PAS 128 explained, the standard for utility surveys
PAS 128:2022 is the British specification for finding and recording buried utilities. Survey types, quality levels, methodology and how to specify a survey to it.
By Terry Bagust, Co-founder and Managing Director
PAS 128 is the British specification for finding and recording buried utilities. It does not tell you what is in the ground; it tells you how well someone looked and how much you can trust what they drew. The current edition is PAS 128:2022, Underground utility detection, verification and location, Specification, published by BSI on 30 April 2022. It replaced the 2014 edition, which is withdrawn. A specification that still cites the 2014 edition is out of date.
For the full guide, with the quality-level table, see PAS 128 explained. This article is the short version.
Four survey types
- Type D: a desktop search of the statutory undertakers’ records. Nobody visits the site.
- Type C: the records checked on site against what can be seen, such as covers, cabinets and reinstatement scars.
- Type B: detection. The site is swept with an electromagnetic locator and GPR, and every service found is positioned.
- Type A: verification. The service is exposed in a trial hole or by vacuum excavation and measured.
Most jobs need D plus B. A needs B first.
Quality levels
Quality levels describe each service on the drawing, from QL-D (records only) through QL-C, QL-B4 to QL-B1, to QL-A (exposed and measured). QL-B1 means position and depth detected to within ±150 mm horizontally and ±15% of depth. QL-B2 is ±250 mm and ±40% of depth. QL-B3 is detected by one method with a horizontal tolerance of ±500 mm and depth not reliable. QL-B4 is expected but not detected, so the route is assumed.
Methodology M1 to M4
The M number sets how dense the survey is, from a sweep along the expected routes (M1) to a full grid at close centres (M4). From M3 upward the GPR data must be post-processed in the office, shown as M3P or M4P and as a P on the quality level (QL-B1P).
What changed in 2022
Type B now requires two detection methods and a full passive sweep. GPR arrays are recognised. Raw data must be kept. Practitioners are expected to hold a recognised utility-mapping qualification. Data sharing to the National Underground Asset Register is encouraged.
How to specify it
“Utility detection survey to PAS 128:2022, Type D and Type B, methodology M3P minimum, deliverable in DWG and PDF with PAS 128 quality levels shown per utility, report to include methodology, equipment and operator qualifications.”
What a survey does not do
A PAS 128 survey records what was detected to the stated quality level. It does not guarantee that every service has been found. Wet clay, made ground and parallel services all limit what detection can separate. Where the drawing says QL-B3 or QL-B4, a trial hole is the next step, and HSG47 still applies. Be wary of any survey that promises to find everything.
Who does it
Our utility surveyors hold the ProQual Level 3 Certificate in Utility Mapping and Surveying and deliver surveys to PAS 128:2022. See PAS 128 utility surveys or PAS 128 utility surveys in London.
What this means for your job
For a PAS 128 survey before you dig, see PAS 128 utility surveys.
