditions. Clay-rich soils and saturated ground attenuate the radar signal, which can reduce the depth at which reliable reflections are obtained. Tightly spaced targets at similar depths can also be difficult to distinguish as separate objects. These are physical limitations of the method, not interpretation errors, and they are reasons why findings are described as detections and interpreted positions rather than absolute measurements.
The FHWA’s framework describes the combination of surface geophysical methods as Quality Level B, or designating. At this level, the horizontal position of utilities is determined through field investigation and tied to survey control, addressing problems caused by inaccurate utility records, abandoned or unrecorded facilities, and lost references.
Using Both Methods Together
On a typical commercial site, electromagnetic tracing and ground penetrating radar complement each other. Electromagnetic tracing follows the live metallic services and any non-metallic lines that have installed tracer wire. Ground penetrating radar picks up the PVC, concrete and abandoned infrastructure a tracer cannot reach. Running both methods across the same work area produces a more complete picture than either method alone.
For private utility locating on commercial and municipal sites across Florida and Georgia, that field investigation also reaches the private infrastructure past the meter point that public 811 notification does not cover. Site lighting feeds, irrigation mains, private sewer laterals and any line installed by prior tenants or contractors all need to be investigated in the field, because they are unlikely to appear on any utility-company record.
How to Think About Records at Each Stage of a Project
Records research is still the right first step. It frames the investigation, identifies which utilities may be present, and helps a field technician know which signals to look for during electromagnetic tracing. The problem arises when records are treated as the finished product rather than the starting framework.
For project planning and early route decisions, records give a general picture. For design decisions involving storm drainage, footings, foundations or utility routing, field investigation at the geophysical level is needed to verify what is actually there and to allow design adjustments before conflicts become construction problems. The FHWA’s guidance is explicit that accurate utility information available early in project development allows designers to work around conflicts rather than discovering them during construction.
The practical sequence on a commercial excavation project looks like this:
- Gather all available utility records, as-builts and permit drawings for the work area.
- Survey visible surface features and correlate them with existing records.
- Conduct field investigation using electromagnetic tracing and ground penetrating radar across the full work area, not only where records show a known line.
- Document and mark findings on the ground before any excavation, trenching or boring begins.
- Review findings against the records to identify discrepancies, unlisted lines and potential depth variations.
A Note on Depth and Accuracy
Horizontal position determined through field investigation is more reliable than depth. Depth estimates from electromagnetic tracing and ground penetrating radar carry uncertainty that varies with soil conditions, signal interference and target geometry. Treating a depth reading as an exact measurement rather than an approximate value is a common point where field information is misapplied in project planning. When precise depth is critical to a design decision, that specific point in the utility path warrants closer investigation.
Ready to Move from Records to a Field Investigation?
If your project is in the planning or early design phase and you are working from utility drawings alone, field investigation using electromagnetic tracing and ground penetrating radar can show you what those drawings do not. Call for a free estimate to discuss the scope of your site and what methods apply.
