What Do PAS 128 Survey Types (M1, M2, B, C, D) Mean – and Which One Do You Need for Your Project?
If you’re planning excavation, utilities installation, or highways works, you will have heard people talk about PAS 128 survey types. These classifications—M1, M2, B, C and D—tell you how your underground utility survey was carried out and how confident you can be in the results. Understanding these categories helps you choose the right utility survey for your project and manage risk from the start.
This guide breaks each type down in plain English so you know what they mean, when they’re used, and how to decide which level you need.
Accurate utility detection starts below ground — PAS 128 surveys provide the clarity contractors need to plan safely and avoid costly service strikes.
Why PAS 128 Survey Types Matter
Keywords: PAS 128 survey, utility mapping survey types
PAS 128 is the UK’s recognised specification for underground utility detection. It standardises how surveys are carried out and the level of accuracy clients can expect. As stated in the HSE guidance on safe excavation practices, understanding where buried services are located is essential before breaking ground.
Choosing the wrong PAS 128 type can lead to:
Unexpected clashes on site
Delays and redesign fees
Strikes on live services
Increased cost due to rescanning or potholing
If you missed last week’s article, here is PAS 128 Explained on the AKN blog — a useful foundation before reading this more detailed breakdown.
PAS 128 Survey Types: What Each One Means
Type D – Desktop Utility Search (Lowest Certainty)
A PAS 128 Type D is essentially a records-based search. No one visits the site. The output combines statutory undertakers’ plans, old drawings, and any information you provide.
Best for: ✔ Early design work ✔ Budget planning ✔ Projects where site access is not yet available
Limitations: ✘ Utility plans are often outdated or inaccurate ✘ Not suitable for excavation.
Type C – Site Reconnaissance + Records Review
Type C combines a desktop search with a visual walkover of the site. No detection techniques are used. Surveyors identify visible evidence such as manholes, lamp columns, substations, or utility markers.
Best for: ✔ Feasibility studies ✔ Early-stage risk assessments ✔ Narrowing down potential problem areas
Limitations: ✘ Still not accurate enough for breaking ground ✘ No verification of depth or route.
Type B – Utility Detection Survey (Most Common)
This is where GPR (Ground Penetrating Radar) and EML (Electromagnetic Locator) are used to detect buried utilities. PAS 128 Type B is the backbone of most pre-construction surveys and the service level AKN Engineering typically delivers for contractors.
Within Type B, there are survey methods M1–M4:
M1 & M2 – The Two Methods You Need to Know
M1 – EML Only
Uses electromagnetic detection to locate metallic utilities and tracer-wired cables.
Strengths: ✔ Quick, cost-effective ✔ Good for power, telecoms, water (if metallic)
Limitations: ✘ No plastic pipes unless a tracer wire is present ✘ Depth readings less reliable.
M2 – EML + GPR (Recommended for Most Projects)
Combines both technologies to produce a more complete picture of the underground environment.
Strengths: ✔ Detects metallic and non-metallic utilities ✔ Improves confidence in depth and alignment ✔ More suitable for excavation and design
Limitations: ✘ Requires accessible surfaces (as noted in HSG47 guidance) ✘ Thick vegetation, cobbles, or reinforced concrete may affect GPR results
This is the survey method AKN Engineering most often recommends for highways, civils, car parks, housing sites, schools, and utility diversions.
PAS 128 Type A – Verification (Highest Certainty)
Although not in your headline list, Type A is worth mentioning. This is where utilities are physically exposed to confirm depth, diameter, and material — usually through trial holes.
As the ICE states, exposure is the only way to confirm exact positions.
Best for: ✔ Complex diversions ✔ Major highways works ✔ Projects with tight tolerances
Which PAS 128 Survey Type Do You Need?
Here is a quick guide:
Table Summary showing Project type vs recommended PAS 128 Type
If in doubt, choose Type B M2. It provides the most reliable results without the cost of intrusive works.
For more detail on how AKN conducts GPR and EML surveys, see our Utility Surveys service page on the AKN Engineering website.