DTIC ADA530738: Bathymetry Retrieval from Hyperspectral Imagery in the Very Shallow Water Limit: A Case Study from the 2007 Virginia Coast Reserve (VCR'07) Multi-Sensor Campaign

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MarineGeodesy,33:53–75,2010
Copyright©Taylor&FrancisGroup,LLC
ISSN:0149-0419print /1521-060Xonline
DOI:10.1080/01490410903534333
Bathymetry Retrieval from Hyperspectral Imagery
in the Very Shallow Water Limit: A Case Study from
the 2007 Virginia Coast Reserve (VCR’07)
Multi-Sensor Campaign
CHARLESM.BACHMANN,1MARCOSJ.MONTES,1
ROBERTA.FUSINA,1CHRISTOPHERPARRISH,2
JONSELLARS,2ALANWEIDEMANN,3WESLEYGOODE,3
C.REIDNICHOLS,4PATRICKWOODWARD,4
0
1
20 KEVINMcILHANY,5VICTORIAHILL,6

h
rc RICHARD ZIMMERMAN,6DANIELKORWAN,1
a
M
5 BARRYTRUITT,7ANDARTHURSCHWARZSCHILD8
2

3
5
9: 1NavalResearchLaboratory,RemoteSensingDivision,Washington,D.C.,USA
1
: 2NOAANGS,SilverSpring,Maryland,USA
t
A 3NavalResearchLaboratory,OceanographyDivision,Stennis,Mississippi,USA
]
y
ve 4MarineInformationResourcesCorporation,EllicottCity,Maryland,USA
r
Su 5U.S.NavalAcademy,PhysicsDepartment,Annapolis,Maryland,USA

al 6OldDominionUniversity,Norfolk,Virginia,USA
c
i
og 7TheNatureConservancy,Nassawadox,Virginia,USA
l
eo 8UniversityofVirginia,DepartmentofEnvironmentalScience,Charlottesville,
G

S Virginia,USA
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[

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d
e Wefocusonthevalidationofasimplifiedapproachtobathymetryretrievalfromhyper-
d
a
o spectralimagery(HSI)intheveryshallowwaterlimit(lessthan1–2m),wheremany
l
wn existingbathymetricLIDARsensorsperformpoorly.Inthisdepthregime,nearinfra-
o
D red(NIR)reflectancedependsprimarilyonwaterdepth(waterabsorption)andbottom
type,withsuspendedconstituentsplayingasecondaryrole.Ourprocessingframework
exploitstwooptimalregionswhereasimplemodeldependingonbottomtypeandwater
depthcanbeappliedintheveryshallowlimit.Thesetwooptimalspectralregionsareat
alocalmaximuminthenearinfra-redreflectancenear810nm,correspondingtoalocal
minimuminabsorption,andamaximuminthefirstderivativeofthereflectancenear
720nm.Thesetworegionscorrespondtopeaksinspectralcorrelationwithbathymetry
atthesedepths.
Keywords Hyperspectral,shallow-waterbathymetry,bottomtype,IOP,Hydrolight,
VirginiaCoastReserve
Received26March2009;accepted14October2009.
AddresscorrespondencetoCharlesM.Bachmann,NavalResearchLaboratory,RemoteSensing
Division,CoastalScienceandInterpretationSection,Code7232,Washington,DC20375.E-mail:
[email protected]
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4. TITLE AND SUBTITLE 5a. CONTRACT NUMBER
Bathymetry Retrieval from Hyperspectral Imagery in the Very
5b. GRANT NUMBER
ShallowWater Limit: A Case Study from the 2007 Virginia Coast Reserve
(VCR?07) Multi-Sensor Campaign 5c. PROGRAM ELEMENT NUMBER
6. AUTHOR(S) 5d. PROJECT NUMBER
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54 C.M.Bachmannetal.
IntroductionandBackground
Numerous institutions, including the Naval Research Laboratory (NRL), have developed
look-uptablesforremoteretrievalofbathymetryandin-wateropticalpropertiesfromhy-
perspectralimagery(HSI)(Mobleyetal.2005).Forbathymetryretrieval,thelowerlimitis
theveryshallowwatercase(heredefinedtobelessthan2m),adepthrangethatisnotwell
resolvedbymanyexistingbathymetriclightdetectionandranging(LIDAR)sensorssuch
as Scanning Hydrographic Operational Airborne Lidar Survey (SHOALS) (Ebrite, Pope
and Lillycrop 2001). The ability to rapidly model these shallow water depths from HSI
directlyhaspotentialbenefitsforcombinedHSI/LIDARsystemssuchastheCompactHy-
drographicAirborneRapidTotalSurvey(CHARTS)(Smith,IrishandSmith2000).Among
themanypotentialapplicationareasistheNationalOceanicandAtmosphericAdministra-
tion (NOAA) LIDARshoreline mapping initiative (White2007), inwhich shallow-water
topo/bathydatasetswithseamlesscoverageinanalongshoreswathencompassingthein-
tertidalzonecanbeusedforextractionofbothmeanlowerlowwater(MLLW)andmean
highwater(MHW)shoreline.Inthisstudy,wefocusedonthevalidationofanearinfra-red
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1 feature corresponding to a local minimum in absorption (and therefore a local peak in
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h reflectance), which can be correlated directly to bathymetry with a high degree of confi-
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ar dence. Our study also concludes that a second near infra-red feature near a peak in the
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5 derivativeofthereflectanceisanotheroptimalregionforretrievalintheveryshallowlimit.
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53 Compared to other VNIR wavelengths, these particular near-IR features correspond to a
:
19 peak in the correlation with depth in this very shallow water regime. In this article, we

:
t develop a simplified computational framework that exploits these near infra-red features
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y] andthefactthatinthenearinfra-red,intheveryshallowwaterlimit,reflectancedepends
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rv primarilyonwaterdepth(waterabsorption)andbottomtype,withsuspendedconstituents
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playingasecondaryrole.
l
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ic The general problem of bathymetry retrieval from HSI is more complicated out of
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lo theselimits.Atvisibleandnearinfra-red(VNIR)wavelengths,reflectanceisdetermined
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e
G byavarietyoffactorsincludingwaterdepth,bottomtype,andthepresenceofsuspended

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[U constituentssuchascolor-dissolvedorganicmatter(CDOM),suspendedsediments,chloro-

y: phyll,phytoplankton,andothersuspendedconstituents(Mobley1994).Thegeneralprob-
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d lem of retrieving depth and other water properties has been approached using spectral
e
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oa look-up tables (Mobley et al. 2005) in which a forward radiative transfer model such as
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wn Hydrolight(MobleyandSundman2000)isexecutedrepeatedlywithvaryingwatercolumn
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properties,depth,andbottomtypes.Tobecomprehensive,theselook-uptablesmustbelarge
andmayneedtobetunedtospecificcoastaltypesbecauseofdifferencesinbottomtypes
and water properties, which may vary significantly with coastal type. Other approaches
to the problem of bathymetry retrieval from HSI have included a semi-analytical model
(Leeetal.1999,2001).BoththeLUTapproachandsemi-analyticalmodelssimultaneously
retrievemorethanoneinherentopticalproperty(IOP)ofthewatercolumn.
RecentcommercialbathymetricLIDARsystems,suchastheSHOALS(Ebrite,Pope
andLillycrop2001),havebeendesignedforimprovedperformanceatthedeependofthe
operatingrange,possiblyattheexpenseoflimitingperformanceinshallowwater(0–4m
depth).Inparticular,thecombinationofarelativelylarge(severalnanosecond)laserpulse
width(whichincreasesthelengthofthetotalsystemresponsefunction),largespotsizeat
thewatersurface(duetodesignrequirementsimposedbyeyesafetyconsiderations with
high-poweredlasers),andgeometricpulsestretchingatthewatersurface(largelydue,in
the case of the SHOALS, to a fixed 20◦ incidence angle) make it difficult to resolve the
bottom in returns from very shallow water (less than ∼2 m). Typical data gaps found in
VeryShallowWaterBathymetryfromHSI 55
Figure1. Typical SHOALS bathymetry coverage superimposed on a Google Earth image. These
datawerecollectedbytheU.S.ArmyCorpsofEngineersinCurrituckCounty,N.C.,inJuly2004.
0
01 Thesensorfails toresolvethebottom inreturnsfromveryshallowwater regions,leading todata
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h gaps.
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2 manyoftheSHOALSstandardproductsareshowninFigure1.Somefairlyrecentstudies
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: have focused on using advanced digital signal processing techniques to estimate bottom
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: depths from LIDAR waveforms in these very shallow regions, with encouraging results
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A (Yangetal.2007).However,alargenumberofexistingSHOALSdatasetsexhibitdatagaps,
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y
e such as those shown in Figure 1. In addition, the very low (few hundred meter) altitude
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u requiredfordataacquisitionbybathymetricLIDARsystemsmakesacquisitionimpossible
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al inmanyregionsoftheworld.Inthisarticle,wefocusonthisveryshallowwaterregime,
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gi specificallyontheretrievalofbathymetryfromaspectralrangewherearelativelysimple
o
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o model forreflectance can beassumed and whereafull-scaleLUT may not benecessary.
e
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S Comparedtothereflectanceatvisiblewavelengths,whereallcontributionsfromthewater
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[ column must be carefully considered, near infra-red wavelength reflectance (beyond 800
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B nm)isprimarilydependentonwaterabsorptionanddepth,alongwithbottomreflectance,

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de withwatercolumnconstituentsplayingasecondaryrole.Wewillfindthatanearoptimal
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lo wavelengthforthisretrievalisapproximately810nm.Asecondoptimalregionalsoappears
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o nearapeakinthefirstderivativeofthereflectancenear720nm.
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WaterColumnReflectanceintheNearInfra-RedandHydrolightModels
In this section, we demonstrate that varying concentrations of suspended constituents
do not significantly affect the reflectance at 810 nm or 720 nm but indeed are a large
contributing factor to the shorter wavelength returns. Likewise, the wavelength ranges
near 810 nm and 720 nm are particularly sensitive to water depth in the very shallow
limits. To demonstrate this theoretical expectation, we have run a series of models using
Hydrolight. This model allows the user to vary the concentration of constituents, bottom
type,depth,andotherparametersandlookattheircontributiontoremotesensingreflectance
as a function of wavelength. Hydrolight supports the ingest of field measured spectra.
To test our model assumptions, a bottom reflectance measured by us in the intertidal
zone at the Virginia Coast Reserve Long-term Ecological Research (VCR LTER) site
(http://www.lternet.edu/sites/vcr/)wasusedasamodelinputtoHydrolight.Totalsuspended
sediment (TSS), CDOM, and chlorophyll concentration were held constant with values
56 C.M.Bachmannetal.
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ey that might be found in coastal lagoons such as those at the VCR LTER. With the water
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u column properties fixed, depth was varied, and the resulting reflectance was calculated
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al fromHydrolight.Figure2showstheresultingreflectanceforafixedsetofwatercolumn
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gi propertiesasthedepthisvaried.Italsoshowsapeakinthevicinityof810nmthatiswell
o
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o correlatedwithdepth.Thespectralregionnear720nm(maximumderivativeofreflectance)
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S is another peak in this correlation. Empirical studies of the visible and near IR spectrum
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[ inlaboratorysettingshaveshownarelativelybroadpeaknear810nm,correspondingtoa
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y
B localminimumintheliquidwaterabsorption(CurcioandPetty1951).

d
de We also used Hydrolight to show that varying suspended constituents are indeed
a
lo expected to make only a small difference near 810 nm and 720 nm, while at shorter
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o wavelengthsinthevisible,thesechangeshaveasignificantimpactthatcannotbeignored
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inmodeling.Figure3aillustratesthispointforacaseinwhichtotalsuspendedsediment
(TSS) is varied while all other inherent optical properties (IOPs) are held fixed and the
depthisconstantforaspecificbottomtype(inthiscase,thefieldspectrumofourbottom
substratetakenfortheoystershellbottomsiteatWreckIsland,Virginia,describedlaterin
thisarticle).Weprovidetwoexamples,oneatafixeddepthof0.4mandtheotherat0.8
m. Hydrolight reflectances are shown for TSS = 2, 3, and 5 g/m3. Note the much larger
variationinreflectanceatwavelengthsbelow700nmcomparedtolongerwavelengths.
Whilethetotalcontributionstoreflectancearegovernedbytheequationsofradiative
transfer(Mobley1994)andareincludedinHydrolight,theHydrolightsimulationssuggest
the possibility of an approximate form in this wavelength regime. The simulations of
Figures2and3showthatinthelongerwavelengths,thedominantfactoristheabsorption
duetoliquidwater,withtheinfluenceofthescatteringbecominganegligibleterm.Under
these assumptions, and ignoring fluorescence and Raman scattering for the moment, the
attenuationisapproximatelyduetotwoterms:oneduetothereflectanceofthebottomand
oneduetothepropertiesofthewatercolumnwiththesecondbecomingsmallerinthenear
VeryShallowWaterBathymetryfromHSI 57
Variation of Suspended Sediment, depth=0.8m,
Variation of Suspended Sediment, depth=0.4m,
chl=3mg/m3, chl=3mg/m3,
cdom(440nm) = 0.1/m cdom(440nm)=0.1/m
0.07
0.06
0.06
drolight Reflectance 0000....00002345 rrreeeffflll(((tttssssss===235ggg///mmm^^^333))) drolight Reflectance0000....00002345 rrreeeffflll(((tttssssss===235ggg///mmm^^^333)))
y y
H H
0.01
0.01
0
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350 450 550 650 750 850 950
350 450 550 650 750 850 950
Wavelength (nm)
Wavelength (nm)
0
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2 Figure3a. SimulatedreflectanceusingHydrolight,forvaryingTSS,withallotherIOP’sheldfixed,
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rc andatfixeddepths:(left)0.4m,(right)0.8m.
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5
2

3
5 infra-redwavelengths:
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At: Rλ ∼ρbottome−α(λ)d +R∞(λ)(1−e−3α(λ)d) (1)

]
y
e
v where R is the reflectance at wavelength λ; ρ is the bottom reflectance; α(λ) is a
r λ bottom
u
S wavelength-dependentattenuationcoefficient,whichatlongernearinfra-redwavelengths
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ca isdominatedbytheabsorptionduetoliquidwaterandislessaffectedbyscatterasshown
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g
o in the Hydrolight simulations; d is the depth (for our purposes we define quantities such
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Geo thatdisincmandα(λ)isincm−1);andR∞(λ)isareflectanceofthewatercolumnwhen

[US thereisnobottomcontribution.Atlongernearinfra-redwavelengths,R∞(λ)isnearzero,

: whereasatshorterwavelengthsthistermissignificantduetothescatteringofsuspended
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d constituents.Eq.(1)issimilartoapproximateformsforthesetwocontributionsfound,for
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ad example,inLeeetal.(1994),whereα(λ)isacompositeoffactorssuchasthequasi-diffuse
o
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n attenuationcoefficient(sumofabsorptionandscatteringcoefficients)andthedownwelling
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o
D distributionfunction.Withthisassumption,welookforaregressionbetweenln(R )andd,
λ
assumingthatinourshallowwaterlimitthefirstterminEq.(1)isthemostsignificant.In
thisapproximation,wehave:
In(R )=−α(λ)d+β(λ) (2)
λ
where β(λ) ∼ ln(ρ ) is a wavelength-dependent offset, depending on the bottom re-
bottom
flectance.
Numerically, we can show that this is further justified by extending the Hydrolight
simulations in Figures 2 and 3a to deeper depths. Figure 3b shows ln(R ) versus d (in
λ
meters)forλ=722.5nmand812.5nm,nearthetwooptimalwavelengthsforthistypeof
retrieval.InthisfigurewehaveplottedtheeffectsofincreasingtheconcentrationofTSS
andchlorophyll.Therelationshipbetweenln(R )anddislinearintheshallowlimit,and
λ
departurefromthislinearlimitvariesdependingonconcentrationsofTSSandchlorophyll.
More important, the linear slope is the same regardless of concentration, and the point
of departure from linearity marks the point at which the second term in Eq. (1) due to
58 C.M.Bachmannetal.
thewatercolumnbecomesmoresignificantandcannolongerbeneglected.Thedeparture
fromlinearityoccurssoonerathigherconcentrationsofsuspendedconstituentsasscattering
andabsorptionbecomemoreimportant.Evenatlowconcentrations,extinctioneventually
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Figure3b. Hydrolightsimulationsshowingthenaturallogarithmofreflectancevs.depthinmeters
at two NIR wavelengths: 722.5 nm (left column) and 812.5 nm (right column) for varying levels
oftotalsuspendedsediment(TSS)atlow,fixedchlorophyllandCDOMconcentrations(toprow),
andforvaryinglevelsofchlorophyllconcentrationatfixedTSSandCDOMconcentrations(middle
row).(Bottomrow)NegligibleimpactoffluorescenceandRamanscatteringfor(bottom,left)low
andhighconcentrationsofchlorophylland(bottom,right)lowandhighconcentrationsofTSSat
722.5nm.
VeryShallowWaterBathymetryfromHSI 59
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ve Figure4. NRLCASI-1500quicklookcompositeoftheVCR’07multi-sensorcampaign.Thesensor
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Su suitecoveredover1880km2,someareasmultipletimesatdifferenttidalstages.Areasusedforshallow

al waterbathymetryvalidationduringtheVCR’07campaignandearlierin2005areindicatedbyred
c
gi arrows.
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[U becomes the most important factor at these longer NIR wavelengths, which is why the

y: curvesreachanasymptoticlimitinallcases.TSShasthebiggestimpactonthedepthof
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d departurefromlinearityinthesecurves,whilechlorophyllhasanimpactbutnotasstrong.
e
d
oa There is virtually no dependence on CDOM concentration at these NIR wavelengths, so
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wn these graphs are not shown. Finally, the impact of fluorescence and Raman (inelastic)
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scatteringisnegligibleasshowninFigure3b.Forthisreason,thefluorescenceandRaman
termswereignoredinthediscussionsurroundingEq.(1).
VirginiaCoastReserve2007(VCR’07)Campaign
DuringSeptember2007,theNavalResearchLaboratory(NRL)andanumberofcollabo-
rating institutions undertook a multisensor airborne data collection and in situ validation
campaign at the Virginia Coast Reserve (VCR’07). Airborne, in-water, and land-based
measurementstookplaceoverthecourseofthe2–1/2weekexerciseandcovereda95km
stretchofbarrierislands,inlets,coastallagoons,tidalflats,andmainlandmarshsystems.
In all, some 1880 km2 were covered by the NRL sensor suite (Figure 4), some areas
multiple times at different tidal stages. NRL flew three sensors on a Twin Otter aircraft:
a Compact Airborne Spectrographic Imager (CASI-1500), which is a visible and near
infra-red(VNIR)HSIsensor(www.itres.com);aSurfaceOptics(www.surfaceoptics.com)
InGaAs SWIR HSI sensor operating in the 0.9–1.7 µm spectral region; and a single-
60 C.M.Bachmannetal.
channelIndigoSystemsMerlinmidwaveIRcameraoperatinginthe3–5µmspectralrange
(www.corebyindigo.com/products/legacy.cfm). This article focuses on the CASI imagery
takenduringtheexperimentandtheinsituvalidationdatacollectedtostudytheNIRfeature
describedearlier.OtherlittoralfocusareasstudiedduringtheVCR’07experimentincluded
the remote retrieval of soil bearing strength (Bachmann et al. 2008), biomass retrieval,
land-covermapping(Bachmannetal.2003),andseagrassandoysterreefmonitoring.
During the experiment, the NRL CASI-1500 was operated at an altitude of approxi-
mately3000m,withanominalgroundsampledistance(GSD)ofroughly1.5m.Atthis
spatialresolution,theswathwidthwasapproximately2.25km.AlthoughtheCASI-1500is
capableofupto288spectralchannels,spectraldatawerebinnedto96channelstoimprove
signaltonoiseratio(SNR).Withthisnumberofchannels,thespectralbinshadasizeof
approximately7nm.SincetheCASI-1500hyperspectralimagerywasacquiredinacoastal
region,withwaterpresentineveryscene,dataweretakeninamannerthatwouldminimize
glintbyconfiningdatacollectiontospecificwindowswhenthesolarelevationanglewas
in the range between 30 and 60◦. A further measure to reduce glint was that flight line
0 headingsduringdataacquisitionwereeitherintooroutofthesun.Thiswasaccomplished
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20 bysetsoflinesinhourlywindowsinsuchawaythatfollowedtheaverageheadingintoor

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c outofthesunforthewindow.
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M Fortheveryshallowbathymetrystudy,fieldvalidationeffortsusedAnalyticalSpectral
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2 Devices(ASD)spectrometers(www.asdi.com)positionedonatripodinashallowcoveon
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: thenorthwesternshoreofWreckIsland,Virginia,andintheshallowstretchesofamuddy
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: tidalcreekandsaltmarshsysteminthenorthernendofParramoreIsland,Virginia.Depth
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A andASDspectralmeasurementswererecordedonafallingtideuntilthesubstratewascom-
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ey pletelyexposed,allowinganinsitubottomreflectancetobeobtainedinthemeasurement
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u suite. Bottom reflectance was also measured by divers using an underwater spectrometer
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al atotherlocations.Previously,duringfieldworkinOctober2005,ASDmeasurementshad
c
gi beencollectedintheshallowwatersontheedgeofasaltmarshsysteminthenorthernend
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o of Smith Island. The ground truth for VCR’07 included postprocessed kinematic (PPK)
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S GPSdatafortheinter-tidalregiononthenorthernendofWreckIslandtosupportproduct
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[ validation.Inaddition,depthsoundingsweretakenintheshallowwatersbehindtheisland
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B andintheadjacentchannelseparatingWreckfromCobbIsland.Attheconclusionofthe

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de study,theNASAExperimentalAirborneAdvancedResearchLIDAR(EAARL)acquired
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lo datainconjunctionwiththeNRLsensorsuite,albeitduringaperiodwhenweathercondi-
n
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o tions were not optimal and turbidity was high. For this study, the PPK GPS ground truth
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dataservedastheelevationreferencedataforbathymetryretrievalvalidation.
Figures5and6showthemeasuredspectralcurvesforeachdepthcollectedduringthe
measurement cycle at Parramore and Wreck Islands, respectively, during VCR’07, along
withinsituphotosandtheregressionat810nm.Asimilarregressionwascarriedoutfor
datatakenbyusonthenorthernendofSmithIsland,Virginia,inOctober2005(Figure7)for
asand/siltbottomonthewaterymarginbetweenbeachandsaltmarshzones.Asdescribed
earlier,thefocuswasonthelongerwavelengthsofthenearinfra-redspectrum,butthefull
rangespectrumfrom0.35–2.5µmalsowascollected.Thevisibleandnearinfra-redarethe
portionsofthespectrumusuallyusedinwaterapplications,sincethereflectancebeyonda
micronisnearzero,exceptwhenconstituentsinthewatercolumnarefullymixedtothe
surface layer. The visible shallow water spectra in Figures 5, 6, and 7 show a relatively
broadpeaknear810nm,correspondingtoalocalminimumintheliquidwaterabsorption
(CurcioandPetty1951).Thereisasimilarpeaknear1.076µm,whichisofasimilarorigin,
thoughofalesspronouncedcharacter.
VeryShallowWaterBathymetryfromHSI 61
Toassesstherelativemeritsofwavelengthselectionattheselongerwavelengthsand
confirmtheoptimalityofthechoiceofthe810nmand722nmfeatures,wecomputedan
R2goodnessoffitbetweenalinearregressionofln(R )andd,whereR isthereflectance
λ λ
atwavelengthλandd isthedepthincm.Thisisbasedonourearlierassumptionthatthe
bottom type and water absorption and depth are the most significant factors in the very
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Figure5. (Top)ParramoreIslandinsituspectrometrysiteonsaltmarshtidalcreekbank,September
12,2007,withmuddybottom;(middle)NRLCASI1500imageofParramoreIslandonsameday,
showing the ASD spectral measurement site (yellow circle) where spectral reflectance vs. depth
profileswererecorded:(top,left)beginning(nearhightide)(top,right)andend(nearlowtide)of
measurement cycle. (Bottom right) Spectral samples at depths varying from 90 cm to 0 cm (wet
substrate).(Bottom,left)ln(R810nm)vs.d(depthincm).