Abstract Segmentation of multispectral remote sensing images using active support vector ma
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Segmentationofmultispectralremotesensingimagesusingactivesupportvectormachines
PabitraMitra*,B.UmaShankar,SankarK.PalMachineIntelligenceUnit,IndianStatisticalInstitute,Kolkata700108,IndiaReceived21October2003;receivedinrevisedform26January2004Availableonline15April2004
AbstractTheproblemofscarcityoflabeledpixels,requiredforsegmentationofremotelysensedsatelliteimagesinsupervisedpixelclassificationframework,isaddressedinthisarticle.Asupportvectormachine(SVM)isconsideredforclassifyingthepixelsintodifferentlandcovertypes.Itisinitiallydesignedusingasmallsetoflabeledpoints,andsubsequentlyrefinedbyactivelyqueryingforthelabelsofpixelsfromapoolofunlabeleddata.Thelabelofthemostinteresting/ambiguousunlabeledpointisqueriedateachstep.Here,activelearningisexploitedtominimizethenumberoflabeleddatausedbytheSVMclassifierbyseveralorders.ThesefeaturesaredemonstratedonanIRS-1Afourbandmulti-spectralimage.Comparisonwithrelatedmethodsismadeintermsofnumberofdatapointsused,computationaltimeandaclusterqualitymeasure.Ó2004ElsevierB.V.Allrightsreserved.
Keywords:Imagesegmentation;Semi-supervisedlearning;Transductivelearning;Querysupportvectormachine
1.IntroductionSegmentationisaprocessofpartitioninganimagespaceintosomenonoverlappingmeaningfulhomogeneousregions.Thesuccessofanimageanalysissystemdependsonthequalityofseg-mentation.Twobroadapproachestosegmenta-tionofremotelysensedimagesaregraylevelthresholdingandpixelclassification(Richards,1993).Inthresholding(Paletal.,2000)onetriestogetasetofthresholdsfT1;T2;...;Tkgsuchthatallpixelswithgreyvaluesintherange½Ti;Tiþ1Þcon-stitutetheithregiontype.Ontheotherhandinpixelclassification,homogeneousregionsaredeterminedbyclusteringthefeaturespaceofmultipleimagebands.Multispectralnatureofmostremotesensingimagesmakepixelclassifica-tionthenaturalchoiceforsegmentation.Intheunsupervisedpixelclassificationframe-work,severalclusteringalgorithmslikesplit-and-merge(Laprade,1988),fuzzyk-means(Paletal.,2000;Cannonetal.,1986),neuralnetworksbasedmethods(BaraldiandParmiggiani,1995),scalespacetechniques(WongandPosner,1993)andstatisticalmethodshavebeenusedforthepurpose
*Correspondingauthor.Address:DepartmentofComputer
Science,IndianInstituteofTechnologyandEngineering,Kanpur208016,India.Tel.:+91-0512-259-7584.E-mailaddresses:pmitra@iitk.ac.in,pabitra_r@isical.ac.in(P.Mitra),uma@isical.ac.in(B.UmaShankar),sankar@isi-cal.ac.in(S.K.Pal).
0167-8655/$-seefrontmatterÓ2004ElsevierB.V.Allrightsreserved.doi:10.1016/j.patrec.2004.03.004
PatternRecognitionLetters25(2004)1067–1074www.elsevier.com/locate/patrecofsegmentation.Statisticalmethodsarewidelyusedinunsupervisedpixelclassificationframeworkbecauseoftheircapabilityofhandlinguncertain-tiesarisingfrombothmeasurementerrorandthepresenceofmixedpixelswhichhavecertaindegreeofmembershiptomorethanoneclass.Ageneralmethodofstatisticalclusteringisbymeansoftheexpectationmaximization(EM)algorithm(Dempsteretal.,1977)anditsvariants(PalandMitra,2002).However,theunsupervisedpixelclassificationmethodshavemanylimitations.Thenumberofclustersareoftenunknown,whichre-sultsinregionmerging/splittingandalsohinderstheinterpretationofthesegmentedimages.Also,unsupervisedmethodsmostlygenerateconvexclusters,whichleadstodegradationinsegmenta-tionquality.Theaforesaiddifficultiesdonotariseinsuper-visedpixelclassification,andseveralmethodsbasedonneuralnetworks,geneticalgorithms(BandyopadhyayandPal,2001)hasbeendevel-opedinthisframework.Recently,supportvectormachinesarebecomingpopularforclassificationofmultispectralremotesensingimages(Brownetal.,2000;Huangetal.,2002).Theprimaryprobleminsupervisedpixelclas-sificationisthepureavailabilityoflabeleddata,whichcanbeobtainedonlyfromgroundtruthsandbycostlymanuallabeling.Recently,activelearninghasbecomeapopularparadigmforreducingthedatarequirementoflargescalelearningtasks(Angluin,1988;Cohnetal.,1994).Here,insteadoflearningfrom‘randomsamples’,thelearnerhastheabilitytoselectitsowntrainingdata.Thisisdoneiteratively,andtheoutputofastepisusedtoselecttheexamplesforthenextstep.Severalactivelearningstrategiesexistinpractice,e.g.,errordriventechniques,uncertaintysampling,versionspacereductionandadaptiveresampling.Supportvectormachines(SVM)areparticu-larlysuitedforactivelearningsinceaSVMclas-sifierischaracterizedbyasmallsetofsupportvectors(SVs)whichcanbeeasilyupdatedoversuccessivelearningsteps.OneofthemostefficientactiveSVMlearningstrategyistoiterativelyre-queststhelabelofthedatapointclosesttothecurrentseparatinghyperplaneorwhichviolatesthemarginconstraintmaximally(Mitraetal.,