TSA Client Guide - Mobile LiDAR Surveys Issue 3_HR

8/19/2019 TSA Client Guide - Mobile LiDAR Surveys Issue 3_HR

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CLIENT GUIDE TOMOBILE LIDAR

SURVEYS

The Survey Association’s Client Guides are primarilyaimed at other professionals such as engineers,architects, planners and clients in general. They are notintended to go ‘in depth’ into practical issues but to actas a basic guide on a particular topic and in particular,

on procedures and regulations which may govern howa particular aspect of the survey is carried out.

1. What is a Mobile LiDAR System?

Mobile LiDAR (Light Detection and Ranging) is a fast and reliable survey method forobtaining 3-dimensional data for the generation of point cloud information, froma moving platform, with a level of precision that will complement existing surveytechniques. A LiDAR system is an active sensor which uses laser to measure distancesto features and the ground. With an integral camera, digital images can supplement theLiDAR data for a full evaluation of the captured features.

Mobile LiDAR systems are ideally designed for corridor mapping and therefore theyparticularly suit both the road and rail network environments. The measurement rangeof the sensor is dependant on the reflectivity and orientation of the objects or surfacebut generally sensors are capable of recording data with a range of up to 200m.

ISSUE 3

MAY 2015

©TSA

Endorsed by:


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CLIENT GUIDE TO MOBILE LIDAR SURVEYS

ISSUE 3 MAY 2015 ©TSA

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Mobile LiDAR provides the ability to capture highly detailed and accurate point cloudinformation, this data can then be utilised to extract valuable information such astopographic mapping products.

1.1 How does it work?

A laser emits a light pulse which is projected to a mirror and is reflected from a vehicleor moving platform. The light pulse travels out until it reaches an object and the pulseis then reflected back to the system. Since the speed of light is a constant, a standard

mathematical equation can be used to determine the distance the light travelled withina recorded time interval. This measurement tells you the range to an object.In addition to knowing the range to the object, Inertial Navigation Systems are usedto verify latitude, longitude and elevation providing the X, Y and Z data points of athree-dimensional data set.

Modern mobile LiDAR systems can produce hundreds of thousands of pulses everysecond which allows for extremely rapid acquisition of data over long lengths of areas,which may be a hostile or risky environment and therefore not suitable for traditionalmethods of surveying. There are very few Health and Safety risks associated with amobile LiDAR survey and the lasers cause no harm to humans as they are manufacturedto be ‘eye safe’.

1.2 What are Point Clouds?

A point cloud is a set of vertices in a three-dimensional coordinate system.These vertices are usually defined by X, Y, and Z coordinates, and are intendedto be representative of the external surface of an object

Figure 1 above shows an example of a height shaded point cloud output.

1.3 Characteristics of Continuous Mobile LiDAR

• Multiple scanners, collecting data forwards and backwards

• Linked to an inertial navigational system (INS) for more precise location of points

• Rapid acquisition of 100km’s of route network per day

• Sensors operate in day or night conditions

• Can operate at speeds up to 100km/hr

“ Motorway junctionscanned from maincarriageways andover bridges.”

Figure 1


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2. Equipment

There are a wide variety of mobile LiDAR system available from specialised profilersused on rail track surveys to more versatile corridor mapping system that utilise multiple360º scanners.

Mobile LiDAR systems are made up from a number of components which are generallyreferred to as the ‘system’, these can vary slightly in detail from system to system butthe main components are:

• Laser Scanners

• Inertial Navigational System (INS)

• Cameras

2.1 Laser Scanner

A Laser scanner is a system for emitting an array of timed laser pulses, and measuringthe elapsed time/phase shift of the return pulse from the reflecting object, along withthe strength of the returning pulse (intensity). A common approach is to have a fixedlaser source, which is then deflected by a rotating or oscillating mirror, to generate a

scanning action.

Alternative approaches are also used, such as the use of a fibre array to generate ascan pattern.

Most mobile laser scanners are capable of recording either multiple returns for a singleoutgoing pulse or the “Full Waveform” return, which then allows discreet returns to beextracted during post processing. Both these techniques allow the laser to measuremultiple targets with one pulse, provided all the targets fall within the footprint of thepulse. The first pulse digital surface model depicts all the features identified by thesensor without any filtering of points, the last pulse digital surface model after filteringof the first and intermediate returns can depict the ground surface. It should be notedthat the last pulse return is not necessarily from the ground and thus needs to beverified by ground truth data.

“ Example oftargets withmultiple pulse

returns.”

Figure 2


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2.2 Inertial Navigational System (INS)

The solution for positioning and orientating the system whilst the laser scannercomponent is recording data is critical to the accuracy of the final output. The INSis reliant on blending information from Global Navigation Satellite Systems (GNSS)receivers and an Inertial Measurement Unit (IMU) to generate highly accurate positionaland orientation estimates for the trajectory of the system - commonly referred to asSBET (Smoothed Best Estimate of Trajectory). This commonly involves the use of thelatest GNSS base station technology to allow differential processing.

2.3 Cameras

Most mobile LiDAR systems can be mounted with multiple cameras. These can collectimagery at various frame rates to ensure overlap and coverage of the area of interest.

If cameras are used with systems that have an Inertial Navigational System (INS) then theimages can be precisely time stamp using the onboard GNSS signal. The point cloudcan be colourised using the information from the imagery which adds extra contextualinformation when interrogating the data.

Figures 3, 4 and 5 below show a typical rail and vehicle mounted mobile LiDAR surveyset-up.

3. Uses of Point Cloud data

“ Mobile laserscanners can bemounted on manydifferent vehicular

platforms. In thisexample thevehicle has beenadapter for therail environment.”

Figure 3

Figure 4 (Left) The primary use of mobile mappingsystems is on the highway

Figure 5 (Right) Compact scanning systemscan be fitted to off road vehicleto reach inaccessible areas


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Applications for the use of LiDAR are evolving, but generally any application thatrequires a volumetric or three dimensional set of data or topographic vector informationcan be considered.

As the result of a 3D scanning process point clouds are generated and used for manypurposes, including creating 3D CAD models and a multitude of visualisation andanimation products.

2.1 Applications

Typical applications include:

• Infrastructure modelling

• Asset inventory

• 2D and 3D Vector Mapping

• Rail infrastructure and asset capture

• Building modeling and facade detail

• DTM generation by meshing or surfacing

• Profiles and cross section generation

• Visualisation / animation

• Volumetric Analysis

An example of infrastructure capture and outputs are shown below in Figure 6.

3.1 Output and deliverables

The outputs of the mobile LiDAR surveys are often dependent upon the application andproject requirements but in general the collected measurements can be supplied in anumber of formats, depending on the end users systems.

“Complex structures

can be surveyedand processed to

point cloud datavery quickly. Inthis example wecan see assets,road surfacesand the bridgestructure withinthe same scan.”

Figure 6


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1. The LAS file format is a public file format for the interchange of 3-dimensionalpoint cloud data between data users; this binary format maintains informationspecific to the LiDAR capture.

2. The generic ASCII file interchange is more familiar with organisations but tendsto be larger in file size and thus slower in processing whilst the use of specialistformats such as POD & PTS is common within the surveying industry.

3. Gridded products such as DTM & DSM can be output in exchange formats such

as ASC Grid, ARC Grid, & GeoTIFF to name a few.

4. Ortho-rectified images can be supplied in common imagery exchange formatssuch as Tiff, GeoTiff, ECW & Mr SID etc.

5. Vector dataset can be output in common vector exchange formats, such asShapefile, DXF, DWG, DGN etc.

6. The raw point cloud can be displayed in a 3D environment, providing a verypowerful tool for straight point to point measurement.

7. Even with multi sensor systems, the point cloud is usually supplied as one datafile which can be defined using an agreed tile naming convention. Unlike aerialLiDAR data which is usually supplied in 1km tiles, mobile LiDAR data is moresuited to smaller subsets due to the density of the data.

Some example outputs are shown below in Figure 7.

4. What level of accuracy can be achieved?

Under optimal GNSS conditions an absolute height accuracy of up to ±35mm to1 sigma ( ) is achievable with most mobile LiDAR systems. These accuracies areusually quantified by comparing laser returns on an open hard, flat surface withsurvey ground control.

Please note that the price of a Mobile LiDAR survey can vary considerably whenrequiring accuracies within the higher ranges.

These accuracies are however dependant on a number of operational conditionsand additional post-processing techniques.

“In environmentswhere GNSSmasking may occur

it is advisable touse additionalground controlpoints to aidaccuracy checkingon the scan data.”

Figure 7


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• Good GNSS signal is essential to generate precise trajectories

• Quality of the Inertial Measurement Unit

• Additional ground truth data may be required

• The range to the target

It should also be noted that the accuracy of the point cloud data should always behigher than the intended accuracy of the derived products.

Because the mobile scanner collects data dynamically, the overall accuracy of thegenerated point cloud is directly limited by the underlying accuracy of the GNSS signaland the quality of the IMU. The generation of an accurate survey can be sometimesdifficult, especially in urban areas due to the unreliability of obtaining a strong GNSSpositioning solution caused by buildings masking the signal. Therefore it is importantto discuss your requirements with the survey firm at an early stage to determine thebest solution.

5. Advantages of Mobile LiDAR System surveys comparedto traditional surveys

Highway corridor surveys are becoming increasingly more and more difficult andexpensive to carry out because of the lane closures, traffic disruptions and safetyhazards posed to the surveyors as well as the public. Roadway workers and surveyorsare at risk, whether in vehicles or not, when working in the traffic corridor if they can’tmove with the flow of traffic. Safety for the travelling public is also a major problemwhen lanes are closed, or blocked by slow moving vehicles. In order to mitigatethese problems, a Mobile LiDAR System can be implemented. It can collect detailedtopographic and infrastructure information without requiring traffic management,lane closures or even a disruption in traffic flow.

As data is collected from the survey platform while it is moving at speeds up to100 km/h the mobile LiDAR System gives opportunity to increase productivitywhilst minimising exposure of staff to high risk environments such as highway

corridors or the railway.

“ Many health andsafety concernscan be minimisedby removing stafffrom the highwayenvironment!”

Figure 8


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6. What information do I need to provide to obtaina quotation?

When procuring mobile LiDAR works the following information should be provided:

• Purpose of the survey

• Expected accuracy of the deliverable products

• Information on the survey environment & specific requirements, such assurveying at specific times i.e. if vegetation penetration is required

• A plan with the survey route or area outlined

• Specification including any special requests

• Deliverable data format

• Access and security arrangements (if necessary for a ground truthing surveyor Traffic Management Scheme)

Reference section 3.1 for deliverables that you might expect.

7. Return of informationA company practiced in mobile LiDAR surveys can turn around a quotation quickly andas a client you should insist on the following details in your quotation, as a minimum:

• A start time for the survey work to commence

• A delivery time of the survey data

• The survey Company’s terms and conditions; so you understand the proceduresthe company uses during the course of the project

• A provision in the fee for a weekly (or daily) progress report; so you can allowfor any changes in the program and effectively plan resources accordingly.

Additional information such as method statements and risk assessments can berequested and should be supplied if required.

Contact the UK Survey Association phone 01636 642840 or visit the TSA websitewww.tsa-uk.org.uk to locate an established mobile LiDAR survey firm.

8. Summary

Mobile LiDAR systems offer an alternative to traditional survey techniques. It provideshigh speed data capture solution of complex surfaces and often inaccessibleenvironments. It is a very effective and time-saving system that can be used on highwayswithout requiring traffic management or lane closures.

There are many possible outputs available from point-clouds from basic measurementsthrough to ortho-images, derived 2D/3D drawings, meshing/surfacing or solidmodelling.

However, it is important to approach this survey in an informed way. This leaflet provides

a basic guide to terminology and an overview.


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The Survey Association

Formed in 1979 as The UK Land and Hydrographic Association, TSA is now establishedas the representative organisation for UK private surveying firms. The Association’saims are:

> To provide a vehicle for members to act effectively together on agreedcourses of action

> To promote the interests of the profession to all those who determinethe economic and social conditions in which the industry operates

> To identify and represent the views of the industry.

Using a TSA member

By using a TSA member you can be assured that your project will get off to the bestpossible start. Whatever the size of project, you can be certain that TSA membercompanies are expert in the provision and management of spatially related data onwhich to base your concept, design and construction.

Professional attention from a TSA surveyor will reduce risk, repetition, possibly save

you money and will ensure that your project receives the best possible attention.

TSA Contact Details

If you would like any more information about the TSA or its members or about otherInformation leaflets then please contact Rachel Tyrrell at:

The Survey Association

Northgate Business Centre, 38 Northgate,Newark-on-Trent, Nottinghamshire NG24 1EZ

Tel: 01636 642 840

Email: [email protected]

Website: www.tsa-uk.org.uk

TSA Disclaimer

Whilst The Survey Association (TSA) makes every attempt to ensure the accuracy andreliability of the information contained in this publication, this information should not berelied upon as a substitute for formal advice from the originating bodies or services ofTSA members. You should not assume that this publication is error-free or that it will besuitable for the particular purpose which you have in mind when using it. TSA assumesno responsibility for errors or omissions in this publication or other documents which arereferenced by or linked to this publication.

In no event shall TSA and its employees and agents be liable for any special, incidental,indirect or consequential damages of any kind, or any damages whatsoever, including,without limitation, those resulting from loss of use, data or profits, whether or notadvised of the possibility of damage, and on any liability, arising out of or in connectionwith the use or performance of this publication or other documents which arereferenced by or linked to this publication.


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Glossary of Terms

TERM EXPLANATION

DSM A Digital Surface Model represents the earth’s surface andincludes all objects on it such as buildings, vegetation, etc

DTM A Digital Terrain Model is a digital representation of ground

surface topography or terrain. It represents the bare groundsurface without any objects like plants and buildings

GNSS Global Navigation Satellite System

INS Inertial Navigational System - combines information from GNSSreceivers and an Inertial Measurement Unit (IMU) to generatehighly accurate positional and orientation estimates for thetrajectory of the system

LiDAR Light Detection and Ranging is an optical remote sensingtechnology that can measure the distance to, or other properties

of target by illuminating the target with light, often using pulsesfrom a laser

Point Cloud A point cloud is a set of vertices in a three-dimensionalcoordinate system. These vertices are usually defined byX, Y, and Z coordinates, and typically are intended to berepresentative of the external surface of an object

Document Revision History

Issue 1 March 2012 Original documentIssue 2 January 2015 Accuracy/pricing related edits

Issue 3 May 2015 TSA disclaimer added

TSA Client Guide - Mobile LiDAR Surveys Issue 3_HR

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