9 Verification (survey type A)

NOTE 1 Verification is the process of exposing a utility and subsequently measuring and recording its accurate location as well as other relevant attributes.

NOTE 2 Verification of a utility that does not have access via a manhole or inspection chamber can be performed using one, or a combination of excavation techniques that protect the integrity of the utility. Such techniques include, for example, vacuum excavation (air only), hydro or dual (air and hydro) and safe hand digging techniques. For guidance on potential dangers of working near underground services and on how to reduce any direct risks to people's health and safety, as well as the indirect risks arising through damage to services, see HSE's Avoiding danger from underground services (HSG47) [3].

9.1 General

9.1.1 Survey type A shall comprise exposing the target utility(ies) within the survey area to confirm and record the location and other attribute data.

9.1.2 The quality level achieved shall be documented as QL-A in accordance with Table 1.

9.2 Methodology

NOTE Where it is not possible to lift a manhole cover because it is stuck or obstructed, this should be reported to the client at the earliest opportunity with a request for the cover to be made accessible or lifted, preferably whilst the practitioner(s) are still on-site. It is good practice not to attempt to lift damaged covers but to report the damage to the client for instructions.

WARNING. This PAS refers to physical entry into confined spaces, which is not to be attempted without suitably trained operatives and safety equipment. Attention is drawn to HSE's publication. Confined space – A brief guide to working safely (INDG258) [1].

9.2.1 For survey type A, the data shall be obtained through visual inspection of the utility:

  • a) at access points such as in a manhole or inspection chamber; and/or
  • b) by its excavation and exposure.

NOTE 1 Damage to the utility and the surrounding infrastructure should be avoided when exposing the utility and also in undertaking the reinstatement of any excavations. Exposing the utility could be by any number of standard of care techniques including:

  • a) vacuum excavation with pressurized air and/or water to expose the utility;
  • b) hand digging techniques to expose the utility;
  • c) other conventional mechanical excavation technologies used in conjunction with a) and/or b).

NOTE 2 Exact intervals of non-destructive verifications are project dependent and should be specified by the client.

WARNING. For all excavations, assume that underground utilities are present and act accordingly.
Attention is drawn to laws, rules and regulations applicable to vacuum excavating or hand digging near or atop dangerous utilities such as electric, gas, fuel or petroleum.

9.2.2 Data gathered from visual inspection at access points shall include as a minimum:

  • a) for foul, surface and combined water drainage systems access points (manholes and inspection chambers):
    • 1) pipe positions and orientation at ground surface;
    • 2) visible pipe diameters;
    • 3) material type;
    • 4) pipe depths (invert levels related to a common datum);
    • 5) directions of flow;
    • 6) manhole/inspection chamber size;
    • 7) manhole/inspection chamber soffit depth and depth to base;
    • 8) connectivity diagram;
    • 9) manhole/inspection chamber layout and/or photographs;
  • b) for telecoms/electrical utility manholes and inspection chambers:
    • 1) ducts position and orientation at ground surface;
    • 2) number and size of ducts on each face;
    • 3) material type;
    • 4) depth to top of ducts;
    • 5) manhole/inspection chamber size;
    • 6) depth to base;
    • 7) manhole/inspection chamber layout and/or photographs;
  • c) for gas/water utility manholes and inspection chambers:
    • 1) depths and diameter of the utility, when visible in manhole/inspection chamber;
    • 2) valve connectivity;
    • 3) material type;
    • 4) manhole/inspection chamber layout and/or photographs.

9.2.3 Excavations shall be:

  • a) a single spot excavation (commonly known as an inspection pit, test hole or pothole) for the verification of an individual utility; and/or
  • b) a trench excavation for the verification of multiple utilities.

NOTE 1 The verification measurements should only be attributed to the utility at the exact point of excavation. Further test holes might be required along the same utility route.

NOTE 2 Verification might not be achievable at all locations within a survey area due to ground conditions or construction methods.

NOTE 3 The thrust block should not be disturbed, e.g. no vacuum or hand excavation behind or around the thrust block.

NOTE 4 Suitable safe systems of work should be planned and applied, including consideration of the installation of shoring equipment or stepped battered excavations when site conditions warrant it.

9.2.4 The location of the utility to be excavated shall be determined by:

  • a) utility detection; and/or
  • b) on-site utility features obtained through site reconnaissance; and/or
  • c) utility records.

NOTE Excavation verification work should be undertaken using utility detection data [9.2.4 a)] undertaken in accordance with this PAS. However, it can be undertaken using data from site reconnaissance [9.2.4 b)] or a desktop utility records search [9.2.4 c)].

9.2.5 The information gathered once the utility has been exposed shall include as a minimum, depth from top of utility to a reference point installed on the surface level.

NOTE 1 Where safe working practices allow, the information gathered should also include:

  • a) nature of utility (i.e. pipe, cable or other);
  • b) configuration of multiple utility layout;
  • c) diameter of utility (external diameter only);
  • d) material type;
  • e) backfill materials used;
  • f) observation of the condition of utility;
  • g) prevailing ground conditions;

Attention is drawn to HSE's Avoiding danger from underground services (H5G47) [3] and in particular to guidance on confined space entry.

NOTE 2 Where information on subsurface ground conditions (i.e. soil/rock type) is requested, it should be in accordance with BS EN ISO 14688-1 and BS EN ISO 14689-1.

10 Location

10.1 General

10.1.1 A location survey shall comprise a topographic survey to:

  • a) locate topographic features to provide new mapping and/or to validate the utility data obtained through the survey type D or C; and/or
  • b) locate the utility data obtained through a survey type B; and/or
  • c) locate the utility data obtained through a survey type A; and/or
  • d) locate GPR and EML transects.

NOTE Location points identified through detection or verification should be given absolute coordinates based on a national grid and datum (e.g. OSGB36 or Irish Grid).

10.1.2 All topographic detail and utility location points identified via detection or verification shall be located by geospatial surveying techniques to an accuracy of ±50 mm horizontally and ±25 mm vertically, using a total station and/or real-time kinematic (RTK) global navigation satellite system (GNSS) equipment.

NOTE 1 The horizontal and vertical accuracies specified are equivalent to survey detail accuracy band F or 1:200 legacy output scale as defined in RICS, Measured surveys of land, buildings and utilities 3rd edition specification and guidance note 2014 [6] 1)

NOTE 2 Examples of topographic detail that might be needed to be located include buildings, kerbs, vegetation, walls, manhole covers, inspection covers, valve covers, cabinets and traffic signals. For further guidance see RICS, Measured surveys of land, buildings and utilities 3rd edition specification and guidance note 2014 [6] 1).

10.1.3 All survey points located shall be referenced to a framework of survey control. The survey control shall be established using GNSS and/or via a total station traverse between survey control points.

10.1.4 Where the framework of survey control uses a local site grid, the local site grid shall be referenced to a national grid and datum (e.g. OSGB36 or Irish Grid).

NOTE The practitioner should consider if the type of location and data capture being used is appropriate to the deliverable data required. For example, GIS systems dictate that certain attributes are captured in order to provide full functionality of metadata.

10.1.5 Where the use of geospatial surveying instruments is restricted due to insufficient satellite signals or obstruction to line of sight, measuring tapes or electronic distance measuring tools shall be used for infill surveys.

NOTE 1 The most accurate practical methods of positioning should always be chosen.

NOTE 2 Infill surveys apply to small areas only, usually ≤ 100 m2.

NOTE 3 For further requirements and guidance on the use of measuring tapes and electronic distance measuring tools, see 10.3.

10.1.6 Three-dimensional coordinates shall be recorded for all identified utility location points. Where no depth or an uncertain depth reading is obtained, a null level shall be recorded.

10.2 Total station and global navigation satellite systems (GNSS)

10.2.1 The total station or GNSS system shall be used in accordance with the manufacturers' instructions, procedures, calibration and any instrument or process limitations.

10.2.2 All survey points shall be either:

  • a) stored digitally within the total station or GNSS system as coded detail points for post-processing; or
  • b) plotted directly onto digital drawings on-site using a precision mobile system for data collection.

10.2.3 The survey detail pole shall be held vertically above the point being surveyed by reference to a plate bubble attached to the pole.

10.2.4 The GNSS system shall be configured to work with either available GNSS correction services or a base and rover system with a post-processed base position.

10.2.5 When using GNSS as a data collection method, this shall be used in accordance with RICS guidance, Guidelines for the use of GNSS in land surveying and mapping [NR2].

10.3 Measuring tapes and electronic distance measuring tools

NOTE 1 The use of measuring tapes and electronic distance measuring tools to accurately depict positions of utilities is not recommended and should only be used for infill areas (see 10.1.5). This is due to the lower accuracy levels and systematic errors of the process versus the accuracy levels that a dedicated survey instrument delivers.

NOTE 2 Relative surveying techniques reference a utility by offset measurements from a permanent physical feature identified on a map (such as a kerb or wall). This technique is susceptible to a number of different types of errors including:

  • poor and inaccurate tape measurement due to factors such as slope, distance, temperature and equipment;
  • movement, realignment or dismantling of the physical feature the utility is referenced from making it inaccurate (and sometimes impossible) to re-establish the location of the utility;
  • scaling and/or transcription errors when plotting and/ or locating the utility from the physical feature it was referenced from.

10.3.1 Measuring tapes and electronic distance measuring tools shall not be used to position a utility for distances ≥ 30 m or ground sloping at a gradient of ≥ 10%. Where using such equipment, minimal gradient measurements or slope measurements shall be adjusted to the horizontal.

NOTE The use of measuring tapes and electronic distance measuring tools do not meet the accuracy requirements of QL-B1 (see Table 1).

10.3.2 Where offset measurements are used, the offset distance shall not exceed 10 m or 1/3 of the length of the baseline whichever is the least.

10.3.3 For utility location points that are > 10 m from the baseline, trilateration shall be from three hard points of detail identified on the topographic base map.

10.3.4 Measurements shall be taken to two decimal places of a metre. Baselines shall be checked and scaled for relative length between two known hard points of detail shown on the topographic base map.

10.4 Survey/mapping accuracies

NOTE GNSS/total station combined with Ordnance Survey base mapping can provide high level relative accuracy but not necessarily absolute accuracy. If Ordnance Survey base mapping is increased in production scale above its base scale (1:1 250 or 1:2 500) the relative accuracy of mapped features might diminish.

10.4.1 Where existing base mapping data are used, checks shall be undertaken on-site to demonstrate the location accuracy of the base mapping provided. These checks shall include as a minimum:

  • a) visual checks that features on-site are represented on the drawings provided (known as currency);
  • b) dimensional checks of site features against those shown at scale on the drawings provided to ensure the correct scale is used when plotting points;
  • с) instrumental checks to demonstrate the accuracy of the survey control.

NOTE 1 Outdated or inaccurate topographic data do not necessarily prevent the practitioner from completing the utility detection/verification and location using either total station or GNSS to obtain an absolute geospatial location, as recorded location data may be incorporated into an updated topographical survey at a later date. Relative positioning should not be undertaken by tape measurements, using inaccurate, incomplete plans containing insufficient important topographical features, as this might lead to irreconcilable and/or intolerable inaccuracies in the deliverables.

NOTE 2 Ordnance Survey base mapping provides topographical representation covering the whole of Great Britain. It is surveyed and reproduced to tolerances appropriate to the scales of 1:1 250 for urban areas 1:2 500 for rural areas and 1:10 000 for mountains and moorlands which might deliver a base framework for some utility surveys and asset records. Ordnance Survey base mapping data does not contain sufficient street furniture or ground levels for a full utility survey. In its base form, it is suitable for the depiction of survey type D utility data. It can be rendered suitable for the depiction of survey type А, В or С utility data if appropriate topographical metadata are attached to the underlying data features.

NOTE 3 The inherent accuracy of Ordnance Survey base mapping should be assessed before use.

NOTE 4 Table 4 shows the expected relative accuracy for topographic features within each scale category of Ordnance Survey data. The values apply up to the stated maximum measured distances quoted in Table 4.

10.4.2 Use of supplied base plan survey data shall be reflected in the title block of the deliverable.

Table 4 – Relative accuracy of map data (informative)
Scale and method of original survey Expected relative accuracy at differing confidence levels m Maximum measured distance m
68% 95% 99%
1:1 250 scale Maps surveyed at 1:1 250 scale using instrumental methods to provide a framework of controlling detail ±0,4 ±0,8 ±1,0 60,0
1:2 500 scale resurvey/ reformed Maps surveyed at 1:2 500 scale using instrumental methods to provide a framework of controlling detail ±0,9 ±1,8 ±2,3 100,0
1:2 500 scale overhaul Maps originally recompiled from pre-1946 County Series mapping ±1,2 ±2,3 ±3,0 200,0
1:10 000 scale Maps surveyed at 1:10 000 scale using instrumental methods to provide a framework of controlling detail ±3,5 ±6,7 ±8,8 500,0
SOURCE: Ordnance Survey, http://www.boundary-problems.co.uk/boundary-problems/osmaps.html.

PAS 128:2014 Specification for underground utility detection, verification and location