On most BC job sites, "line locating" and "GPR survey" get used almost interchangeably. A supervisor asks for a locate, a technician shows up with a GPR cart, and everyone assumes the two terms mean the same thing. They don't, and knowing the difference matters when you're scoping a project and deciding what coverage you actually need.
This guide breaks down what utility line locating covers, where it stops, and when a full GPR survey needs to be added on top of it.
What Utility Line Locating Actually Is
Utility line locating, often just called line locating or a line locate, is the process of tracing and marking individual underground lines using electromagnetic (EM) locating equipment. A technician induces a signal onto a conductive line, metal pipe, electrical conduit, or a cable with a tracer wire, and follows that signal from the surface, marking the line's path and depth with paint and flags as they go.
This is the fieldwork most people picture when they think of a utility locate: flagged and painted lines running across a street or lot ahead of an excavation crew. It's fast, it's accurate for the lines it can trace, and for a huge share of everyday digging, gas services, electrical feeds, water mains with tracer wire, it's exactly the coverage a project needs.
What Line Locating Can't Catch
Electromagnetic equipment only works on lines that carry or can be induced with a signal. Older plastic water main, clay tile sewer, concrete conduit, and fibre optic duct without a tracer wire don't respond to EM at all, an EM-only locate simply won't see them. Voids, abandoned tanks, and utilities missing from any record are invisible to line locating for the same reason: there's no conductive signal to trace.
This is the gap that causes strikes on sites where a line locate was performed correctly but treated as a complete subsurface picture. It wasn't incomplete work, it was the wrong tool for what was actually underground.
What a Full GPR Survey Adds
Ground Penetrating Radar fills exactly that gap. GPR sends radar pulses into the ground and reads the reflections, so it detects both conductive and non-conductive targets, plastic pipe, concrete structures, voids, buried tanks, and utilities with no trace signal at all. A full GPR survey typically means a systematic grid scan of the work area, not just tracing a handful of known lines.
The trade-off is time and scope. A grid scan takes longer than tracing a few known lines, and it's the right call when the site history is unclear, the ground has been disturbed or redeveloped before, or the excavation is deep or large enough that an unknown utility or void would be a serious problem.
When You Need Both
Most BC projects end up using EM line locating and GPR scanning together, which is why our standard utility locating service includes both as one combined visit rather than two separate calls. Common situations where that combination matters most:
- Sites with older infrastructure where plastic, clay, or concrete utilities are likely present
- Pre-construction and redevelopment sites where existing records are incomplete or outdated
- Environmental drilling and monitoring well installations, where an unknown void or tank changes the plan
- Any excavation deep or large enough that a missed utility would be costly to hit
The Bottom Line
Line locating and a full GPR survey aren't competing services, they're complementary ones, and knowing which you're asking for (or which you're being quoted) is the difference between a locate that covers your site and one that only covers part of it. If you're not sure which your project needs, contact GPR Surveys and we'll scope it properly before we show up.