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Flowchart

Underground Utility Detection

Survey Workflow Flowchart

EML-primary workflow · GPR for verification & non-metallic detection · Click any node for details

Pre-Survey

Passive EML

Active EML

Trace & Record

Non-Metallic / GPR

Success

Fail / SNS

👆Click any node on the flowchart to see detailed explanation and source

Key Facts

EML detectsMetallic utilities only

GPR detectsAll materials incl. non-metallic

Sonde used forGravity drains with manhole access

Best active methodDirect connection (most accurate)

Reliable accuracy±150mm or ±15% of depth

Desktop recordsIndicative only — not exact

GPR limitationCannot ID utility type alone

EML limitationSignal bleed in congested areas

PRACTITIONER NOTE

In real-world practice, ~95% of detection is completed through EML. GPR's primary role is to verify EML results and detect non-metallic utilities. EML is the workhorse — GPR is the safety net.

Sources: PAS 128:2014/2022 · LSGI 2019 · Leica/Hexagon Geosystems · Met Consultancy Group · SUMO Services · GEOTEC · KB Surveys · Research: Wallace W.L. Lai (PolyU)

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