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TetraVision#3,2014
Tech Talk
Michael Grossman
LIGHT DETECTING AND RANGING (LIDAR)
The toolsof science have accelerated exponentially the speedandscope of inquiries tofacilitateproject
work. They make the worldmore transparent,helpsolveourmostpressingproblems, andsometimes
take scientificknowledge toahigherlevel.
One tool,lightdetectingandranging(LiDAR), atechnology thatusesahighpoweredaerial laser,
has made prodigiouscontributionsto science, engineering,andurbanplanning since the 1980s. Within
the past tenyears,advancesin airborne LiDARhave givenTetraTech’s GeomaticTechnologies Groupa
meansto apply thisremote sensingtechnology togetherwithafull suite of otherservicesto projects
involvingwater,energy,mining, transportation, remediation,andconstruction.
Geomatics—alsoreferredtoas geospatial technology—involvescollecting,integrating,and
providinggeographicdataor, regardingLiDAR, spatiallyreferenced data. LiDARmeasuresdistance using
pulsesof light (laser) toilluminate terrainandrenderingacalculationbasedonthe time the lighttravels
to and fromthe (first) objectinitspath. Since we know the speedof light—186,000 milespersecond—
a pulse’s travel time canbe convertedtoa range measurement. Inconjunctionwiththe positionand
orientationinspace of the sensor,thisyieldsa3D coordinate of the targetpoint. Laseremissionrates
range from a fewpulsespersecondto hundreds of thousands persecond,thuscollectingmillionsof 3D
pointsina short time.
“LiDARis commonlyused formanyapplicationsandmarkets,”saysTetraTech’sRenee
Walmsley,RemoteSensingProgramManagerbased inLafayette,California. Initially,LiDARhelped
generate basic3D data setsfor topographical mapping. Technological advances have increasedits
applications innumerably andhave eliminated muchof the laboriouswork scientistsand fieldteams had
beendoingfortheirprojectwork.
Archeologists canmapmore acreage indense jungles more accurately inthree hours than
groundsurveyingcaninthree years. Geologists,generatinghigh-resolutiondigital elevation maps,can

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detectminute,oftenimperceptibletopographicfeatures,suchasriverterraces,thatwouldnotbe
visible fromthe ground;anditcan detectsubtle declinesinglaciersand,combinedwithGPS (global
positioningsystem),faultsinthe earth’scrust. Biologistscanuse itfor biomassmeasurementsin
forests. Renewableenergyspecialists canmeasure windspeedsandwindturbulence more accurately
to helpsite new windfarms orimprove the energyoutputof existingones. Andeveryone benefitsin
areas of lowsurface visibility,steepslopes,anddangerousterrain.
How doesLiDARinstrumentationachieve such precision? Whenairborne,areascanningoccurs
throughprofile measurementsinthe directionperpendiculartothe flight path(“cross-track”) and
parallel tothe flightpath(“alongtrack”). By measuringthese profiles,positionsandelevationsof a
meshof points—LiDARpointclouds—arecreated. Combiningthe laserrange,laserscanangle, laser
positionfromGPS/IMU(inertial measurementunit),andlaserorientation,accurate groundcoordinates
can be calculatedforeachlaserpulse.
“Pointmeasurementisprecise andpointspacingcanbe extremelydense. Essentially point
spacingisthe numberof groundLiDARpointsthat fall withinacertainsquare meterof land. Usually,the
more groundpointsthe more accurate the elevationmodel,”Walmsleyexplains.
But it ismore complicatedthanthat. “We have to account for the aircraft’svelocity,
orientation, groundtopography,vegetative conditions, andthe particularspecificationsforthe project,”
saysTetra Tech’sMatt Coleman, Technical ProjectManager,whohas loggedmore than 2,000 hours of
flighttime performingLiDAR scans. “Combiningthe latestGPS,laser,andIMU technology,we can
eliminateorat leastminimizeerrorscreated bythe aircraft’smovementand atmosphericdisturbances.”
Coleman’saerial excursions have beenconducted fromasmall single-engineCessnatoa
Learjet. Each aircraft hasa customizedsensorporton itsbelly ora sensorpod attachedto the
acquisitionplatform.
In recentyears, LiDARsystems have increasedlaserrepetitionrates dramaticallyandare built
smallertofitinsmalleraircraftand UAVs. A decade ago,the sensorPRF (pulse rate frequency) peaked
around100kHz (100,000 times/second);more recentonesoperate upto500kHz.

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Flightmanagementtechnologyhas advancedsignificantly. LiDARflightplans occurinan office
and are providedtothe flightcrewona USB device fortransferto the LiDARsystem. Withflightlines
predetermined,the surveypilotusesa Yoke-mountedguidance screenthatensuresthe LiDARsystemis
flownatthe correct altitude,airspeed,andproperheading. Typically, aerial survey missions range from
fourto six hours, two missionsperday.
Colemansaystroubleshooting‘on the fly’isthe mostimportantpartof an aerial system
operator’sjob.“Replacingcables,harddrives,andothersensorcomponentsinthe airsavestime and
money,since you canavoidreturningtothe base of operationsforrepairs.” He says thisisimportant
whenworkingin remote partsof the world withlimitedaccesstoairportsnearthe projectarea.
Brownsville,Texas
LiDARhas provento be amongthe besttechnologiesformappingelectrical transmissionlines.
In Texas,BrownsvillePublicUtilities isconstructinga345 kV,double-circuittransmissionline,extending
approximately13 miles. TetraTechsupportedthe routing,structure spotting,structure design,and
otherapplications. Fromahelicopter,TetraTechcollectedhighdensityLiDARdata(over20 pointsper
sq meter) andhighresolution3"-GSD(groundsamplingdistance) imageryoverthe proposedroute.
“The data forthe new andexistinglineshelpbuild anaccurate 3-D model toperform
simulations,takingintoaccountdifferentelectrical loads, windspeedsandtemperature,toensure there
isno obstructionbetweentransmissionlines,vegetation,orotherman-made structures,”MattColeman
says. “Creatinga highaccuracy productin a short periodof time isa bigadvantage overtraditional
methodsof transmissionline mapping.”
Buckhorn Mountain, Washington
In 2009, Tetra Tech performedaLiDARstudyto supportthe Buckhorn MountainExploration
ProjectBaseline StudyProgram onfederal andstate lands innorthernWashingtonState. A mining
companysoughtto expandoperationsinanarea where goldhasbeenminedforthe past100 years.

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LiDARhelpeddetermine where tobuildanaccessroadon difficultterrainforbigrigsand facilitated
cultural resource investigationsforenvironmental documentationunderthe federalNationalHistoric
PreservationActandthe State Environmental PolicyAct. The TetraTech teaminterpretedthe datato
classifyvegetation,identifycultural featuresobscuredbythe forestgrowth,andlocate oldroadsfrom
previous miningoperations.In2010, TetraTech wildlife biologistsusedLiDARtoidentifystructuresthat
were potentially suitableforbathabitat.
Susitna River,Alaska
In south-central Alaska,the SusitnaRiverflowssouthwest intothe northside of the CookInlet, a
highlyvegetated areawithruggedterrain. The AlaskaEnergyAuthorityestimatesthat the SusitnaDam,
a hydroelectricproject120 milesnorthof Anchorage, will deliver2,800 GWh/yearof electricityto half of
the state’sRailbeltarea, whichincludes AnchorageandFairbanks.
LiDARwas a vital tool for developing the Federal EnergyRegulatoryCommission(FERC) License
application. The newlycollecteddatawascomparedwiththe historical photographsto determine
geomorphicchange. Itsupported environmental studies onthe project’s potential effectsonthe fluvial
geomorphology,aquatichabitat,andriparianhabitatalongthe riverfor185 milesdownstreamof the
dam. LiDAR data helped defineanaccurate geometryof the riverchannel andadjacentfloodplain to
develop useful modelsforhydraulics,sedimenttransport,waterquality,andice processes.Itidentified
faultsonthe earth'ssurface fora seismichazardstudyunderwayfordesignof aseconddam.
“Old-style aerial photosdon’tgetthroughthe [vegetation]density,”saysTetraTech’sBill
Fullerton, aprojectmanageronthe project. “WithLiDAR,thingsjustpop rightout at you. It’s amazing.”
Fullerton addsthattime gettingdataismuch fasterbutnot instantaneouswithLiDAR. “There’salotof
processing—billionsof points—runningthroughcomputers. There’smuchhumanworkwithpointsyou
don’twant. Renee’steamdoesanexceptional jobsortingthisout.”

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Walmsleysaysprocessing usefulpointdatadependsonthe area’ssize andotherfactors. The
time forSusitnaRiver,BuckhornMountain,andBrownsville PublicUtilities rangedfromafew weeksto
a fewmonths.
The Mergingof Art and Science
The imageryLiDARproduces—itshighcolorcontrastsandintense saturation—rivalsapainter’s
brushor an illustrator’spen. Discerningthe heightandtexture of objects andthe contoursof terrain
usingmulti-spectral images takenfromanaerial path choreographedprecisely,makesthe artof science
no philosophical musing andnotrivial partof empirical inquirieswhose resultssupport manyscientific
disciplines. LiDAR,however, contributes somethinginvaluable tothe art-science aesthetic. Whereasart
inmany forms usesscience to representthe intangible;the science of LiDARusesartto representwhat
is tangible. Andall practitionersof science are betterforit.