Comparison Of Elevation Collection Technologies

www.geovision.co.uk Global Solutions

Flood modelling and management

Glasgow University

8 September 2004

Paul Shaw - GeoVision

Format for morning session

• 9-30 – 10.30 Comparison of modern data collection technologies

• 10.30 – 10.45 Coffee• 10.45 – 11.45 Comparison of modern data collection

technologies (cont)• 11.45 - 12.00 Coffee• 12.00 – 1.00 Remote sensing for environmental monitoring• 1.00 – 2.00 Lunch

How important are heights in flood modelling?

Comparison of data collection technologies

• GPS - Global Positioning Systems• Aerial Photography• LIDAR - Light Detection and Ranging

• SAR– Synthetic Aperture Radio Detection and Ranging

• Multi Spectral Satellite Imagery

For each technology a summary on• Process of data collection• Potential sources of error• Benefits and concerns

• General comments

GPSProcess• Use technique called RTK or kinematic• Requires base station and rover• Requires radio link• Software converts to grid and geoid heights• Collect mapping data (as points,

polylines,polygons and attributes) • May require post-plotting using survey infill

techniques

Potential sources of error • Grid transformation and geoid model – Link• Satellite azimuth, number, quality and spread

(measured as DOP’s) - Link• Radio range• Base station problems

Benefits• Accurate to 10mm• Certainty with

ground heights• Accurate definition of

break lines• Data collected in

useable format

Concerns• Slow at 0.2 sq km/day • Not allowed in all

countries• Radio strength• Foliage and infrastructure• No image• Access restrictions

GPSGeneral comments• Many UK companies - Link• In GB OS have active and passive stns – Link• OS trig and BM system redundant• NAVSTAR (US) – 29 operational at present - Link• GLONASS (Russian) - 8 operational at present – (

Link)• GALILEO (European) – operational from 2008 – 30

in total – (Link)• Started becoming available in the mid ninetiesFurther details on how GPS works at (Link)

Aerial PhotographyProcess• Route planning; weather forecasting; flying

permission; possible pre-mark; control network• Film processing• Possible Scanning• Photo/Imagery control• Aerial triangulation• DEM generation; possible vector digitising; post

plotting; integration• Ortho production/mosaicing

Aerial Photography

Potential sources of error • Crabbing, fore and aft overlap

• Lens calibration• Film quality• Verticality• Scale problems through large terrain change• Forward Motion• Photo control errors

Concerns• Flying permission

required• Seasonal• Weather dependency• Heights to top of foliage• Restrictions by

government• Matching of orthos • Automated DEM’s

Aerial Photography

Benefits• Large areas

covered in one flight

• Height generated from the imagery

• Infrared available • Orthophotos• 3D visualisations• Automated DEM’s

Aerial PhotographyGeneral comments• Was used by the EA – replaced by LIDAR in 1998• Increasingly sophisticated on-board systems • Approx 10 UK companies• Fixed Wing or helicopter, hard copy or digital • Large archive at larger scales through BKS,

Simmons Aerofilms, OS, OSNI and others• Getmapping have GB covered at 1:10,000 – Link

Imagery £25 per sq km at 1:10,000 scale, heights extra

Process • Route planning; weather forecasting; flying

permission; control network• Processing – last reflection • DEM generation;• Post survey infill• Integration

Potential sources of error include• GPS and IMU calibration• IMU and laser head calibration• Lazer pulse rate• GPS baseline length • Level of uncertainty with ground height• No break line definition

Details available at (Link)

Benefits• Accurate to 50mm• Large areas covered

in one flight (30 sq km in one hour)

• Can be flown at night• Not dependent on

sun• Water height

Concerns• No image• Weather

dependency• Flying permission

required• Processing time –

up to 4 months for a few days data collection

General comments• 4 UK companies – global directory at (Link)• QinetiQ have combined with digital cameras to

create ATLAS system – (Link)• General information about LIDAR can be found at

(Link and Link)• Used extensively by the EA - archive coverage -

Link. Catalogue - Link

Coffee break

SARProcess (InterMap)• Aircraft has 2 antennae, receive slightly different

return signals, phase difference determines height• Synthetic aperture digitally enhances the data in

real time• Rigorous QC processing checks • DEM• DSM by interpolationIntermap SAR description at (Link)

Potential sources of error• Shadow• Rain shadow• Signal saturation• Missing data• Specular reflection • DSM generation

Details at (Link)

Benefits• Can cover 100 sq

km in a few hours• Generates height

(up to 0.3m) and image (up to 2.5m)

• All weather capability

SARConcerns

• Flying permission required

• Gives height to first reflection

• Accuracy of DSM• No break lines -

Nextmap products are 5 and 10m grid spacing

General comments• No UK companies• Accuracy comparison by EA available at (Link)• Nextmap data for UK has 2 accuracy levels – 0.6m

for the SE and 1.5m for the rest• QinetiQ have just gained reseller status• Nextmap archive at Link• Intermap global archive and prices at Link

Multi Spectral Satellites Imagery

Process • Order and collect basic imagery• Image control points• Rectification• DEM generation• Possible vector digitising• Map completion

Potential sources of error • Cloud• Other atmospheric interference• Off-nadir angle of separate image• Sun• Shadow• Incorrect sensor calibration details

Benefits • No restrictions• Large archives• Large area

coverage• Repeatability

Concerns•Problems with automated DEM’s•Weather dependency•Heights to top of foliage/buildings•Height accuracy still at research stage

Satellites

General comments• Wide range of sensors with stereo capability • On-going program of launch of new satellites• Quickbird (Link), IKONOS (Link), SPOT (Link), IRS

(Link).• Recent research by a Japanese University has shown

0.5m height accuracy possible – LinkQuickbird archive at Link

Comparison Technology Cost/sq km Delivery Accuracy

(No image)

£5-10k 50 days/sq km new

Aerial Photography £3-5k 50 days new

Global archive

(No image)

£0.35k 120 days new

GB archive

SAR £0.064k

(Hts only)

Global archive 30cm

Quickbird £0.03k 50 days new

Min 100 sq km

Summary

There is an increasing amount of height data for flood modelling. The skill is to select the correct one for a given application. Considerations include:-

• How accurate do you want the heights?• How regular do you want it monitored?• What intensity of information do you want?• How much money have you got?• How quickly do you want it?

but.................

Other considerations

• Global warming – 10 - 20cm msl rise last century, estimated 50 cm this century - (Link)

• Glacial Isostatic Adjustment – 2 mm per annum (- in South, + in North) – (Link). General Info at (Link)

• OS recently adjusted UK geoid – up to 10cm variance

• Spring tides – up to 12m range - (Link and Link)• Tidal loading – up to 6cm variance in 1 day

Coffee break

Comparison Of Elevation Collection Technologies

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