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  • Emma Chetwynd Jarvis

What is PAS 128 surveying and how does it work?

What is PAS 128?

British Standards Institute (BSI) specification PAS 128 is a specification for underground utility detection, verification and location, which allows surveyors to offer standardised surveys with qualitative results.

Unlike other survey specifications, PAS 128 focuses on a hierarchical classification of surveys, on a levels-based system. This hierarchy is focused around clients’ needs and requirements and allows flexibility for different budgets and situations.

How does PAS 128 work?

PAS 128 is divided into several category types of survey, ranging from ‘D’ up to ‘A’. These quality levels (QLs) all require different levels of effort and expertise, and all yield different degrees of accuracy in their results.

Type D – Desktop utility surveying

Type D surveys are desktop utility record searches, where existing paper or online utility records are used to identify the presence and reported location of underground utilities.

As records are often either incomplete or inaccurate, type D surveys usually produce the lease accurate results of the four types and as such are used mostly as a preliminary to further surveys.

Type C – Site reconnaissance

Site Reconnaissance, PAS 128 survey type C allows surveyors to physically assess a proposed building or excavation site, and validates findings and locations shown on records with physical evidence, for example a stop tap over reported waterworks.

Type C surveys bring more accurate results than desktop searches, although it is often used in conjunction with Type D, to verify findings.

Type B – Detection using geophysical tools

Type B surveys are on-site mapping surveys which use multiple geophysical instruments to detect underground utilities, for example ground-penetrating radar (GPR) or electromagnetic location (EML).

These require a lot of effort and time to complete, as well as someone trained in using these instruments to operate and carry out these surveys. Despite the amount of effort and expertise needed, these are preferential to lower survey types due to the accuracy they provide.

Type A – Physical verification

Physical verification is where the location of underground utilities is confirmed in person, through either a manhole cover or inspection chamber or by careful excavation of the structure (trial pitting).

This type of survey is the most time-and-effort-consuming of the four levels, but it also gives the clearest and most accurate results.

Physical verification surveys are also often used to confirm the findings of lesser searches, such as a Type B or C.

What are the benefits?

By carrying out these surveys, on-site safety is maximised and utility strikes can be prevented, along with serious injury to contractors or staff.

A properly carried out PAS 128 survey will provide precise, detailed results to protect your workers, save you time and money and avoid damage to your reputation.

How can Rock Surveying help?

We are experienced and competent surveyors and can carry out a full range of PAS 128-compliant surveys.

Rock Surveying is experienced in providing accurate underground utility surveys in high risk, urban and rural locations. Our team of Surveyors can carry out a full range of PAS 128 surveys and guarantee accurate results.

To learn more or to speak to one of our professional surveyors, contact us today at or call 01905 456 384.


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