nr/l2/trk/3100 - bridgewayhouse.com app 6 - forms... · figure 9 – two-peg test 24 7.4 laser...

Ref: NR/L2/TRK/3100/MOD 3

Issue: 2

Date: 02 September 2017

Compliance date: 02 September 2017

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Copyright 2017 Network Rail.

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Published and Issued by Network Rail, 2nd Floor, One Eversholt Street, London, NW1 2DN.

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NR/L2/TRK/3100

Module 03

Topographic, engineering, land and measured building surveying – Survey and mapping techniques


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User information

This Network Rail standard contains colour-coding according to the following Red–Amber–Green classification.

Red requirements – no variations permitted

• Red requirements are to be complied with and achieved at all times.

• Red requirements are presented in a red box.

• Red requirements are monitored for compliance.

• Non-compliances will be investigated and corrective actions enforced.

Amber requirements – variations permitted subject to approved risk analysis and mitigation

• Amber requirements are to be complied with unless an approved variation is in place.

• Amber requirements are presented with an amber sidebar.

• Amber requirements are monitored for compliance.

• Variations can only be approved through the national variations process.

• Non-approved variations will be investigated and corrective actions enforced.

Green guidance – to be used unless alternative solutions are followed

Guidance should be followed unless an alternative solution produces a better result.

Guidance is presented with a dotted green sidebar.

Guidance is not monitored for compliance.

Alternative solutions should be documented to demonstrate effective control.


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Contents

1 Purpose 8

2 Scope 8

3 Definitions 8

4 Surveying and GRIP 8

Table 1 – Surveying and GRIP 9

5 Introduction 10

6 GNSS surveys 10

6.1 Introduction 10

Figure 1 – Dual frequency GNSS receiver 11

6.2 General 11

Table 2 – PGM hierarchy 12

6.3 Methodology 13

6.4 Computations 14

6.5 Deliverables 15

7 Levelling 15

7.1 Introduction 15

Figure 2 – OSBM 15

7.2 Instrumentation 16

Figure 3 – Examples of Engineer’s levels 16

Figure 4 – Example of digital level and bar-coded staff with conventional “E”staff 17

Figure 5 – Conventional levelling staff and view through the instrument 17

Figure 6 – Variations on cross-hairs as seen through an instrument 18

Table 3 –Accuracy of levelling equipment 18

7.3 Site procedure for levelling 18

Table 4 – Survey book example 20

Table 5 – Calculated reduced levels for the example site 21

Table 6 – Readings taken during a fly back run and the calculations 21

Figure 8 – “Rise and fall” example 22

Figure 9 – Two-peg test 24

7.4 Laser levelling 24

Figure 10 – Leica Rugby 610 and sensor 25

7.5 Comparison between GNSS heighting and levelling 25


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8 Selective and non-selectable surveying techniques 26

9 Aerial techniques 26

9.1 General 26

9.2 Costs 27

9.3 Comparison of aerial techniques 27

Table 8 – Comparison of aerial techniques 27

9.4 Aerial photography 28

Figure 11 – NR Geo-RINM Aerial survey data capture extents 28

9.5 Digital mapping 30

9.6 Details to be surveyed 30

9.7 Field completion and site verification 31

9.8 Aerial LiDAR survey 33

Figure 12 – Typical LiDAR system (TopEye) 33

Figure 13 – Marker boards 34

9.9 Accuracy 34

9.10 Deliverables 34

Figure 15 – Abstract of a Digital surface model (DSM) 36

9.11 Ground truthing sites 36

10 Terrestrial laser scanning 37

10.1 General 37

10.2 Instrumentation 37

Figure 16 – Leica P40Scanner 37

Figure 17 – Faro Focus Scanner 38

Figure 18 – Trimble TX8 Scanner 38

Figure 19 – Riegl VZ400i Scanner 39

Figure 20 – GEB-REVO hand held scanner with added camera 40

Figure 21 – FARO Freestyle3D 40

10.3 Accuracy 40

10.4 Data collection 41

Figure 22 – Laser scan shadow or void 42

10.5 Data derived from terrestrial laser scanning 42

Table 9 – Modelling output types (MOT) 43

10.6 Software 43

Figure 23 – Scanner set-up locations 44


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Figure 24 – Point cloud data as viewed in Leica TruView software 45

10.7 Benefits of terrestrial laser scanning 45

Figure 25 – Image from point-cloud data of Blackfriars Railway bridge 46


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Issue record

Issue Date Comments

1 June 2017 First issue

2 Sept 2017 Second issue updated table 2

Reference documentation

NR/L2/TRK/2102, Design and Construction of Track.

NR/L2/TRK/3100, Topographic, Engineering, Land and Measured building surveying – Strategy and general

NR/L2/OPS/251, Air operations manual .

NR/L1/INI/CP1010, Policy on working safely in the vicinity of buried services.

NR/L2/INI/CP1030, Working safely in the vicinity of buried services

NR/L2/INI/EDT/CP0091 Issue 2, Specification for Computer Aided Design.

NR/L3/MTC/PL0094, Planning and documenting the Safe System Of Work Arrangements

NR/L3/TRK/3101, Topographic, engineering, land and measured building surveying – Track.

Benchmarks to GNSS heighting – Virtually level. RICS (Royal Institution of Chartered Surveyors) Guidance note.

OSGB36. A guide to Co-ordinate systems. Produced by the OS.

BS 1192:2007, Collaborative production of architectural, engineering and construction information– Code of practice

PAS 1192-2:2013, Specification for information management for the capital/delivery phase of construction projects using building information modelling. It specifies requirements for achieving building information modelling (BIM) Level 2

PAS 1192-3:2014, Specification for information management for the operational phase of assets using building information modelling. It specifies the requirements for information management to achieve building information modelling (BIM) Level 2 in relation to the operation and maintenance of assets (buildings and infrastructure).

BS 1192-4:2014, Collaborative production of information. Fulfilling employer’s information exchange requirements using COBie. Code of practice. This British Standard defines a methodology for the transfer between parties of structured information relating to Facilities, including buildings and infrastructure. It defines expectations for the design and construction project phases prior to handover and acquisition and the subsequent in-use phase.

PAS 1192-5:2015, Specification for security-minded building information modelling, digital built environments and smart asset management

Survey4BIM, Digital plan of works (DPOW)

BIM Forum – Level of Development Specification, version October 2016


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Surveying for Engineers. Uren, J. and Price, W.F. 5th edition 2010. ISBN 9780230221574

GPS Guidelines for the use of GPS in Surveying and mapping. Published by RICS Books 2010.

RICS Guidance Note, Network RTK Best Practice Guide and Guidelines for the use of Network RTK GPS in Land surveys. Newcastle University in association with the Survey Association, Ordnance Survey, Leica Geosystems, Trimble and RICS.

Virtually Real: Terrestrial laser scanning: RICS Geomatics client guide series

ASPRS (American Society for Photogrammetry and Remote Sensing) LAS Specification v1.2


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1 Purpose

This module has been written to enable a general understanding of the techniques for Topographic surveys minimising the risks of poorly executed Topographic survey works being undertaken.

2 Scope

This module provides different approaches to evaluate on surveying activity for which a co-ordinated engineering, land, topographic or measured building survey is required.

It ensures that project managers, project engineers and other project team members have an appreciation of a range of techniques that may be used. However, this is no substitute for appointing a competent Client survey manager for all projects.

An overview and recommendations on the GRIP stages that the various techniques are best suited for is provided.

It is intended to be used in conjunction with NR/L2/TRK/3100, Topographic, engineering, land and measured building surveying – Strategy and general. Other modules specify information to enable a specification for survey data collection to be developed for the disciplines of Track (Module 1), OLE (NR.L3/TRK/3105 to become Module 5) and Asset data extraction and Topographic surveying - Signalling.(NR.L3/TRK/3104 to become Module 4).

It specifically excludes ground investigation, geotechnical, building condition, dilapidation surveys and surveys associated with Network Rail owned and operated measurement trains.

The concept of a survey risk register is covered in NR/L2/TRK/3100.

3 Definitions

For the purposes of this standard, the terms and definitions in NR/L2/TRK/3100 apply.

4 Surveying and GRIP

Table 1 gives guidance on the types of survey techniques and where these techniques are best suited for use within the GRIP process. Techniques may be used in a GRIP stage other than that shown in Table 1 when this meets the project needs in a more appropriate manner, with acceptance of the CSM. Topographic surveys are often required at the earliest stages of GRIP from GRIP 2 onwards.


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Table1 – Surveying and GRIP

All projects will benefit by utilising Building information modelling (BIM) methods which can be related to the GRIP stages. Details of BIM can be found in PAS 1192-2:2013 and subsequent documents given in the references clause.

The needs at the earlier stages of the GRIP process (2 and 3) and the accuracies required are better suited to remote sensing techniques such as aerial photography and LiDAR. Other aerial mounted sensors may also be used to provide data such as oblique imagery, infra-red imagery (vegetation management, heat loss uses) and hyperspectral data (soil moisture analysis, soil mineral content, vegetation type, archaeology).

Survey

GRIP Stage

1 2 3 4 5 6 7 8

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Survey project strategy*

Aerial photography

Aerial LiDAR

Terrestrial Laser Scanning or LiDAR

Train borne systems with positional accuracy better than +/-12mm

Ground land and topographic survey

Geographic Information Systems


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At GRIP stage 6, during implementation and commissioning, setting out using ground survey techniques is appropriate.

At scheme handback and project closeout, data may be required for the Building Information Modelling (BIM) process allowing data for information systems such as GIS and asset information systems.

5 Introduction

A phase often used in surveying is to work from the “whole to the part”. In practice this means that surveys are often conducted in two parts: PGM survey and detail survey.

The PGM survey is a framework of ground markers that are surveyed to high accuracy to find their location on the survey grid.

During the detail survey, instruments are established over the ground markers and the features that we want to record are measured. The detail survey may be carried out to a lower accuracy than the PGM survey and adjusted to fit the survey PGM framework. The detail survey also has a role in detecting gross errors in the survey PGMs.

This same idea of working from the “whole to the part” applies with surveying using other methods such as the train mounted systems like RILA, TAS and Omnicom systems. In these cases, the scan data or the video is the detail survey and the survey framework information comes from the GNSS and inertial measurements that are used to determine the location of the train.

NOTE 1: As the survey PGM network is often not required by the engineer for their work, its purpose is often not understood. It should be considered to be like the foundations of a bridge: if it is ignored, problems will follow.

Provision of a witness diagram (an example is given in NR/L2/TRL/3100 Appendix C) should mean that the PGM can be found again and used for further work without needing to re-observe the whole framework.

NOTE 2: A witness diagram App (Survey Witness) is available for internal NR use on an iPAD and a similar App is also available commercially via the Apple Store (PGM manager).

For example, additions might be required to an original survey, or a design based on this survey might need to be marked out on the ground for construction. However, extending a survey PGM network because a new adjacent area is now required might cause errors when a small site is extended to cover a much larger area.

When a small site is extended to cover a wider area, a new network shall be re-computed and the original survey adjusted to fit the new framework.

6 GNSS surveys

6.1 Introduction

This explains GNSS uses for high precision survey PGM networks and use for survey detail capture on site.

NOTE 1: Other types of GNSS techniques may be used for the collection of Asset Information for input into a GIS.


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NOTE 2: Several different methodologies are in regular use but only the most used Static and Real Time Kinematic concepts are explained.

For PGM surveys, ETRS89 co-ordinates shall be required. These are then capable of being converted into the project’s chosen survey grid.

A typical dual frequency GNSS receiver, capable of receiving signals from GPS, Glonass or Galileo is shown as Figure 1.

Figure1 – Dual frequency GNSS receiver

6.2 General

GNSS survey “Good practice” shall be adhered to as detailed in the following documents (or their future updated versions):

a) GPS Guidelines for the use of GPS in Surveying and mapping. Published by RICS Books 2010.

b) RICS Guidance Note, Network RTK Best Practice Guide and Guidelines for the use of Network RTK GPS in Land surveys.

c) Guidelines for the use of Network RTK GPS in Land surveys, Newcastle University in association with the Survey Association (TSA), Ordnance Survey, Leica Geosystems, Trimble and RICS.

d) Network RTK Best Practice Guide.

NOTE 1: The last two documents available for download from: www.tsa.org.uk.

Surveying using GNSS methods are best visualised by the measurement of baselines of several kilometres in length to make up a survey framework.

NOTE 2: The accuracy of these baselines is given below for “Static” observation techniques. GPS Guidelines for the use of GPS in Surveying and mapping gives more information on the expected accuracies.

http://www.tsa.org.uk/


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Table 2 shows the hierarchy of PGMs from Primary to Tertiary and information about their layout and expected accuracies.

PGM hierarchy type

Longitudinal spacing Plan accuracy Maximum error

over 200 m

Primary (First order)

5 to 10 km

Pair >400 m

1 in 100,000 (10 mm per km)

2 mm

Secondary (Second order)

1 to 2 km

Pair >400 m

1 in 75,000 (13.3 mm per km).

3 mm

Tertiary (Third order)

Not usually located using GNSS techniques

200 m 1/50 000

(20 mm per km).

3 mm

Table 2 – PGM hierarchy

NOTE 3: Tertiary (Third order) not usually located using GNSS techniques.

Some additional general points are highlighted as follows.

a) Dual frequency GNSS receivers with choke ring antenna, for reduction of mulltipath, capable of receiving signals from GPS, Glonass, Beidou or Galileo satellites, should be used.

b) A reconnaissance and desktop GNSS planning campaign should be undertaken.

c) Static or rapid static GNSS survey techniques shall be utilised at Primary PGMs (First order). Taking note of the PGM hierarchy, these PGMs will form a properly surveyed network, working from “the whole to the part”, i.e. starting from the Primary PGMs and creating Secondary PGMs (Second order) between them, keeping the primary as fixed in the calculations. Tertiary PGMs (Third order) are created between the Second order, keeping both the First order and Second order fixed in the calculations. Terrestrial observations should be used as a check of the fitness of GNSS positions e.g. a distance and height check between the two points of the pair that are intervisible.

d) Once the 10 km primary network has been established, in filling of the secondary network every 2 km should take place, with four pairs of Second order PGMs between the 10 km pairs.

e) When new PGMs are selected they shall be selected in a GNSS friendly location, where there exists a clear view of the sky to minimise cycle slips, multipath and other interference.

f) The most frequent source of error when observing GNSS baselines are incorrect measurement of the instrument height. Consequently, the survey contractor shall use at least two different tripod set-ups at each PGM. Further checks of the relative levels of GNSS heighted PGMs shall be incorporated in the surveying system. A suitable approach to be adopted is given in Clause 6.7.


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g) The PGMs shall also be linked to the OSNet CORS..

NOTE 4: Heights derived from GNSS surveys are two to three times less accurate than that of plan co-ordinates. So, although the absolute accuracy of GNSS heighting is high – each point may have a standard error of +/- 20 mm – the relative accuracy between nearby points is low.

NOTE 5: Heights for engineering purposes are required to be orthometric heights – vertical distance above defined “level” surface (the geoid).

Ellipsoidal heights derived from GNSS survey shall be converted to orthometric heights by the use of a geoid model that consists of differences (separation) between the geoid and the ellipsoid defined at latitude and longitude grid. In the UK the height datum usually used is defined as Ordnance Datum Newlyn (ODN) based on the mean sea level at Newlyn. The OSGM15 geoid model developed by Ordnance Survey shall be used for GNSS heights calculations.

NOTE 6: This calculation may be undertaken using GridInQuest software.

To ensure consistency between adjacent PGMs created using GNSS, spirit levelling between adjacent PGMs shall be undertaken to obtain level values that may be utilised for engineering construction.

All PGM surveys in GNSS shall be recorded as raw data and post-processed to include CORS reference stations and precise ephemeris downloads.

When using Network RTK GNSS surveying (up to 2m pole height) for detail surveying of Band 2B or lower accuracy features, a check observation to PGMs established by higher accuracy GNSS techniques, shall be included.

NOTE 7: For accuracy bands refer to NR/L2/TRK/3100 Table 4.

6.3 Methodology

Whenever possible, before commencing the observations, the contractor’s lead field surveyor for the project should carry out a reconnaissance, accompanied by the Client’s Survey Manager (CSM).

For every set up, a booking sheet or a field book, to record antenna heights at the start and at the end of observations should be used.

The PGM shall be connected to the CORS to achieve accurate ETRS89 coordination and to provide an independent check on the PGMs.

A minimum of five CORS locations should be included in the observation campaign and calculations.

The length of baselines to the CORS shall be considered in the pre-planning of the works to determine the time duration of the baselines observations.

To observe a PGM framework, three or more GNSS receivers should be used simultaneously, occupying at least three PGMs at any one time, using baselines of between 20 min and one hour depending on satellite coverage and baseline distance.

By holding one station fixed and moving the two other GNSS units on to the next two stations, a traverse run shall be observed with GNSS static observations, with each PGM fixed relative to at least two others in the network. The primary PGMs shall be

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occupied at least twice, once tying in the primary points only, the second run when including measurements to the Secondary PGMs.

The survey contractor should provide a detailed methodology that will detail the manner of moving receivers around the network to achieve accurate results – synchronous shifting, leap-frogging, etc., for acceptance by the CSM prior to commencement of the site work.

6.4 Computations

All GNSS survey data shall be downloaded to office computers.

Survey data processing shall be carried out using GNSS processing software.

Prior to processing the PGM network, antenna heights should be checked twice, once on import and once during set up of processing parameters. As much data as possible collected for each PGM should be used in the processing to maximize the reliability. Where bad data, for whatever reason, was encountered, it should be removed from the adjustment.. This is achieved by editing the receiver timelines.

All processing shall be carried out in the ETRS89 co-ordinates. Precise ephemeris where available shall be used for GNSS processing.

Baselines should be processed and checked,.

NOTE 1: The tropospheric and jonospheric effect on measurements are factors that are the most difficult to eliminate. These are difficult to model and are source of random errors.

Person undertaking the calculations shall have appropriate knowledge and experience when calculating baselines to use an appropriate modelling solution for these factors to minimise effect on accuracy of results.

An unconstrained computing solution of shall be performed before the final adjustment is calculated. Such a solution holds a single known CORS location fixed in the calculation allowing a comparison with the published coordinates for other CORS locations.

NOTE 2: This gives a measure of the accuracy and quality of the baselines.

Loop closures should be computed and a network adjustment (using “least squares methods”) shall be performed.

The maximum error ellipse at 95% shall not exceed +/-15 mm for the primary PGMs.

NOTE 3: Most non-scientific GNSS processing software are known to give ‘over optimistic’ quality results. This should be considered when judging the accuracy of the final resulting coordinates.

Co-ordinates shall be output in the ETRS89 co-ordinates along with their precision values.

These ETRS89 values in Latitude and Longitude may now be converted into the project survey grid.

NOTE 4: Further discussion on the intricacies of GNSS computation are beyond the scope of this document and reference should be made to GPS Guidelines for the use of GPS in surveying and mapping.

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6.5 Deliverables

The survey report shall be similar to that within NR/L2/TRK/3100 Appendix E1 but excluding that for “Detail survey” and the Appendix covering Traverse PGM Observations. However, the following additions shall be included:

a) quality reports giving how many times the network has been adjusted to derive the final results;

b) a GNSS network adjustment that provides the error ellipses for each point, and the residuals of each baseline;

7 Levelling

7.1 Introduction

NOTE 1: The vertical is the direction which a plumb bob takes when it hangs freely, under the effect of gravity A horizontal surface is a surface at right angles to it...

NOTE 2: The height of a point is defined as the distance up or down the vertical through a point associated with a reference horizontal surface or datum.

The datum used is related to a point known as a “Bench mark” (BM). This may have a simple local value or be related to the Ordnance Survey Newlyn datum (OSBM) from which all heights in the UK were defined. Figure 2 shows a typical OSBM.

Figure2 – OSBM

The network of OSBM have now been superseded by the use of GNSS levels.

At its simplest, levelling is measuring between the ground and a horizontal plane established by a spirit level or more usually by a survey instrument known as a level. In addition to the level, a tripod to place the level on and a measuring staff is needed.


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7.2 Instrumentation

Levels fall into several categories.

a) the Tilting or “Dumpy” level where before each reading a spirit level bubble has to be adjusted (now generally superseded);

b) an automatic level (like an Optic in use for track levelling) that has a suspended compensator to define the horizontal plain. The tolerances of the compensator should be satisfied when the circular bubble on the automatic level is centralised. The compensator is a free hanging prism within the telescope, of both the automatic and digital levels which creates the necessary refraction so that the line of sight of is horizontal. When used near heavy machinery, the compensator can vibrate making readings impossible. Most instruments available today fall into the latter category and are classed as “Engineer’s level”. There is also a variation on the Engineer’s level called a “Digital level”. These instruments may be further split up depending on the accuracy needs. Engineers’ levels are usually read to 1mm with an accuracy of 1 to 2 mm. A more accurate and precise level is also available that may be read to an accuracy of 0.1 mm and estimated to 0.001 mm;

c) laser levels where a plain is defined by a rotating laser and a sensor is used on a measuring staff to provide a reading.

Conventional Engineer’s levels are used to sight directly upon an E-type levelling staff and the readings are read off (as shown in Figure 5) by the observer which allows direct measurement to the order of 10 mm with estimation to the order of 1 mm. Examples of the different types of levels are shown in Figure 3.

.

Figure 3 – Examples of Engineer’s levels

A digital level reads a bar-coded levelling staff (as shown in Figure 4). It is sighted onto the staff and focussed then, with the press of a button, it observes the bar-code in the field of view and calculates the observed measurement. This is stored in the on-the instrument’s memory for download at a later time. If a value for the start BM is


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known, a level may be computed on the instrument. This has the huge advantage of removing booking error with the disadvantages of the extra costs associated with buying a digital level and the reliance on batteries. Some digital levels do not function correctly in low light levels or in darkness and a backlit staff is required.

Figure 4 gives an example of a digital levels and the associated bar-coded levelling staff alongside a conventional “E” reading staff, that the operator has to read and also estimate to get the mm value.

Figure 5 shows the view through the level telescope of a conventional levelling staff and the reading estimated to 1mm.

Figure 6 shows a range of different cross-hair arrangements for different level instruments.

Figure4 – Example of digital level and bar-coded staff with conventional “E”staff

Figure 5 – Conventional levelling staff and view through the instrument


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Figure 6 – Variations on cross-hairs as seen through an instrument

In Figure 6, the two small “cross-hairs” are known as “Stadia hairs”, If readings of the staff for each are taken, the lower subtracted from the higher and multiplied by 100, the distance to the staff will be known to within 100 mm. It is a good way of estimating distances.

Table 3 gives an indication of the accuracy that may be achieved using various configurations of equipment.

Equipment Maximum sight distance to staff

Maximum misclosure

Geodetic level and invar staves (stabilised)

50 m 2 mm √k

Digital level with barcode staves

60 m 5 mm √k

Automatic optical level with E-pattern staves

60 m 15 mm √k

Key

k is the distance levelled in kilometres.

Table 3 –Accuracy of levelling equipment

7.3 Site procedure for levelling

Levelling shall always use a closed loop, starting and finishing on a benchmark or known PGM. Any misclosure gives an indication of the quality of the levelling.

Benchmarks and PGMs shall be at a stable location where the level will not change. Where permanent benchmarks are not present a bolt on the base of a stanchion or signal is particularly suitable for a temporary benchmark (TBM) but a cable route troughing lid or catch pit corner are not suitable.

The levelling instrument should be set up between two benchmarks in a “place of safety” that has clear sighting to both benchmarks. The maximum distance between benchmarks should be 100 to 120 m meaning that sighting with the levelling instrument's telescope would be around 50 to 60 m. Backsights and Foresights should be equal. A levelling run between two benchmarks should always be done twice (double levelling) so that no gross errors in staff readings occur. In practice on


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site it is often possible to observe both forward (foresight – FS) and back (backsight – BS) readings twice by resetting the instrument after the first set of readings saving time.

NOTE: For further information on site operation for levelling, reference should be made to a good surveying textbook such as Surveying for Engineers by J.Uren and W.F.Price.

7.3.1 Calculating reduced levels

There are two well-known methods for calculating reduced levels from site measurements. The “rise and fall” method and the “height of collimation” (HofC) method.

Use the “rise and fall” method of computation when control levelling, as this shows arithmetic errors. The “height of collimation” method should only be used when taking many intermediate observations and particularly on building sites for setting out.

The plane is called the height of collimation (HofC) and is defined by the height at which the telescope of the levelling instrument is set up.

The rise and fall method has the advantage of being able to compute the differences in height as you work and allows direct comparison with the second set of readings when double levelling.

Figure 7 shows an example of a site with three TBMs and two locations where an automatic level was set up. Table 4 shows what was written in the survey book.

Figure 7 – Site Example with three TBMs and two level set up locations

BS IS FS HofC RL Remarks

2.900 TBM1, Bolt at base of signal SN71

2.401 A

1.980 B

0.747 1.545 TBM2, Corner of LOC cabinet base


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1.228 C

2.156 D

1.896 TBM3, Corner of access steps

Table 4 – Survey book example

7.3.2 H of C method

The height of collimation for each set up is the horizontal plane that is viewed through the telescope. This is shown in Figure 7 by the dotted line passing horizontally through the levelling instrument’s telescope.

As in this example, we don’t know the height of any of the TBMs and we are only interested in the relative heights throughout the site, we can assign an arbitrary height of 10.000 metres to TBM1. As the staff reading between TBM1 and the height of collimation is our backsight, adding the backsight value to TBM1’s reduced level value will give us the height of collimation for this initial set up.

TBM Reduced Level + Backsight to TBM = Height of Collimation equation [1]

In this case, we obtain a value for height of collimation of 12.900 m. This height of collimation will remain the same for this set up. For each of the intermediate sights, we have measured the staff reading between the ground and the height of collimation. Therefore, to calculate the reduced level of the ground, we subtract the intermediate sight’s value from the height of collimation.

Height of Collimation – Intermediate Sight = Reduced Level equation [2]

So, for point A of this survey, we should subtract 2.401 m from 12.900 m, giving a reduced level for point A of 10.499 m. We can apply the same equation to calculate the reduced level at point B. Table 5 gives an example fieldbook for the Height of collimation method of computation.

We have taken a foresight at TBM2 from which we can calculate the reduced level at this location in much the same way as we would treat an intermediate sight.

Height of Collimation – Foresight to TBM = TBM Reduced Level equation [3]

In setting up and levelling the levelling apparatus at another location, a new height of collimation will have been set and will need calculating. As we know the reduced level of TBM2 (from using equation [3]) and we have taken a backsight to TBM2, we can use equation [1] to calculate the new height of collimation. This new value is then used to calculate reduced levels for the intermediate sights and for the next TBM.


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After performing these calculations, our survey book appears as shown in Table 5.

Table 5 shows calculated reduced levels for the example site shown in Figure 7.

BS IS FS HofC RL Remarks

2.900 12.900 10.000 TBM1, bolt at base of signal SN71

2.401 10.499 A

1.980 10.920 B

0.747 1.545 12.102 11.355 TBM2, corner of LOC cabinet base

1.228 10.874 C

2.156 9.946 D

1.896 10.206 TBM3, corner of access steps

3.647 3.441 0.206 0.206 Check calculations

Table 5 – Height of collimation computation of levels for the example site

Checks on the calculations shall be done by subtracting the sum of the Foresights (FS) from the sum of the Backsights (BS) and comparing this value with difference in Reduced level (RL) between start RL and final RL as shown in Table 5 in italics.

It is not possible to work out the misclosure error in this case as the true heights of the TBMs are not known. However, by repeating the observations (flybacks or double levelling) a misclosure may be obtained. In a situation where the benchmarks had known heights the misclosure could be calculated from comparing actual and calculate reduced levels.

The backsight and foresight values read on the flybacks and calculated heights of collimation and reduced levels are shown in Table 6.

BS FS HofC RL Remarks

1.865 12.071 10.206 TBM3

1.627 0.716 12.982 11.355 TBM2

2.984 9.998 TBM1

Table 6 – Readings taken during a fly back run and the calculations

There is a misclosure of –2mm as the calculation of the reduced level of TBM1 for the flyback is 2 mm less the originally assigned reduced level. To ascertain if this is an acceptable Misclosure, equation [4] should be used.

Comparison of the reduced levels of the TBMs may be obtained and normal practice is compute mean results.

Acceptable misclosure n3 mm equation [4]

where

n is the number of set ups.

As this site involved two set ups, the allowable misclosure is ±4mm so the survey is acceptable. Due to the misclosure, an adjustment should be made to the reduced


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levels originally calculated. This is applied by making an equal and cumulative adjustment to each set up. In this case a –2 mm misclosure of two set ups can be rectified by adding 1 mm to each set up as follows.

Table 7 shows the adjustments necessary to correct for the misclosure and the adjusted reduced levels for this example site.

Table 7 – Adjustments to correct for misclosure and final adjusted reduced levels

7.3.3 “Rise and fall” method

Figure 8 – “Rise and fall” example

In Figure 8, the intermediate sighting is subtracted from the backsight and the resultant rise is written in the rise column on the same line as the observation.

2.856 – 1.432 = 1.424

In the next line, the intermediate sight is subtracted from the foresight to arrive at a fall of 2.111. It is written in the fall column on the same line as the observation.

To derive the reduced level, the TBM value of 35.688 has first the rise of 1.424 added to it to arrive at 37.112 and then the fall of 2.111 taken from that value to get 35.001.

The advantage of the “rise and fall” method is that checks are easily obtained of the calculation by summing all the rises and all the falls and taking one from the other. This value should then equal the final reduced level taken away from the start reduced level. This is also shown in Table 5.

Initial RL Adjustment Adj RL Remarks

10.000 10.000 TBM1

10.499 +0.001 10.500 A

10.920 +0.001 10.921 B

11.355 +0.001 11.356 TBM2

10.874 +0.002 10.876 C

9.946 +0.002 9.948 D

10.206 +0.002 10.208 TBM3


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7.3.4 Errors with levelling and ways to avoid them

The collimation error of the instrument shall be checked for by using the two-peg test (see 6.6.8).

Parallax of the instrument should be checked for and eliminated whenever a staff reading is taken. This is achieved by unfocusing the instrument telescope and focusing the eyepiece so that cross-hairs are sharply focused before refocusing the main telescope.

NOTE: The staff itself can introduce errors if the writing on it is worn, or the sections in a multi-section staff do not fit together well, or the foot of the staff is damaged.

It is very important that the operator on the staff shall check that when putting up an extra section of the staff that the staff goes up fully and locks into position.

If the staff is not vertical, the observer shall not be able to take an accurate reading from it. In this case, rocking the staff to and fro or using a specially designed spirit level bubble is necessary.

The level of some surfaces will change depending upon weather conditions. For example, a tarmac surface will expand in heat and muddy ground, when wet, can encourage the tripod feet to sink into it. This effect can be minimised by treading the tripod legs well into the surface and leaving a minimum of time between setting up the apparatus and taking readings.

Errors in recording readings can be avoided by sticking to a rigorous double checking procedure. Using a digital level will remove this common source of error.

Although errors in TBMs or change points (CP) are given by the misclosure of the levelling, intermediate sights are not checked. If they are observed using a digital level, the chances of an error are small.

7.3.5 Collimation error and the two-peg test

Under use and transportation, the parts of a level are usually disturbed. For instance, the compensator or the cross-hairs moving out of alignment might lead to the line of collimation not being truly horizontal. Many levels have a collimation error associated with them, which shall be tested so that it is within an acceptable tolerance.

The collimation error is found by carrying out a two-peg test. The two-peg test shall be done at least weekly with the results recorded, or more frequently when the accuracy of the results is imperative.

A two-peg test should be carried out on fairly level ground.

First, two points, A and B, should be defined a known distance, L (metres) apart. L should be at least 40 m. These points can be defined either by using pegs in soft ground or paint on hard ground. Another point, C, is defined exactly halfway between these points and the levelling instrument is set up at this point. Pacing out these distances is accurate enough.

The levelling staff is placed at each location and the readings are recorded. The reading at B is S1 and the reading at A is S2.

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The level is set up at a new point, D, that is L/10 (metres) away from point B, as shown in Figure 9. The levelling staff is placed at points A and B and the readings recorded. This time, the reading at B is S3 and the reading at A is S4.

Figure 9 – Two-peg test

Using the values for S1, S2, S3 and S4, the collimation error, e, can be calculated using equation [5]

Collimation error e = (S1 – S2) – (S3 – S4) per L metres equation [5]

If the collimation error is found to be less than ±1 mm per 20 m, the instrument is accepted as being in tolerance. If the collimation error is greater than this, then the instrument should be calibrated by a trained technician under laboratory conditions.

7.4 Laser levelling

Laser levelling instruments are also known as rotating lasers. They generate a plane of laser light and may be supported on various mountings from tripods to column and wall brackets.

Usually they are Laser classes 1 or 2 and present only a minimal hazard to the user.


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Figure10 – Leica Rugby 610 and sensor

An example of such an instrument is shown in Figure 10. It is self-levelling and has

an accuracy of 1.5 mm at 30m. It utilises a red visible beam that is picked up by a photoelectric sensor that may be mounted on a staff to provide level details as with conventional levelling.

The advantage of these rotating lasers are that they are a one man-operation system and more than one sensor (several operators) can be used at the same time enabling many intermediate measurements to be undertaken quickly within a small area. However, the drawback is that the range providing good accurate of results is limited to 40 m.

Such systems are often used for setting out (marking up a design on site for construction) and some other types of system may be set to provide a certain grade and are often used for controlling earthworks and ballast construction using automatic sensors linked to the blades on bulldozers.

NOTE: These concepts have been utilised on track renewal sites for many years. Manufacturer’s literature and websites can provide further details.

7.5 Comparison between GNSS heighting and levelling

The results of GNSS have high absolute accuracy in relation to the height datum, but low relative accuracy between points that are close together.

Levelling has a high relative accuracy, particularly where points are connected directly.

To solve this problem, two or three points shall be established on the site using GNSS and then level between them also creating other PGMs on the site. The levelling shall be adjusted to be internally consistent and then all the levelled points can be moved up or down to make a best mean fit on the GNSS-derived levels.

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8 Selective and non-selectable surveying techniques

NOTE 1: Total station and GNSS detailing are classed as selective techniques as the surveyor makes a decision about what is being collected for the site as they move over the site. Consequently, there is always a compromise to be made between gathering all the data needed, time and cost.

Selective and non-selectable surveying methods shall require a clear specification and scope of work.

Total station surveying is not described in detail here since these methods are very well covered in many surveying textbooks and in NR/L2/TRK/3100 Module 1. However, non-selective techniques like, LIDAR, aerial photography other less accurate methods, and terrestrial laser scanning are developing very rapidly into the technique of choice in the railway environment to gather high quality data, where repeated access to site is difficult due to safety constraints. These techniques measure everything within the area defined without the surveyor visiting each point and the data is extracted as needed from the “point cloud” using software in the safety of the office.

NOTE 2: A development of the total station now allows a laser scan of a small defined area to be undertaken with the same instrument rather than needing a unique terrestrial laser scanner.

9 Aerial techniques

9.1 General

Aerial techniques are all known as remote sensing and do not require direct access to the track side.

They fall into two categories:

a) Aerial photography may use visible imagery or remote sensed imagery to record a different part of the spectrum such as infrared, ultraviolet or hyperspectral to detect various ground or vegetation characteristics. The infrared spectrum is used by Network Rail to detect hot spots on OLE and third rail by using video from the Network Rail helicopter to detect the failure of insulators. Other uses include the checking of point heater operation during the winter season. For a topographic survey, it is more usual to utilise the visible spectrum and we will concentrate on this here.

b) LiDAR – Light detection and ranging. This is an alternative methodology to mapping from imagery in that this is the systematic measurement of three-dimensional co-ordinates point data producing a point cloud. It consists of a laser rangefinder, GNSS and inertial navigation system mounted on a fixed wing aircraft or helicopter. A laser pulse from the source is bounced off the surface and the time taken to travel to the ground and back is measured very precisely and converted into height data knowing the velocity of the pulses. More on LiDAR can be found later in the document.

NOTE 1: The NR GeoRINM team has collected aerial imagery and LiDAR data covering the whole NR owned and operated railway corridor that is available to all working on NR projects.

NOTE 2: The following data sets were collected as part of this project: LiDAR Data, Orthophoto true to scale Imagery, Oblique Imagery, Ground control points, ground truthing sites (for validation


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of LiDAR) and derived data from the LiDAR as DSM/DTM and feature identified mapping from imagery and LiDAR.

9.2 Costs

The cost “per line kilometre” varies depending on location, corridor width, specification and length of corridor. Mobilisation costs for both aerial survey and ground survey are “diluted” by longer route corridors, i.e. “economy of scale”.

9.3 Comparison of aerial techniques

A comparison of aerial techniques is given in Table 8.

Technique Strengths and applications Weaknesses

Fixed-wing Unmanned Aerial System (drone) aerial photography.

Rotary-wing UAS aerial photography (more manoeuvrable than fixed wing)

Accuracy (horizontal and vertical – +/-20 mm). Improved to Band 1A or better depending on low flying height and sensor quality.

Established technique engineering works.

Ideal for rapid collection of infrastructure corridors

Asset mapping.

Stereo photography enables additional datasets to be extracted to support future applications.

Low cost but significant operational constraints depending on weight of UAS.

500m range from operator and 18 to 150m flying height, line of sight required from operator, CAA approval of flight plan. Not within 50m of “building not under operator control.”

Requires GCP approx. every 20 to 50m.

Limited interpretation of features obscured by vegetation.

Rotary-wing aerial photography

High accuracy (vertical – +/-10 mm).

Established technique for high accuracy engineering works with flying heights of around 100m..

Asset mapping.

Stereo photography enables additional datasets to be extracted to support future applications.

Higher cost

Requires ground control approx. every 50 m.

Limited interpretation of features obscured by vegetation.

Fixed-wing aerial photography

Low cost

Ideal for rapid collection of infrastructure corridors

Wide area captured >500 m band.

Stereo viewing of images for mapping and interpretation.

Weather dependant.

Low accuracy (50 mm – 300 mm).

No visibility through dense vegetation

‘Leaf-off’.

Rotary-wing LiDAR (e.g. TopEye)

UAS LiDAR (weight >7kg)

Ideal for geotechnical, asset

Well-established, proven technique for corridor.

Mapping ,e.g. rail, highways, power lines, rivers.

LiDAR laser penetrates vegetation providing a full ground surface and heights of other features.

Interpretation of the point cloud requires specialist skills.

Narrow band width <100 m. on single pass.

Multiple return signals allow creation of digital surface model (DSM) (ground) and digital elevation model (DEM) (highest features e.g. vegetation and buildings).

Table 8 – Comparison of aerial techniques


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NOTE 1: The terms UAS has been used here but other terms such as “remotely piloted aerial system (RPAS), “unattended aerial vehicle (UAV) and “drone” may also be used.

NOTE 2: The arrangements for use of UAS are covered in the Air operations manual NR/L2/OPS/251.

NOTE 3: Categories of UAS are based on weight. The operational constraints differ for UAS weighing less than 7Kg, 7Kg to 20Kg and above 20Kg.

9.4 Aerial photography

9.4.1 General

Aerial photography may be collected utilising “rotary wing” or “fixed wing” support systems for the cameras including SUA (Drones).

Rotary wing equipment can fly at lower flying heights (100 m) where this is authorised and thereby produce imagery at 1:600 scale to produce vector mapping to an accuracy of 10 mm in height by direct measurement.

Height measurement on rails is less accurate due the shiny reflective surface.

9.4.2 Process

The locations and the extent of each site to be digitally mapped shall be clearly defined on appropriate existing mapping or CAD/GIS file where new imagery is being procured. An example is given in Figure 11 of the NR Geo-RINM survey data capture area.

Figure 11 – NR Geo-RINM Aerial survey data capture extents


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The flying conditions, exposure and processing shall be selected by the survey contractor so that critical railway detail such as toes of points can be surveyed even where the railway tracks are in shadow.

9.4.3 Survey ground control points (GCP)

GCP shall be provided by GNSS techniques to establish PGMs, in order to provide photocontrol for the mapping, and to connect the site to be digitally mapped to the project survey grid.

However, this should be outside of the railway boundary at easily accessible locations.

The photography may be imported into software to enable the generation of further tie points by aerial triangulation.

All GNSS photocontrol points should be measured where they are located on the photography and a bundle block adjustment undertaken to derive final co-ordinates to set up individual stereo models.

Reconnaissance and design of horizontal and vertical control including connections to project survey grid, identification of locations of permanent ground markers and GCP shall be carried out by the contractor.

The proposed design of the horizontal and vertical control nets and pattern of GCP, methodology, instrumentation, and methods of adjustment shall be submitted to the Client’s Survey Manager for acceptance.

9.4.4 Photograph documentation and annotation

The images should be named in a logical manner with the following metadata information added to the digital files:

a) producer's name;

b) project name or reference;

c) date of photography;

d) nominal scale of photography;

9.4.5 Deliverables

The following shall be delivered:

a) one digital copy of each orthophoto based on Ordnance Survey 500m grid squares for site in ECW and Geo Tiff formats giving;

1) the date and scale of photography,

2) the camera type, focal lengths and serial number,

b) a technical report as specified in NR/L2/TRK/3100, Clause 12 and using report template given in Appendix E3..


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9.5 Digital mapping

9.5.1 General

Sites specified to be digitally mapped as a feature identified mapping, should be surveyed by photogrammetic techniques, based on the controlled aerial photography, to provide large scale site plans at a nominal 1:500 scale (or 1:200 as agreed with the Client’s Survey manager).

Field completion of obscured areas shall only be required by specific request.

The standards of accuracy and content specified below should be achieved where the ground and other features are visible stereoscopically on the aerial photography.

9.5.2 Accuracy of digital mapping

Well-defined points of detail including running rails should be surveyed to better than

50mm root mean square error (Band 3A from NR/L2/TRK/3100 Table 4), on the ground, when compared with co-ordinates determined by precise measurement within test areas of approximately 30m square, no less than every 20Km (90% of a

representative sample of well-defined points should be within 83 mm. 99% of a

representative sample of well-defined points should be within 150 mm.)

Spot heights on hard surfaces including running rails should be correct to better than

30 mm root mean square error (Band 3A from NR/L2/TRK/3100 Table 4), when compared with heights determined by GNSS heighting within test areas of approximately 30m square (5m grid), no less than every 20Km. (90% of a

representative sample of spot heights should be within 50 mm. 99% of a

representative sample of spot heights should be within 90 mm.)

Spot heights in edges of cesses, tops and bottoms of embankments and cuttings

should be surveyed to 50 mm root mean square error on the ground when compared with heights determined by GNSS heighting. (90% of a representative

sample of spot heights should be within 82 mm. 99% of a representative sample of

spot heights should be within 150 mm.)

Alternatives to these accuracy statements should be considered by the Client’s Survey manager.

9.6 Details to be surveyed

Any levels shall be related to the project grid. However, these are usually now derived from LiDAR data.

The following details should be surveyed.

a) Street furniture and service features such as transmission lines, poles, pylons, manhole inspection covers, drains, gullies, signs and lamp posts should be shown and identified where possible.

b) In open country, overhead utilities crossing the railway should be shown.

c) Rivers, streams, canals and ditches, passing under or over the railway should be shown by the water line at the time of photography.


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d) Woodland, isolated trees and large shrubs should be shown by the extent of their canopies.

e) All names of roads, streets and stations should be included.

f) Three-dimensional strings should be recorded at significant points and at intervals not exceeding 10 m along the following features and breaks of slope.

g) The following should be included:

1) running edges of all railway tracks recorded at toes of switches, noses of crossings, knuckles of diamonds, beginning and end of curves, and changes of gradient. In areas of S&C, the interval between points may be decreased so that a true representation of S&C is established.

2) edges of station platforms.

h) Three-dimensional strings should be recorded at significant points and at intervals not exceeding 25 m along the following features and breaks of slope:

1) ballast lines and cess lines;

2) tops and bottoms of embankments, cuttings, retaining walls and other substantial breaks of slope;

3) railway property boundaries as indicated by fences, walls, hedges, etc. Where ground level along the property boundary is obscured on the photography, the boundary feature should be recorded as a 3D string with –999.0 in the level field;

4) drains, water courses, canals and lakes shown by three or four strings at water or bed level and tops of banks,.

5) road edges and back of pavements or highway boundaries of all roads and tracks at changes of gradient, junctions and intersections and at intermediate points not more than 25 m apart;

6) on substantial bridge parapets, above each rail track and at the ends and crown;

7) all railway furniture such as boxes, cable routes/ducting, etc.

i) Additional spot heights should be recorded to define the ground surface between 3D strings, on humps and hollows, in open spaces and courtyards.

j) Where the ground surface is not visible on the photographs in these locations because of vegetation, overhanging structures, vehicles or other obstructions, alternative locations should be heightened nearby.

9.7 Field completion and site verification

Annotations and names shall be added by the contractor.

Verification for completeness of around 10% of the mapping should be carried out by the client using an alternative data source.

The vector mapping should be captured as specified in NR/L2/INI/EDT/CP0091 Issue 2, Specification for Computer Aided Design.


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The DTM should be captured at a density and accuracy suitable for the generation of contours at an interval of 0.50 m if they are needed.


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9.8 Aerial LiDAR survey

9.8.1 General

The data collection for this is similar to that for aerial photography except that the density of points derived by the field work is far greater. Such systems are capable of collecting over 60,000 three-dimensional points per second and are powerful enough to penetrate vegetation for ground surface detail. An intensity value, related to the surface is also recorded from the return signal, assisting object/surface classification at the processing stage of the project.

This technique may also be combined with high resolution imagery.

This data may be collected from “Rotary wing” or “Fixed wing” platforms, including SUA (drones) and the sensor equipment may be easily mounted on these platforms. The Network Rail helicopter may be modified to collect such data.

A typical LiDAR system is shown in Figure 12.

Figure12 – Typical LiDAR system (TopEye)

The LiDAR data or point cloud data and associated imagery may then be used for mapping and digital terrain model production. A DTM is produced by modelling the point cloud classified as ground.

9.8.2 Ground control points (GCP)

The LiDAR data shall be referenced to the ground by GNSS and an inertial navigation system on the sensor platform. The GNSS co-ordinates shall also be used to connect the site to be digitally mapped to the project survey grid.


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On the ground “Marker boards” (as shown in Figure 13) may also be used for GCP and quality assurance when the surveyed positions of the marker boards are compared with the plan, height and orientation from the processed data.

Figure 13 – Marker boards

9.9 Accuracy

Aerial LIDAR surveys should be produced providing detail with an accuracy as given in NR/L2/TRK/3100 Table 4 Band 3A or better.

9.10 Deliverables

9.10.1 Point cloud

It is assumed that a point cloud is the main deliverable from a LiDAR survey at a maximum point density of 40 points per square metre (psq.m). This data should also be rationalised to provide data at say 5 or 10 psq.m to enable easier use in computer software.

Such data may then be interpreted to provide the following. However, LiDAR data is often combined with imagery to provide a mapping deliverable as given in section 9.5.

Co-ordinates and levels should be obtained by direct measurement of each point except as follows:

a) where the ground surface is built over e.g. building with a large area e.g. warehouses ;

b) where the ground surface is obscured by very thick vegetation;

c) where there are expanses of water. In this case, the water level at the time of the survey should be used, unless soundings have been specified.

When height points are to be interpolated from a point cloud, details of the Contractor's proposed method, to demonstrate that the ground surface will be


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defined to the same accuracy as direct measurement, should be provided to the Client’s Survey Manager for acceptance.

9.10.2 Digital Surface and Elevation models

In addition to a point cloud two other types of data are often requested to be derived from the point cloud:

a) A digital terrain model (DTM - ground excluding vegetation and man-made features). The DTM should be captured at a density and accuracy suitable for the generation of contours at an interval of 0.50m if they are needed.

b) A digital surface model (DSM – the first return signal to the sensor including where the ground is exposed, vegetation and manmade features).

Examples of a similar area showing a DTM is shown in figure 14 and a DSM in figure 15. Both figures are shown from the Geo-RINM Viewer (GRV) available through a Connect link.

Figure 14 – Abstract of Digital terrain model (DTM)

The final deliverable to Network Rail should be presented in a format specified in NR/L2/INI/EDT/CP0091 Issue 2, Specification for Computer Aided Design, for a Microstation .dgn, *.las or *.pod files conforming to the ASPRS (American Society for Photogrammetry and Remote Sensing) LAS Specification v1.2 or above .


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Figure 15 – Abstract of a Digital surface model (DSM)

Additional information may be provided in the form of:

a) long- or cross-sections.

b) cross-sections from the DSM in .an AVI movie format;

c) 3D airborne fly-throughs;

d) 3D oblique views of imagery draped over the final DEM in .jpg format.

The Project manager or Designated Project Engineer shall define what additional information is needed.

9.11 Ground truthing sites

To ensure that the LiDAR data is providing height data that meets the specification, separate independent surveys using ground survey techniques shall be undertaken at location along the corridor where there are well defined flat surfaces, e.g. car parks. The techniques used shall be capable of producing results that are more accurate than the expected heighting accuracy of the LiDAR height data.

A grid of spot levels shall be taken every 5 metres and a direct comparison made with the point cloud data. The differences should be better than the accuracies defined in the specification for the work.


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10 .1Terrestrial laser scanning

10.1 General

In simple terms, this is the systematic measurement of three-dimensional co-ordinates of an object surface, at high speed and in near–real time by laser light. The measurement process is similar to the use of a total station in that angles and distances are measured, but very much faster Mainstream surveying grade scanners are able to capture at a rate of around million points per second (from the specifications of the laser scanners illustrated below).

NOTE: It is particularly suitable for areas where there are huge amounts of detail such as buildings, and it is also useful for inaccessible locations such as railways.

The scanning collects a point cloud but the accuracy is usually better than that collected from Aerial techniques.

For accuracy bands see NR/L2/TRK/3100 Table 4. For Survey purpose, techniques and GRIP stages see NR/L2/TRK/3100 Tables 5, 6, 7 and 8.

As the following explains, laser scanning is a complex technique and competent personnel are needed to understand the purpose of the point cloud to be collected by laser scanning.

A Client survey manager shall be appointed for the project, they can review the proposed methodology and ensure that the collected point cloud satisfies the express purpose required.

10.2 Instrumentation

NOTE: Software provides a full set of geo-referencing, surveying, and CAD-integrated engineering tools for creating accurate deliverables and managing large scan data sets.

Figures 16 to 19 below, show examples of Leica P40, Trimble TX8, Faro Focus and Riegl VZ400i scanners. There are also other manufacturers of scanners and this is rapidly evolving technological area of operation.

Figure 16 – Leica P40Scanner


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*

Figure 17 – Faro Focus Scanner

Figure 18 – Trimble TX8 Scanner


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Figure 19 – Riegl VZ400i Scanner

Laser scanners are becoming miniaturised more and more and one recent developments have resulted in a hand held scanners, the GEB-REVO as shown in Figure 20 or FARO Freestyle3D as shown in Figure 21.

Other instrument manufacturers also produce similar equipment.

The GEB-REVO consists of a rotating LiDAR scanner that can be mounted onto a vehicle or drone, or used as a hand held scanner with a pistol grip. The sensor can also be attached to a pole or even a rope (e.g. for shafts) to scan environments which are hard to reach or difficult to access. The survey area is passed through to record more than 40,000 measurement points per second.

For some requirements, no additional survey permanent ground (or wall) markers are required but the addition of some additional co-ordinated points would provide a degree of assurance to the point cloud..

This scanner collects data which is send to an internal SD card for storage or an external storage device in a back pack. The scanner can be combined with a camera (GoPro session) to provide context to any point cloud data and to make feature extraction from the point cloud easier.


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Figure 20 – GEB-REVO hand held scanner with added camera

Figure 21 – FARO Freestyle3D

This type of equipment is particularly suited to the indoor environment of buildings, complicated wiring installations or any constrained site with many obstructions that would mean that tripod mounted scanners would require many set-ups to capture a full point cloud.

Specific software such as GeoSLAM Desktop or the FARO equivalent should be used to process the data.

10.3 Accuracy

Accuracy on its own is a complex subject and further detail on this may be found in NR/L2/TRK/3100 - Strategy and General Clause 9.

Laser scanners capture millions of measurements per second and many factors are important to understand.

10.3.1 Accuracy, precision and resolution of a single measurement

These are provided by manufacturers in the instrument specification).

NOTE: Important factors associated with the kit are:

• Range,

• Accuracy of around +/-2 to 3mm relative to the scanner, for mainstream survey grade scanners.

The GEB-REVO has an accuracy of 10mm (Band 2) and a range of 30m indoors and 15m


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radius externally. The FARO Freestyle3D

has a quoted 3D point accuracy, by the manufacturer of

+/-1mm.

• Systematic error (parts per million),

• Angular accuracy (usually around 10” of arc or more for mainstream survey grade scanners),

• Noise,

• Reflectivity,

• Atmospheric conditions,

• Calibration of the instrument,

The last two result in corrections which if not applied appropriately are the base of systematic errors.

10.3.2 Relative accuracy

This relates to points side by side, within one single scan or within a registered cluster of scans. How accurate is the registration accuracy resulting from a least squares adjustment of the merged scans?

10.3.3 Accuracy compared to the survey PGM network

The Absolute accuracy, how well do the registered scans fit to the any PGM network and survey grid..

In most cases it is the information derived from the resulting point cloud that is being used by laser scanning client (i.e. design and engineering). Therefore, the accuracy of the modelled interpretation of the point cloud needs to be “fit for purpose.”

NOTE: A scanned surface will have millions of points and among them there may be many points with gross errors. However, when a plane or surface is fitted during the modelling process based on ‘best fit’ concept (usually least squares method), then these gross errors will be considered as outliers and ignored giving a better outcome.

10.4 Data collection

It is now possible to register scans together without the use of targeted points previously co-ordinated by a conventional total station. However, this provides a direct link to a predetermined co-ordinate system and may result in a higher accuracy of registration.

NOTE 1: As an alternative, when using certain scanners, they may be set up and orientated over a PGM, where they already exist, as would be done with a total station, and the scans orientated to the grid system in that manner.

NOTE 2: Software can now bring the point clouds together to give a best fit using a “least squares” algorithm. In such a manner, a series of scans may be linked together, eventually closing back (like a traverse) and the results adjusted to give the best fit.

Where this process is undertaken, an indication of the closing error of the traverse or an average standard error of the scan fit shall be provided.

Data voids shall be minimised during the scanning process by the selection of appropriate scanning positions and minimising temporary obstructions to the scanner during operation (vehicles or pedestrians). Figure 22 shows a dark void or shadow in a point cloud.


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Figure 22 – Laser scan shadow or void

A required point density shall be specified at the outset and the minimum size object that shall be recorded in the scan.

Point densities shall be equal in both scanning axes.

NOTE 3: Point densities depend on the range to the object and it is therefore not possible to maintain a constant point density over an entire object during scanning.

The point density specified should be understood as the maximum value for the subject in question. The appropriateness of the point density of a cloud may be obtained by using equation [6].

Q = 1 – (m/λ) equation [6]

where

Q is data quality

m is point density on the object in mm

λ is the minimum feature size in mm.

NOTE 4: For example, a point density of 10mm when smallest feature is 5 mm 5 mm gives:

Q = 1 – (10/5) = –1

This is an unacceptable fit.

However a point density of 2 mm would provide:

Q = 1 – (2/5) = 0.60

Therefore there is a 60% chance object will be detectable.

If a density of 5 mm were used, the Q value will be zero, indicating that the density will not consistently detect the feature over the entire length of the project.

When a single scan is unable to pick up the detail required, a series of scans shall be needed providing at least 30% overlap. The scans shall be linked together by a process known as registration so that these scans are linked correctly in 3D space.

10.5 Data derived from terrestrial laser scanning

The data should be delivered in the dgn format conforming to NR/L2/INI/EDT/CP0091 Issue 2.


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In addition, the items specified in NR/L2/TRK/3100, Clause 12.3 and Appendix F should be provided.

To keep data manipulation costs to a minimum, this data should be defined in the project deliverables in terms of the following modelling output types (MOT) in Table 9.

NOTE 1: The MOT is not the same as “Level of detail.” See NOTE 3 and the BIM Forum – Level of Development Specification, version October 2016.

NOTE 2: The costs increase from MOT 1 to 4, with MOT 4 being probably approximately 20 times as expensive as MOT 1.

MOT Name Description

1

Wire frame

Simply a line that produces a framework, e.g. the outline of the side of a cube. This may be difficult to interpret simply for what is needed, does give a good impression of complexity.

2

Surface to be constructed Joining lines in a series of blocks to define surfaces, e.g. all the surfaces of a cube given and linked to one another. This should allow sections and simple floor plans

3

Surface plus

As with Type 2. but with the addition of main features such as doorways, windows, main structural members. Types 2 and 3 could be combined.

4

Full detail

This should be carefully and fully defined so that enough data is provided but not too much. It should be considered on a site specific basis, almost on a wall by wall or room-by-room basis.

Table 9 – Modelling output types (MOT)

NOTE 3: In order to maintain a manageable file size the BIM model is to include a basic as-built model with outline of major structural elements. Ornate detailing on steel work and masonry shall be simplified to give a basic visualisation of the main elements.

It is assumed that 2D sections and elevations can be extracted as a report from the 3D BIM model and will not be produced until the 3D model is completed.

NOTE 4: The pan industry group Survey4BIM is defining exactly what Level of Detail (LOD) means in relation to BIM and reference to the output from this group in the Survey4BIM, Digital plan of works (DPOW) will assist in further understanding.

10.6 Software

Software plays a critical role in handling the high-definition point clouds and aids in the speedy extraction of engineering information. The data is stored in a database of information. The scan data in its raw form are simply co-ordinates in a text file but millions of points.


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NOTE 1: Software such as Leica TruView, FARO Scene and others such as PointTools and Bentley Descartes enable the point cloud data model to be viewed. Leica TruView is a simple add-on to Microsoft Internet Explorer web browser and can be requested to be loaded on a PC by reference to the NR IT Helpdesk via NR Intranet “Connect”.. Scan data is stored on a server and requires a good broadband link to use properly.

Figure 23 shows all locations where individual point clouds have been created by setting up the scanner for this area of survey (example from Leica Truview). By “clicking” the mouse over an individual location, the image–point cloud at that location is displayed.

Figure 23 – Scanner set-up locations

Using such software direct measurements on the point cloud dataset may be made as well as abstraction of co-ordinates and addition of notes to aid the use of the data by the data modellers.

Figure 24 shows a screen shot view of the point cloud data, using Leica TruView, from an individual scanner location. When linked via broadband to the server the image may be rotated through 360 to see the view in each direction.


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Figure 24 – Point cloud data as viewed in Leica TruView software

NOTE 2: The software utilised with the GEB-REVO scanner is either GeoSLAM Cloud or GeoSLAM Desktop and allows the full registration of all the scans to produce a full 3D point cloud model. With the GeoSLAM Desktop 2017, click on an image of interest from the camera and you are automatically transported to that positon within the scan data. The data can then be exported to software such as the Bentley products for analysis and design.

This is a rapidly evolving area of surveying practice. Proposals by survey contractors for using terrestrial laser scanning shall be accompanied with details of the scanner and software they propose to use.

NOTE 3: A useful document to explain in simple terms about terrestrial laser scanning has been produced in the RICS Geomatics client guide series: Virtually Real: terrestrial laser scanning.

10.7 Benefits of terrestrial laser scanning

10.7.1 Introduction

The benefits and problems of using this technique, to aid an informed decision by the Project Manager or Designated Project Engineer, are described in clauses 9.7.2 to. 9.7.4.

10.7.2 Less time on site needed

Laser scanning collects the site data relatively quickly but it does not preclude the careful planning and execution of the site works. Less time is spent on site compared to a conventional survey but this time saving should be offset against greater time spent in the office, e.g. on a scanning job, 20% is site, 80% is office processing.

Less time on site also equates to less time exposed to a more dangerous working environment with clear safety benefits.

Greater speed of data collection means less possessions and costs to set them up. The additional problems associated with late starts or earlier finishing of possessions is also overcome.

Laser scanning also does not depend on daylight.


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For example, Figure 25, which shows point cloud data of the underside of Blackfriars Bridge where scanning was undertaken from a platform suspended underneath the bridge over the tidal River Thames. No other technique could have produced such rich data in such a short time span.

Methods that combine traditional total station, discreet point surveys and laser scanning, may be used to provide combined data sets and total stations now exist that combine conventional total station and a laser scanning capability in a single unit..

Figure 25 – Image from point-cloud data of Blackfriars Railway bridge

10.7.3 Cut back on “returns to site”

How often, after the site survey has been completed, does a new requirement become apparent? With laser scan data, the information is already “in the can” and simply needs to abstracted from the point cloud data.

Laser scanning provides greater confidence in the accuracy, completeness and correctness of the data collected.

Things are not omitted by human error if the surface required is not obstructed.

The data, once collected, may be used across multiple disciplines following the strap line of “Survey once use many times.”

The snapshots as shown in Figures 24 and 25 are available as “added value” information from the datasets and can provide background to a conventional 3D vector survey.

Laser scan data supports the BIM process from first survey through design to finally As-built BIM models.

Such BIM models may be used to generate animations to help with spatial awareness and project stageworks planning.


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Infrastructure surveys may be undertaken with the scanner attached to a track measuring device (TMD) as described in NR/L2/TRK/3100 Module 1 Appendix A and NR/L3/TRK/3105 Appendix A.

10.7.4 Disadvantages

Laser scanners are not capable of detailing points that do not actually exist in space such as the running edges of rails or the corner of the top of platform copers. However, there are ways to interpolate these utilising special target or software algorithms.

The subject measured needs to be accessed ideally from all sides so that it can be scanned fully.

Scanning should not be performed in adverse weather conditions where the quality of data may be affected.

For example, scanning in heavy rain can lead to data voids due to rain drops or erroneous data points due to the returns from the raindrops or refraction of the measurement beam.

Huge amounts of data will need to be stored and managed. Consider that three hours work during 17 scans collected 515 million points and created a database of 9.7Gb in size.

The spacing of points is much closer than conventional surveying. Enough computing power is needed to allow the manipulation of the data into a handleable format.

Skilled Data Modellers are needed to derive the greatest benefits from such data.

Scanner locations should be stable because the data is not collected instantaneously. However, this does not prevent the use of scanners located on hoists when weather conditions or local site conditions allow. Clearly the purpose of the survey and the required accuracy needs are important in deciding the most appropriate scanner locations.

A system for checking data collected by laser scanning shall to be developed by the CSM.

NOTE: The concept of Q numbers combined with scan metadata and registration residuals should give a measure of if the dataset conforms to the specified accuracy requirement.

This is a fast-developing technique. Proposals utilising these methods shall be reviewed in the light of the most recent developments by consulting with the project CSM.

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