Resources · PAS 128 guide
PAS 128:2022 simplified guide
A simple visual guide to Survey Types, Quality Levels (QL) and Methodologies (M1–M4).
The key message: PAS 128 does not simply mean “GPR survey”. It provides a structured way to define what information was available, what was detected, how it was detected, how confidently it was located, and where physical verification is required.
1. The PAS 128 workflow
Think of PAS 128 as a journey from information → site evidence → geophysical detection → physical verification.
| TYPE | WHAT IT MEANS IN PLAIN ENGLISH | OUTPUT / QUALITY |
|---|---|---|
| D | Desktop Utility Records Search Existing utility/asset records are gathered and reviewed. This is information from records — not proof of what is physically underground. | QL-D Location is not independently verified in the ground. |
| C | Site Reconnaissance Records are compared with visible evidence such as chambers, covers, street furniture, topographic features or reinstatement scars. | QL-C Useful evidence, but not a geophysical detection of the buried utility. |
| B | Detection Survey Geophysical techniques are used to detect and locate utilities. PAS 128 requires a combination of GPR and EML for Type B detection. | QL-B4 → QL-B1 Confidence and positional accuracy increase depending on what was detected and the supporting evidence. |
| A | Verification Survey The utility is physically exposed/inspected and its position is measured. This can confirm critical design or clash points. | QL-A Highest PAS 128 quality level; typically ±50 mm horizontal and vertical location accuracy. |
Remember
Survey Type ≠ Quality Level ≠ Methodology. D/C/B/A describe the type of survey. QL-D/QL-C/QL-B4...B1/QL-A describe the resulting quality of utility information. M1–M4 describe the effort/search methodology used for Type B detection.
2. Quality levels — the simple version
Each utility segment should be assigned the appropriate quality level based on how it was established and the confidence/accuracy achieved.
| QUALITY | PLAIN-ENGLISH MEANING | LOCATION ACCURACY / CONFIDENCE |
|---|---|---|
| QL-D | Records only | Undefined — record information only. |
| QL-C | Supported by site reconnaissance | Undefined — position supported by visible/site evidence, not geophysical detection. |
| QL-B4 | Suspected / assumed route, not detected | Undefined. Detection was attempted but the utility was not detected; it may remain an assumed route. |
| QL-B3 | Detected horizontally, but no reliable depth | Horizontal position: ±500 mm. No reliable depth measurement. |
| QL-B2 | Detected horizontally and vertically with reasonable confidence | Horizontal: ±250 mm. Vertical: ±40% of detected depth. |
| QL-B1 | Higher-confidence 3D detection | Horizontal: ±150 mm. Vertical: ±15% of detected depth. |
| QL-A | Physically verified utility | Horizontal: ±50 mm. Vertical: ±50 mm, where exposed/inspected and measured. |
Important: these are simplified client-facing descriptions. PAS 128 contains detailed conditions, notes and requirements affecting how quality levels are determined and reported.
What does “P” mean?
B1P, B2P and B3P indicate that post-processing has been applied to the Type B detection data. Post-processing can support more detailed analysis and interpretation of collected GPR data. QL-B4 does not receive a P suffix.
3. M1–M4 — how much search effort?
The M-code is about the search methodology and coverage effort. It is not a substitute for the QL achieved.
| METHOD | TYPICAL SEARCH RESOLUTION | PLAIN-ENGLISH USE |
|---|---|---|
| M1 | EML transects generally ≤10 m; when following a trace ≤5 m. GPR generally ≤5 m grid/appropriate array approach. | Lower-density / undeveloped areas. Lower search effort than M2–M4. |
| M2 | EML generally ≤5 m; when following a trace ≤2 m. GPR generally ≤2 m grid/appropriate array approach. | Suburban or moderately serviced areas. |
| M3 | EML generally ≤2 m; when following a trace ≤1 m. GPR generally ≤1 m grid/appropriate array approach. | Busy urban areas or higher-risk clearance applications. |
| M4 | EML generally ≤2 m; when following a trace ≤0.5 m. GPR generally ≤0.5 m orthogonal / suitable multichannel full-coverage approach. | Highly congested areas where greater search effort is justified. |
A useful way to remember it
M1 → M4 = increasing search effort / tighter coverage. B4 → B1 = increasing confidence/quality of detected utility information. A = physical verification.
Post-processing
The “P” suffix denotes post-processing for applicable Type B methodologies. PAS 128 also provides guidance on when post-processing should be used; higher-effort surveys may warrant more detailed processing and interpretation.
Method selection should reflect utility density, project risk, intended use of the data and the client's required confidence — not simply the cheapest or highest code.
4. What should a client ask for?
Instead of simply asking for “a PAS 128 survey”, define the information you actually need.
| PROJECT QUESTION | WHAT TO SPECIFY / DISCUSS |
|---|---|
| What stage is the project at? | Planning, feasibility, detailed design, construction, excavation/permit-to-break-ground or asset management? |
| How congested is the site? | Consider whether M1, M2, M3 or M4 search effort is appropriate for the expected utility density and risk. |
| What confidence is required? | Specify the required QL outcome where appropriate: B1/B2/B3/B4, with QL-A verification at critical locations where needed. |
| Are records available? | Provide available utility records and other relevant information. Type D is the foundation for Type B planning. |
| Are there critical clashes? | Identify piling, foundations, crossings, tie-ins, deep excavation, drilling or other areas where physical verification may be required. |
| What deliverables are required? | Agree CAD/GIS/PDF formats, survey control, utility attributes, depths, annotations, quality levels, metadata and limitations. |
A simple specification example
Undertake a PAS 128:2022 Type B utility detection survey using an agreed M-code appropriate to site conditions, with the required QL levels identified for the project. Identify locations requiring Type A verification and report all limitations, assumptions and quality levels on the deliverables.
This wording is deliberately generic. The final scope should be agreed with the competent survey practitioner and tailored to the project.
5. PAS 128 cheat sheet
| D | C | B | A |
|---|---|---|---|
| RECORDS | RECONNAISSANCE | DETECTION | VERIFICATION |
| QL-D | QL-C | QL-B4 → B1 | QL-A |
| TERM | REMEMBER IT AS… |
|---|---|
| QL-D | Records |
| QL-C | Records + visible/site evidence |
| QL-B4 | Expected but not detected / assumed route |
| QL-B3 | Detected horizontally; no reliable depth |
| QL-B2 | Detected horizontally + vertically, ±250 mm / ±40% depth |
| QL-B1 | Higher-confidence 3D detection, ±150 mm / ±15% depth |
| QL-A | Physically verified, ±50 mm horizontal/vertical |
| M1–M4 | Search effort / coverage methodology — increasing from M1 to M4 |
| P | Post-processing applied to applicable Type B survey methodology |
Source note
Based on PAS 128:2022 and the TSA Client Guide to BS EN PAS 128:2022, Issue 3 (May 2025). This document is an explanatory aid and is not a replacement for the PAS 128 specification. PAS 128:2022 is the current BSI edition; PAS 128:2014 was superseded in April 2022.
Document created by Mr Michael Brett @ Cadmap Ltd — www.cadmap.co.uk 2026.