Why PAS128 uses quality levels
PAS128 is the specification for detecting, verifying and locating underground utilities in the UK. Its most useful feature for designers and contractors is the quality level. Instead of a drawing that shows every line as if it were equally reliable, each service is labelled to show how its position was established. That tells you how much you can rely on it when you design, plan excavations or price risk.
The current version, PAS128:2022, describes four survey types – D, C, B and A – that produce the matching quality levels.
QL-D: desktop records (Survey Type D)
Positions taken from utility records only. In Scotland that means records from Scottish Water, SGN, the electricity network operator (SP Energy Networks across most of the central belt, SSEN further north), Openreach and other telecoms and fibre operators, and any private or site records. A records search is the first step of every PAS128 survey, but it is the lowest quality level: records are often schematic, out of date or missing depths altogether.
QL-C: site reconnaissance (Survey Type C)
Records checked against what can be seen on site: chamber covers, valve and hydrant covers, marker posts, cabinets, poles and reinstatement scars. Where a record can be matched to surface evidence it is upgraded to QL-C. It is still not detection – nothing below ground has been located.
QL-B: detection (Survey Type B)
Services located using geophysical techniques – typically electromagnetic location (EML) and ground penetrating radar (GPR) – and positioned with survey equipment. PAS128:2022 splits QL-B into four levels so you can see how confident the surveyor is in each line:
| Level | What it means |
|---|---|
| QL-B1 | Position and depth detected, with a high degree of confidence. |
| QL-B2 | Position and depth detected, with reasonable confidence. |
| QL-B3 | Position detected, but no confidence in the depth. |
| QL-B4 | Position assumed – the service is suspected, for example between two detected points, but was not detected along that length. |
PAS128 also sets out methodology types (M1 to M4P) for detection surveys, which describe how intensive the survey is, from open undeveloped land to congested city streets. A more intensive methodology generally gives better results in busy areas.
QL-A: verification (Survey Type A)
The highest quality level. A service is seen and its position and depth recorded – for example inside a chamber, or where it has been exposed in a trial hole or slit trench. QL-A is usually targeted at critical crossings and clash points rather than a whole site.
Which quality level do you need?
- Early feasibility: a QL-D records search, often with QL-C site reconnaissance, is enough to spot major constraints.
- Design and planning excavations: a Type B detection survey, giving QL-B results, is the normal standard for design and before digging.
- Critical clashes: QL-A verification where a new route or foundation crosses a high-risk service.
If you are not sure, tell us what the survey is for and we will recommend the survey type and methodology that fits – there is no point paying for more than the project needs, or relying on less.
Need a utility survey? Call 0345 548 5451 or email us with your site plan for a fixed quote.
