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					Highways Agency


  Guidance Note
 on use of LiDAR



Rev 3 February 2008

                      Rev 3_080208_Arup67519-42
Contents and Introduction



 Introduction                                    2    This Guidance Note provides information on the application and business
                                                      benefits of using airborne laser scanning (LiDAR) – one of the remote
 HA remote sensing research                      3    sensing techniques widely used by the HA.
 HA National LiDAR Framework                     4    LiDAR data is being captured by the HA for significant lengths of the
                                                      network through the HA ‘LiDAR Framework’ and used for a range of
 LiDAR systems                                   5    network management applications. The data is stored in a central archive
 LiDAR data capture and processing               6    (‘HA Archive Storage Service’) and available to any organisation working
                                                      on HA business activities.
 LiDAR datasets                                  7    LiDAR surveys are procured through the Framework, using a bespoke HA
 LiDAR imagery                                   8    technical specification. Advice on LiDAR surveys, their potential application
                                                      and costs is provided by a dedicated HA Framework support team.
 Applications : topographic survey               9
                                                      This Note describes the different types of LiDAR systems and the range of
 Applications : asset inventory surveys          10   output formats available. The various current applications of the data are
                                                      also given, together with details on how to obtain data through the LiDAR
 Applications : noise assessment studies         11   Framework and existing data from the archive, with some cost information.
 Applications : earthworks                       12   The guidance note focuses mainly on airborne LiDAR techniques although
                                                      some information on terrestrial systems is also given.
 Applications : natural slopes                   13
                                                      The HA are seeking to expand the uses of these datasets to other activities
 Applications : geomorphological mapping         14   and feedback is welcomed.

 Applications : archaeological studies           15
 Glossary                                        16




                  Guidance Note - Use of LiDAR                                                                                        2
HA remote sensing research
Remote Sensing                                              HA Research
                                                            ‘Remote Assessment’ Project 3/303
Remote sensing is the acquisition of data and imagery
of the earth’s surface and sub-surface by sensors which     In 2000 the Highways Agency commissioned a research
are not in contact with the subject area. Most sensors      project to investigate the potential offered by remote
are mounted on airborne or satellite platforms but some     sensing for providing survey information of the network.
ground based systems which operate at a distance can        The project was sponsored by the Safety, Standards
be included. Seabed bathymetry surveys from boats           and Research (SSR) directorate and managed by the
with sonar devices could similarly be described as          HA geotechnical group (APD(G)) as, initially, the focus
remote sensing.                                             of the research work was on improving the data
                                                            collection process for principal inspections of
Remote sensing technology has advanced considerably
                                                            earthworks.
over the last ten years and its application has changed
from niche markets to widespread use by the general         The project comprised a scoping study and two stages
public through websites such as Google Earth. The           of remote sensing trials over sites around the HA
advent of differential GPS has improved data accuracy       network. During the course of the study it became clear
and the declassification of military sensors for civilian   through consultations across the HA, with managing
use has significantly increased image resolutions. In       agents and design teams that many of the remote
parallel with this the increased computing power of         techniques investigated had a wider application than
desktop PCs now allow processing and handling of the        just earthworks assessment and could provide a
larger file sizes generated.                                resource of considerable benefit to the HA.
Within the fields of engineering and civil infrastructure   Accordingly, during the research a parallel workstream
management the use of Geographic Information                was initiated with HA Major Projects Directorate as part
Systems (GIS) to store, view and analyse data is now        of their ‘Speeding-Up Delivery’ initiative to arrange a
common and geo-referenced data is generally preferred       framework contract to centrally manage the
to linear systems of asset or feature referencing. Most     procurement of remotely sensed geospatial data. This
remotely sensed data and imagery is compatible with         framework was tendered in 2003 and awarded to three
GIS and CAD systems and provided in digital, geo-           contractor organisations in Feb 2004. This is discussed
referenced format.                                          further on the following sheet.
                                                            Further focused research has been undertaken by the
                                                            HA since 2003 to address specific areas of business
                                                            benefit such as automated change detection and use of
                                                            data for speeding up Road Traffic Accident investigation
                                                            surveys.




                       Guidance Note - Use of LiDAR                                                                    3
HA ‘LiDAR Framework’

HA Nationwide Framework Contract to Undertake LiDAR Surveys
                                                                             First Time (Bespoke) Surveys
The HA Nationwide Framework Contract to Undertake LiDAR Surveys
(‘LiDAR Framework’) was set up in 2004 to collect all types of               Chris Jackson : HA Project Sponsor, Birmingham
geospatial data from satellite imagery and aerial photography, through       Tel : 0121 678 8027
LiDAR / laser scanning using airborne or ground sensors, to traditional      Email: chris.jackson@highways.gsi.gov.uk
ground survey techniques. Three specialist contractors have been
engaged to provide this data to a common technical specification             Ronnie Bain : LiDAR Framework Project Manager
developed specifically for HA applications. The result of this is that       Mouchel, Manchester
data quality, consistency and formats are uniformly controlled.              LIDAR.framework@mouchel.com

Data from first time (bespoke) surveys is stored in a centralised
archive (HA Archive Storage Service) which can then subsequently be
reused by any other interested party at a later date, ensuring the
                                                                             HA Archive Storage Service (Archive Data)
continual availability of this resource. This service facilitates multiple
use of data (‘collect once – use many times’) preventing ‘loss’ of data
                                                                             Steve North (Archive Manager)
for example at scheme completion or handover of area management
                                                                             Tel : 0116 273 2300
to new agent teams.
                                                                             Email : steve.north@infoterra-global.com
The framework is project managed for the HA by Mouchel and
overseen by Chris Jackson (HA, Birmingham). Arup, the original
research project contractor, are the framework Technical Advisors.

Costs for obtaining first-time survey data or imagery can be obtained
from the Framework Team and the initial contact should be via Chris
Jackson (see right).

Archive data is obtained via the HA Archive Storage Service, run by
Infoterra Ltd. A ‘shop window’ indicating available data can be viewed
in HAGIS and HAGDMS (‘remote sensing layer’). Further details of
current coverage around the network can be provided by Infoterra.
Data is provided on CD or DVD in 1km tiles, referenced to OS National
Grid. Archive data is currently available free of charge to anyone
involved on HA business.

To date (Feb 2008) detailed LiDAR surveys have been captured for
over 1500km of the HA network.


                       Guidance Note - Use of LiDAR                                                                           4
LiDAR systems
LiDAR is an acronym for Light Detection And Ranging but it is also referred     The density of laser point data per square metre on the ground or object
to as laser scanning. The technology is not new but the relatively recent       surface is dependent on factors such as pulse frequency, flying speed and
integration with dGPS and inertial measurement data has provided a step         height, scanning pattern, nature of reflecting surface and number of returns
change in the accuracy of airborne sensors over the last few years.             per pulse. Some systems can have more than one return (points) per pulse,
                                                                                with in some cases up to four which gives much improved penetration of tree
The laser scanning unit is mounted either in a pod below a helicopter or        cover / foliage.
through a hole in the fuselage of fixed wing aircraft. The laser source emits
light pulses towards a rotating mirror which re-directs the pulses down in a    The accuracy of the laser point data can vary depending on factors such as
scanning pattern. This pattern can either be a simple side-to-side motion or    flying height, pulse divergence and GPS satellite availability. There is a
in some systems as a helical spiral. The frequency of laser pulses emitted      difference between absolute and relative accuracy and these are usually
was typically in the range from 5000 to 55,000 per second but the latest        quoted in terms of RMSE (root mean square error). Vertical accuracy is
systems are now operating at up to 100,000 pulses per second.                   always better than horizontal accuracy with values generally quoted for hard
                                                                                surfaces and soft surfaces.
The pulses are returned to a receiver in the sensor after reflecting back
from both the ground surface and objects above ground such as structures,                                     RMSE
trees and buildings. Pulse travel times are recorded and the distance from                                    With some qualifications, the RMSE can be related to the
                                                                                                              probability that any particular data sample lies within
the sensor to the scanned object or ground surface can then be determined
                                                                                                              specific data limits. For example, a quoted RMSE of 1m
using speed of light calculations. As the exact position of the sensor is                                     in vertical level implies that 68% of the data points should
known from dGPS and inertial data the xyz co-ordinates of each point of                                       lie within +/- 1m of the true absolute elevation and that
                                                                                        -1m       +1m
reflection can be derived.                                                                                    99% should lie within +/- 3 m.
                                                                                        ‘True’ Elevation




                                                                                Fly higher and faster than helicopters and survey a wider corridor but at a lower point
Fixed wing                                                                      density, commonly 0.5 - 1 point / sq.m. Accuracy is typically around +/-80 mm
                                                                                RMSE vertical. Only laser data points captured but can be merged with other aerial
                                                                                imagery later.


                                                                                Fly lower and slower than fixed wing aircraft and survey a narrower corridor but at
Rotary wing                                                                     a higher point density, commonly 5 - 25 points / sq.m. Accuracy is typically
(Helicopter)                                                                    around +/-30 to 40 mm RMSE vertical. Most systems have digital cameras and /
                                                                                or video cameras collecting imagery synchronously with the laser data. One
                                                                                system has dual laser scanners, angled slightly forward and back.



                                                                                Scanners can be mounted on static or moving platforms and provide the highest
                                                                                point densities with the highest accuracy. Can merge airborne laser data with
Terrestrial                                                                     terrestrial data and commonly used at specific locations where very high accuracy
                                                                                survey is required or where airborne data not possible, such as under bridges and in
                                                                                tunnels. Current systems are one return per pulse only. Some systems provide
                                                                                synchronous imagery.

                      Guidance Note - Use of LiDAR                                                                                                                           5
LiDAR data capture and processing
For Highways Agency survey work fixed wing aircraft fly at heights of around 800m above the
carriageway, well beyond the field of view of drivers. Data capture is also often carried out at night.
The data swath width is typically in the range from 600 to 800m which covers the highway corridor
and around 300m each side. Where required, the density of data points may be increased by flying
several passes with the aircraft. Survey lengths of up to 100km per day can be achieved.

The helicopter mounted systems fly much lower, at heights of around 100 to 400m. Even at 100m the
helicopter causes no distraction to road users and police forces in previously surveyed areas have
been satisfied that driver safety is not compromised. The data swath width is much narrower than
fixed wing systems and generally around 60 to 100m. Multiple passes are required to capture the full
width of motorway corridors and junctions with a 30 to 50m border outside the highway boundary.
Survey lengths of up to 50km per day can be achieved.

During LiDAR surveys no access to the carriageway is
required at all, although temporary GPS base reference
stations are set up, offline, at 15km intervals along the survey
route. These reference stations are connected to the OS
Active GPS Network around the country and together with the
on-board GPS and inertial data recorded in the helicopter are
used to provide the high accuracy of the laser point data.                                                  Helicopter LiDAR 100m above M25

The xyz laser point data is captured by the LiDAR sensors in
WGS84 format. On completion of the survey the point data is
meshed together with the GPS ground survey data and
transformed into OSGB36 format for National Grid
referencing.
                                                                             GPS base reference station
Further processing of the laser data is carried out by the
LiDAR contractor through a series of automated routines and
manual editing using specialist software. The point data is
classified into ground and above ground points with the latter
further classified as low, medium and high vegetation,
buildings, structures etc.

The laser data can then be issued either in this ‘raw’ format
(.LAS) or further processed into a range of other outputs
depending on the end requirement of the survey. Examples              LiDAR survey ground control network
of these are shown on the next page.


                       Guidance Note - Use of LiDAR                                                                                           6
LiDAR datasets
LiDAR data can be viewed and output in a number of different formats

Point Cloud : The xyz data can be viewed as a 3D image using point
cloud visualisation software which can be panned though and rotated in
space. The point cloud can be shown in grey scale (see below), coloured
by elevation of the points or as a surface reflectance intensity image.




                                                                              Cross-sections : This 2D section is taken through a point cloud and
                                                                              shows how LiDAR captures details of the terrain and above ground
                                                                              features. Sections at regular intervals along the survey route can be
                                                                              set to run sequentially within a movie file.




Digital Terrain Model : 3D surfaces can be created from the laser point data from which
a Digital Terrain Model can be derived in grid, TIN or contour format. The accuracy of
the DTM depends on the accuracy of the laser points.
Contour models : One of the most useful formats for engineering purposes is the laser
data output as a 2D or 3D contour model in CAD or GIS format. The contour interval can
be set to suit the terrain and the usual range is 0.25m, 0.5m, 1m and 5m sets. Here (see
right) the contour model has been draped over digital aerial photography.

3D surface : Using image processing software such as ERMapper the 3D surface can be
shown as a shaded relief model to highlight ground surface features or elevation coloured
to show ground level changes. The surface can be viewed as a ‘bare earth model’ (Digital
Terrain Model) or with above ground features (Digital Elevation Model).




                                                                                                             Fly / drive through ‘movie’ files :
                                                                                                             Movie files playable in Windows Media Player or
                                                                                                             similar can be generated for point clouds, cross
                                                                                                             sections or 3D surfaces. Aerial photography can
                                                                                                             be draped over the 3D surface.

                       Guidance Note - Use of LiDAR                                                                                                        7
LiDAR imagery
Most helicopter mounted LiDAR systems have a digital camera within
the sensor pod which captures synchronous imagery with the laser
data. In addition, some systems have one or two video cameras
orientated vertically down and / or obliquely forward.




The digital photography collected by helicopter systems is of a very
high resolution due to the low flying height of the helicopter during the
survey. The resolution can be as good as 2 to 4cm per pixel. Bright
conditions are needed to obtain imagery of this quality and where
photography is specifically required (e.g. for topographic surveys) this
can introduce a level of weather dependency. Laser data capture, on
its own, is much less dependent on good weather conditions.

The imagery is processed with the laser data, which is used to
orthorectify the imagery and create a series of regular image tiles. The
images are available in TIF, ECW, BMP or JPEG formats and can be
viewed independently of the laser data, although the combination of the
laser datasets and high resolution imagery creates a very powerful tool
for many applications.

An example of helicopter LiDAR imagery at 4cm resolution is shown on
the right and features such as LRPs (Location Reference Points or
‘chart nodes’) and drainage grills are clearly visible. As a digital image
it is possible to pan and zoom around the photography at a range of
scales.

Fixed wing LiDAR systems fly too high to take imagery of this resolution
and also commonly capture the laser data at night. However digital
imagery can be obtained from other sources for use with fixed wing
LiDAR data.



                      Guidance Note - Use of LiDAR                           8
LiDAR applications : topographic survey
1:500 scale topographic mapping to SD12/96 (MCHW Vol 5 Section 1)
LiDAR point data and high resolution aerial imagery can be used together to
produce 1:500 mapping for highway schemes. The horizontal or positional
alignment (x,y) of the derived 3D line-strings are extracted from the imagery
and the vertical or elevation (z) alignment provided by the LiDAR point cloud
data. Development of this application was undertaken during the HA research
project when the accuracy of mapping derived from LiDAR was verified
through a series of independent ground proving surveys. These showed that
the accuracy met the requirements of 1:500 scale mapping (+/- 50mm RMSE
horizontal, +/- 100mm RMSE vertical) and that the point data accuracy is
significantly better than this (around +/- 30mm RMSE vertical). In addition,
relative accuracy between points is even higher.

Different LiDAR systems have different capabilities with respect to collecting
ground surface data below tree cover and the HA Technical Specification
defines the laser point density required for particular survey accuracies.
Where more detailed surveys are needed, such as at structures, then a local
ground survey can be undertaken through terrestrial LiDAR or traditional
methods and the survey data merged with the airborne LiDAR data.

Technical advice on LiDAR mapping is available from the Framework Team.
                                                                                 Benefits from use of LiDAR for topographic mapping :
                                  Outline ground model
                                  A simplified 3D model at 1:500 scale showing   •    Minimal or no roadside exposure of surveyors to traffic
                                  selected features such as boundaries, roads,   •    Minimal or no Traffic Management costs and delays
                                  top and bottom of slopes, bridges etc. It is   •    Increased speed of data capture and processing
                                  much quicker to produce than the detailed
                                                                                 •    Reduced survey costs – often over 50% cheaper than ground survey
                                  mapping. Can be issued in different formats
                                  including MX GENIO.                            •    The survey accuracy can be varied across the route corridor to suit different
                                                                                      applications such as route selection, alignment, earthwork volumes, design,
                                                                                      noise mapping etc
                                                                                 •    The outline ground model can be produced relatively quickly after data
                                  Detailed ground model                               capture. The detailed ground model can be processed for selected areas as
                                  Outline mapping as above but also including         and when required.
                                  an increased range of features such as         •    High accuracy LiDAR data already collected for over 1500km of the network
                                  vegetation and highway furniture, specified         for motorway widening, junction improvements and bypass schemes. This
                                  according to the project requirements.              is now available to all through the HA data archive free of charge.
                                                                                 •    Imagery and data can be re-processed for other applications – increases
                                                                                      value of data set
                                  Formats include :
                                  DXF, DGN, DWG, MX GENIO, SHP, XML etc.

                       Guidance Note - Use of LiDAR                                                                                                                   9
LiDAR applications : asset inventory surveys
The high resolution and accuracy of the LiDAR datasets can be used to identify
and geo-reference assets along the highway corridor. In addition the laser
height data can be used to make detailed height or clearance measurements.

Some examples of this are shown below :



                                                               Identification of assets in 4cm
                                                               resolution imagery. National
                                                               Grid geo-referenced locations
                                                               easily extracted


                                                                                                   Vegetation density, height and extent or encroachment can
                                                                                                   be viewed and / or accurately measured. Tree types can be
                                                                                                   identified in places.




                               High density point clouds provide a good
                               representation of assets and other features




                                                                                      Structure measurements such as bridge clearances and parapet heights
                                                                                      can be extracted from LiDAR data. The extent and heights of crash
                                                                                      barriers or safety fences at road over rail crossing can be determined.
                                                                                      Lamp post heights or catenary clearance also possible.

                      Guidance Note - Use of LiDAR                                                                                                        10
LiDAR applications : noise assessment studies
Noise assessment studies use source-pathway-receptor analyses to map          Accurate locational and / or height data can be attributed to all pathway
the traffic noise level and travel extent adjacent to and away from the       and receptor features. This information is then input to the noise
carriageway. LiDAR data can be used to provide digital elevation models       assessment programme to calculate the noise level spread from the road.
for information on the nature of the pathway and the noise receptors.         This can then be output as contoured noise levels, as shown below.
For the pathway this includes details on the spatial location and height /    LiDAR data for areas of the network already surveyed (e.g. for topographic
depth of earthworks, retaining walls, noise barriers, vegetation, buildings   mapping) is stored in the central archive in a raw format suitable for
and other structures. Receptors are typically buildings where the noise       processing for multiple applications. Point data is already classified into
levels arriving at each floor need to be quantified.                          ground points and various types of above ground features.
Laser scanning provides a blanket cover of point data across the surveyed     This data can be retrieved and processed for noise mapping. The
corridor, including ground levels below tree cover. Feature detection to a    Framework Team has considerable experience with this application and
high resolution is possible.                                                  advice can be provided.




                      Guidance Note - Use of LiDAR                                                                                                     11
LiDAR applications : geotechnical asset management                                                                                                   (1)
Earthworks                                                                      Slope Instability
                                                                                0.25m interval contour datasets can be used to detect and delineate a wide
Laser point data outputs such as contour sets together with the high            range of earthwork slip features from larger crest-to-toe slips (A) to smaller
resolution aerial imagery can be used to populate many of the feature           or early stage slips (B,C). Spreading of the contours (B,C) highlights the
fields in the HD41/03 Principal Inspection Form for earthworks. Site            midslope decrease in angle and the slight toe bulge in photo B is also visible
inspections are still required but can be focused just on detection of          in the contours. Backscars to 25-30cm height are detectable by contour
smaller features such as tension cracks and seepages.                           bunching (C).

                                                     Earthwork Inventory
                                                     Contour data provides
                                                     good definition of
                                                     earthwork location,
                                                     extent and geometry –
                                                     particularly useful at                                                       C
                                                     grade separated
                                                                                                             B
                                                     junctions and where
                                                     vegetation is extensive.   A
                                                     Geo-referenced locations
                                                     can be extracted.

Vegetation : can be determined both from laser data and associated
imagery with a high resolution of type, density and heights. Current
LiDAR systems have good penetration through most vegetation types to            A                            B                        C
provide ground surface data below canopy.
                                                                                Use of the LiDAR datasets reduces engineer and TM time on site by allowing
Drainage : High resolution imagery can show drainage features visible at
                                                                                pre-population of many of the feature fields and improving the planning /
ground surface. Cannot detect seepages and waterlogged ground.
                                                                                prioritising of inspections. LiDAR provides information in restricted access
Animal burrows : If arisings present in mounds outside burrows
                                                                                areas, below most vegetation cover and for features outside the HA boundary
Remedial Works : Retaining walls visible in contours and imagery.
                                                                                which can affect earthwork stability.
Repairs detectable where some surface expression present.
                                                                                LiDAR data and imagery can be used by MAGLEs to assist the approvals of
                                                                                PIFs and defect classifications as well as providing base plans and survey data
                                                                                for completion of GMFs including defect definition, SI design, stability
                                                                                analyses, remedial works design, site access planning etc.




                                                                                section through point data

                      Guidance Note - Use of LiDAR                                                                                                               12
LiDAR applications : geotechnical asset management                                                                                                              (2)
Natural Slopes
Natural slopes are traversed as sidelong ground by highway routes in some      M6 Lune Gorge, Cumbria
parts of the network. They are more common adjacent to trunk (‘A’) roads
than motorways and can extend for a considerable distance above and                                                          A40, Leys Bend
below the highway boundary. Natural slopes may be mantled by shallow                                                         Monmouth
relict instability features or have deeper underlying potential mass
movement mechanisms. Shallow and deep-seated instability can be
(re)activated by many different trigger factors and may affect the
carriageway years after construction. Natural slopes present a significant
source of geo-hazards to the highway.

Access to these slopes for inspections may be hindered by slope
steepness, traffic management restrictions and dense vegetation including
mature trees. Where there is a tree canopy present aerial photography
cannot be used to detect signs of slope instability and photogrammetry or
ground surveys are not suitable for topographic mapping.

Airborne LiDAR is able to provide detailed mapping of these slopes and
can detect slope failures to a high resolution – where there is some surface
expression of these features. Penetration of the tree canopy to obtain
ground points is possible with multiple return laser systems which allow
‘hits’ to be made at all levels from the top of canopy to the ground. This
enables good definition of the trees and other vegetation in addition to
ground profile data.

The laser point data can be processed into a number of output formats          1m interval contour dataset for very
                                                                               high steep natural slope. Good
such as contour models, cross sections, digital elevation and terrain models   definition of gulleys, rock slopes, river
and 3D surfaces, with some examples shown opposite.                                                                        Slope instability features detected and delineated
                                                                               alignment. Closer interval contours
                                                                                                                           on steep inaccessible slope below tree canopy.
                                                                               can be generated for local detail
                                                                                                                           Outputs here are point cloud cross section and
As well as providing information on the slopes for interpretation and                                                      3D shaded relief surface with contour overlay.
analysis LiDAR also captures data on other relevant features such as rivers
and smaller water courses, highway alignment across the slope and
proximity of highway structures and furniture to hazards.
                                                                               Where there is no tree cover the high resolution imagery
Both fixed wing and helicopter systems have been used by the HA to             provided by helicopter systems can be used with the
                                                                               laser data outputs to assist interpretation and feature
provide data for natural slopes and system selection depends on the nature     detection. Carriageway cracking at A66 Crackenthorpe.
of the site and the end requirements of the survey. These can be
discussed with the LiDAR Framework team.

                      Guidance Note - Use of LiDAR                                                                                                                        13
LiDAR applications : geomorphological mapping
Geomorphological surveys have been carried out using data generated from LiDAR
surveys. The selection of the appropriate LiDAR system will depend on the site
characteristics, the area of survey and end requirements. Contour models at a range of
intervals – typically 0.25m, 0.5m, 1m and 5m are a useful dataset for this type of study
but the 3D surfaces from which the contour models are generated also offer some
additional benefits.

3D surfaces can be coloured by elevation (see
opposite). The colour ramp and elevation interval
can be set to suit the terrain. Here the purple to
blue elevation approximately represents the chalk
escarpment adjacent to the M25.

Shaded relief surfacing uses a virtual illumination                                        Part of M25 geomorphology study area. Total length
source at a specified dip azimuth (angle and                                               approx 40 x 2km corridor. Relict slope instability features
                                                                                           visible in both contour model and 3D surfaces.
orientation) to apply shading to the surface to
highlight changes in relief. The illumination                                              Collected by fixed wing LiDAR system at 1 point per sq.m.
                                                                                           Higher densities required under vegetation canopy.
azimuth can be rotated to change the shading
and assist the interpretation. These surfaces are
usually grey scale but can be produced to any
colour gradational schema to suit the terrain and
feature relief.

3D surfaces coloured by changes in slope angle
are another possible output. Slope areas inclined
above critical angles can be quickly determined.

These surfaces are usually generated using
specialist image processing software or GIS.
However they can be provided in TIF, ECW or
JPEG format for use by engineers with basic
desk top image browsers such as GeoViewer
(available through the Framework suppliers).

Use of LiDAR datasets in this way can save
much field mapping time by directing the survey
towards key locations for field verification.



                       Guidance Note - Use of LiDAR                                                                                               14
LiDAR applications : archaeological studies
The 3D surfacing output from laser point data can be used to detect
changes in ground surface relief. The resolution of this depends on the       Aerial photograph
original laser point density during data capture but even at 1 point per
square metre a reasonably high degree of detail is visible. This is the
usual density provided by fixed wing aircraft flying a wider corridor than
the helicopter LiDAR systems, and data is typically provided up to
around 300m each side of the carriageway. This can of course be
extended where needed.

A shaded relief 3D surface, either with or without elevation colouring,
derived from a 1 point per sq.m dataset can reveal ridge and furrow
patterns, earth mounds, ditches, sunken ground etc. The shading can
be changed by rotating the illumination azimuth to help delineate feature
extent and morphology. The surfaces can be output in JPEG or TIF
format for use with standard desktop viewers by non GIS users.




                                                                              Shaded relief surface model




     LiDAR data for M1 j10-13
                                                                             Remains of Roman Camp, near York


                     Guidance Note - Use of LiDAR                                                               15
Glossary

 DGN                             Data format for Microstation files

 DXF                             Drawing eXchange Format

 DWG                             Data format for AutoCAD files

 ECW                             ERMapper Compression Wavelets - a proprietary format of image file produced by ERMapper
 (MX) Genio                      Generalised Input Output – string based file format for transfer of geometric data

 GIS                             Geographical Information System
 GMF                             Geotechnical Maintenance Form (HD41/03)
 HA Archive Storage Service      Centralised HA Archive Storage Service for geospatial data sets – run by Infoterra Ltd. for the HA
 HAGDMS                          Highways Agency Geotechnical Data Management System
 HAGIS                           Highways Agency Geographical Information System

 JPEG                            Joint Photographic Experts Group – an image file format

 LiDAR                           Light Detection and Ranging

 LiDAR Framework                 HA ‘National Framework Contract to undertake LiDAR Surveys’

 MCHW                            Manual of Contract Documents for Highway Works

 MAGLE                           Managing Agent Geotechnical Liaison Engineer (HD41/03)

 PIF                             Principal Inspection Form (HD41/03)

 RMSE                            Root Mean Square Error

 SHP                             Shape file - a proprietary format of GIS file produced by ESRI software

 TIFF                            Generic Raster File Format

 TIN                             Triangulated Irregular Network – a form of digital surface model

 XML                             EXtensible Markup Language




                   Guidance Note - Use of LiDAR                                                                                       16
Guidance Note - Use of LiDAR   17

				
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