AS 5488 Quality Levels Explained: QL-D to QL-A for Subsurface Utility Mapping

AS 5488 quality levels tell you how much to trust the position of an underground service on a drawing. There are four, from QL-D (taken from records, unverified) to QL-A (physically exposed and measured). Anyone commissioning subsurface utility mapping in NSW will be asked which level they need, and the answer decides the method, the cost and how the result can be used. This is a plain explanation of each.

What AS 5488 is and why it exists

AS 5488 is the Australian Standard for the classification of subsurface utility information. The current edition, AS 5488-2022, is published by Standards Australia in two parts: one covering how utility information is classified, the other covering subsurface utility engineering.

Before the standard, a line on a services plan could mean anything. It might have been surveyed in an open trench or copied from a sketch made decades earlier. The quality level system fixes that by attaching a label to every service that says how the information was obtained. A designer can then see at a glance which lines are measured and which are assumptions.

The four quality levels at a glance

• QL-D: from existing records or anecdotal information. No site verification.
• QL-C: records matched to visible surface features such as pits, valves and hydrants.
• QL-B: traced on site with electromagnetic or ground penetrating radar equipment.
• QL-A: exposed, seen and measured directly, usually by potholing.

Each level includes the work of the one below it. A QL-A point still starts with records and a surface inspection.

QL-D: records only

QL-D is desktop information. It comes from Before You Dig Australia plans, asset owner records, old drawings and what people on site remember. Nothing has been checked in the field.

It has real uses. QL-D is enough for early feasibility, for scoping a locate, and for knowing which asset owners are present. It is not enough to dig on or to design to. Records show services in roughly the right street and often on the wrong side of it.

QL-C: records matched to what is visible

QL-C adds a site walk. Pit lids, valve covers, hydrants, pillars and meters are surveyed, and the recorded alignments are adjusted to fit between them.

This catches gross errors cheaply. If the plan shows a water main on the northern side and every hydrant is on the southern side, QL-C finds it. What it cannot do is tell you where the pipe runs between two lids. The line between surface features is still an interpretation.

QL-B: detected on site

QL-B is where specialist equipment comes in. Services are traced using electromagnetic locators and ground penetrating radar, marked on the surface and, where a drawing is needed, picked up by survey. Under the standard, a QL-B position is expected to be within about 300 millimetres horizontally and 500 millimetres vertically of the true position.

Most utility locating before excavation is done to this level. It is fast, it does not break ground, and for the majority of design and construction work it is accurate enough.

The honest limit: QL-B depth is an estimate. Electromagnetic depth readings drift where signals couple onto neighbouring services, and radar depth depends on a soil velocity assumption. Treat a QL-B depth as an indication, never as a clearance.

QL-A: seen and measured

QL-A means the service was physically exposed and its position measured, with a tolerance of about 50 millimetres in both plan and level. It also records what the detection methods cannot: the size, material and condition of the asset.

Exposure is normally done by non-destructive excavation, where a vacuum truck removes soil with water or air instead of a bucket. QL-A applies to the point that was exposed, not to the whole length of the service. A pipe potholed at two locations is QL-A at those two locations and QL-B in between.

Which quality level does your project need?

Match the level to the decision being made:

• Feasibility and early planning: QL-D, sometimes QL-C.
• Concept and detailed design: QL-B across the site.
• Design near critical or congested services: QL-B generally, QL-A at clash points.
• Excavation, boring or piling close to an asset: QL-A at the crossing.

Asking for QL-A everywhere is the common mistake. It is expensive, disruptive, and on most sites unnecessary. The better brief is QL-B across the area with QL-A at the specific points where the design depends on an exact depth. A good locator will suggest where those points are after the QL-B pass, not before it.

Transport and water authorities often specify quality levels in their own requirements. If the work is for Transport for NSW or Sydney Water, check the project specification first, because it may set the level for you.

How quality levels appear in the deliverable

A compliant deliverable labels every service segment with its quality level, usually through line type, layer naming or an attribute in the model. It should also state the survey datum and list anything that could not be located.

LCG Locating classifies and reports every investigation to AS 5488-2022, with utility survey and mapping delivered as 12D or DWG models on GDA2020. If a drawing you have been given shows services with no quality level attached, treat all of it as QL-D until someone tells you otherwise.

Next step

Work out which decisions on your project depend on knowing where services are, then brief for QL-B across the site and QL-A where those decisions are tight. Talk to our team with a plan of the work area and we will recommend the level for each part of it.