PAS 128 defines four quality levels that describe how a buried utility was found and how far its position can be trusted. QL-D is a desktop records search, QL-C adds a site walkover, QL-B is a geophysical detection survey using methods such as GPR and EML, and QL-A is physical verification by exposing the service. Confidence rises at each level, from a paper exercise at QL-D to a service you have actually seen and measured at QL-A.
Those letters are the whole point of the standard. Before PAS 128, a utility drawing was just a drawing, and you had no consistent way of knowing whether a line on it came from a records search or from someone standing over an open trial hole. The quality levels give the entire project team a shared language for confidence, so a utility map now carries a stated reliability rather than an implied one.
The four PAS 128 quality levels
PAS 128:2022 sets out four levels of increasing confidence. They are not a menu where higher is always better, but a scale you match to the risk of the work in front of you.
| Level | What it involves | Confidence |
|---|---|---|
| QL-D | Desktop records search only, no site work. Statutory plans and asset records collated into a drawing. | Lowest |
| QL-C | Site reconnaissance added. Records visually correlated against surface features such as covers, valves and marker posts. | Low |
| QL-B | Geophysical detection survey on site using GPR and EML to locate services. Subdivided B1 to B4. | Medium to high |
| QL-A | Physical verification. The service is exposed by trial hole or vacuum excavation and its position, depth and type confirmed. | Highest |
QL-D and QL-C are essentially records-based. They are useful for early feasibility and design awareness, but neither involves detecting a service in the ground, so both depend entirely on the accuracy and completeness of existing records, which are often neither.
Why QL-B is subdivided into B1 to B4
QL-B is where the survey moves from paper to physics, and it is the level most projects actually commission. Because detection results vary with ground conditions, the equipment used and how thoroughly an area is covered, PAS 128 subdivides QL-B by confidence rather than treating every detection survey as equal.
- QL-B1 is the highest detection confidence, where a service has been located and its position and depth are well resolved.
- QL-B2, B3 and B4 step down as confidence in position, depth or interpretation reduces, for example where a signal is weak, where services are congested, or where only horizontal position can be established with confidence.
The practical value is honesty. A good QL-B survey does not pretend every service is equally certain. It records where the confidence is high and flags where it is not, so you know which parts of the drawing to lean on and which to treat with caution.
GPR and EML: why QL-B uses both
A QL-B survey almost always combines two detection methods, because neither finds everything on its own.
- Electromagnetic location (EML) detects metallic and conductive services such as electricity cables and metal water and gas pipes, and can trace live cables. It cannot see non-conductive services.
- Ground penetrating radar (GPR) detects non-metallic services that EML misses, including plastic pipes, ducts and fibre optics, along with voids and other subsurface features.
Used together they give far more complete coverage than either alone, which is why a credible QL-B survey pairs them rather than relying on a single technique. Our guide to GPR surveys explains how radar detection works in more detail.
QL-A: the only level you have actually seen
QL-A is verification. A service located at QL-B is physically exposed, usually by trial hole or by vacuum excavation, so its exact position, depth and type can be confirmed by eye and by measurement. It is the highest confidence the standard offers because it removes interpretation altogether.
Because it involves excavation, QL-A is used selectively rather than everywhere. It is reserved for the points that matter most: high-pressure gas mains, high-voltage cables, or the exact spot where a new pile or foundation will land next to a critical service. Many projects run a layered approach, mapping the whole site at QL-B and then verifying the handful of positions where certainty is worth the dig.
Which quality level does your project need?
There is no single right answer, only a right answer for a given stage and risk.
- Feasibility and early design are often served by QL-D or QL-C, giving an initial picture of what is likely below.
- Detailed design and pre-construction typically call for QL-B, the most widely commissioned level, giving a detected, coded map you can design and plan safe digging against.
- High-risk excavation near critical services warrants QL-A verification at the specific points of concern.
Specifying the level up front matters, because it sets both the cost and the reliability of what you receive. Asking for QL-A across an entire site is rarely proportionate; relying on QL-D before breaking ground is rarely safe.
Common questions
What is PAS 128?
PAS 128 is the British publicly available specification for underground utility detection, verification and location. It defines a consistent methodology for surveying buried services and a set of quality levels that describe how each service was found and how reliable its recorded position is, so utility surveys can be specified and compared on a common basis.
What is the difference between QL-B and QL-A?
QL-B is a geophysical detection survey that locates services from the surface using methods such as GPR and EML, giving a good but interpreted position. QL-A goes further and physically exposes the service, usually by trial hole or vacuum excavation, to confirm its exact position, depth and type. QL-A gives the highest confidence because the service has actually been seen rather than inferred.
Do I always need the highest quality level?
No. The right level depends on your project stage and the risk of the works. Early design may only need a records-based QL-D or QL-C, most detailed design and excavation work uses QL-B, and QL-A verification is applied selectively at high-risk points such as near high-pressure gas or high-voltage cables. Matching the level to the risk keeps the survey both safe and proportionate.
What does QL-B1 mean?
QL-B1 is the highest confidence subdivision of a QL-B detection survey. It means a service has been detected and both its horizontal position and depth are well resolved. The lower subdivisions, B2 to B4, indicate reducing confidence in position, depth or interpretation, for example in congested ground or where signals are weak.
Is PAS 128 a legal requirement?
PAS 128 is a specification rather than a law, but it has become the recognised industry standard for underground utility surveys in the UK. Specifying a survey to PAS 128 gives you a defined methodology and a stated confidence level, which supports safe digging duties and sits alongside guidance such as HSG47 on avoiding underground services.
Knowing the quality levels is really about specifying the right survey for the risk in front of you. If you are planning ground works and want a coded, PAS 128 classified map of what lies beneath, our underground utility surveys are carried out to the level your project needs, and it is worth reading alongside whether you need a utility survey before excavation and our practical notes on avoiding utility strikes under HSG47.