专业英语.docx

专业英语.docx

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专业英语

Unit25DigitalTerrainModeling

Thecreationofdigitalmodelsoftheterrainisarelativelyrecentdevelopment,andtheintroductionofthetermDigitalTerrainModel（DTM）isgenerallyaccreditedtotwoAmericanengineers,MillerC.andLaFlammeR.A.,workingattheMassachusettsInstituteofTechnologyduringthelate1950s（PetrieandKennie,1990）.Thedefinitiongivenbythemisasfollows:

“AstatisticalrepresentationofthecontinuoussurfaceofthegroundbyalargenumberofselectedpointswithknownX,YandZcoordinatesinanarbitrarycoordinatefield.”

DigitalTerrainModelingistheelectronicprocessofrepresentingtopographyinthreedimensions.Itconsistsofanumberofsurfacepointsthatarerepresentativeoftheterrainanddesignatethepositionsofpointsinrelationtoacommonreferenceframe.Incommonusage,theXandYcoordinatefixthehorizontalpositionofthepoint,andZreferstotheelevation.ADTMmayinvolvearangeofgeographicalelementsandnaturalfeaturessuchasriversorridgelinesandmayincludederiveddataabouttheterrainsuchasslope,aspect,visibility,etc.

ThetermDigitalElevationModel（DEM）specificallyrelatestotheheightaboveadatumandtheabsoluteelevationofthepointscontainedinthemodel.Inanycase,thetermusuallyreferstothecreationofaregulararrayofelevations,normallyinasquaregrid,overtheterrain.ThemanipulationofthedatainsuchaforminacomputersystemisstraightforwardsinceaDEMisessentiallyatwodimensionalmatrix.DTMisamultistepprocessthatismadeupofthefollowingsequenceoftasks[C.P.LoandAlbertK.W.Yeung,2002]:

（1）Digitalterraindatasamplingisthestructuringandacquisitionsofdigitalterraindatabyphotogrammetric,cartographic,andfieldsurveymethods.

（2）DigitalterraindataprocessingisthemanipulationofdigitalterraintoensuretheirusabilitybyGIS.

（3）Digitalterraindataanalysisinvolvestheuseofalgorithmsandproceduresthatrestructuredigitalterraindataintousefulgeographicinformation.

（4）Digitalterraindatavisualizationentailsthedevelopmentofalgorithmsandmethodsthatallowtheeffectivedisplayoftheterraintoassistinspatialproblemsolvinganddecisionmaking.

（5）DigitalterraindataapplicationscomprisethepracticaluseofDTMindifferentfieldsofscienceandtechnology.

DigitalTerrainDataSampling

Therearetwoapproachestodigitalterraindatasampling:

systematicandadaptive.Insystematicterraindatasampling,elevationpointsaremeasuredatregularlyspacedintervals（RegularGrid）.

Theresultisamatrixofelevationvaluesthatisusuallyreferredtoasadigitalelevationmodel（DEM）.ThelocationsofelevationpointsinDEMareimplicitinthedatamodel.Whenadaptivesamplingmethodisused,elevationmeasurementsaremadeatselectedpointsthatareassumedtoberepresentativeoftheterrain.Theresultisacollectionofirregularlydistributedelevationvaluesthatmustbeproperlystructuredbeforetheycanbeusedforfurtherprocessing.Sincethemethodoftriangulationisusedtobuildthespatialframeworkforstoringtheelevationvalues,thedatacollectedbythisapproacharereferredtoasatriangularirregularnetwork（TIN）.TINisamodeloftheverticesoftrianglesgeneratingfromthedistributeddataset,whichformuniquelyspacednodes.Unlikethegrid（regularlyspacedintervals）,theTINprovidesfordenseinformationincomplexareasandsparseinformationinhomogeneousareas.TheTINdatasetsprovidetopographicalvalueamongpointsandtheirneighboringtriangles.TheTINdatamodelisdistinctfromtheDEMdatamodelintwoimportantways:

eachandeverysamplepointinaTINhasan（X,Y）coordinateandanelevation,orZvalue;theTINdatamodelmayincludeexplicittopologicalrelationshipbetweenpointsandtheirproximaltriangles.

Digitalterraindatamaybeacquiredbyavarietyofmethods,dependingonfactorssuchasthelocationandsizeoftheareaofinterest,thepurposeoftheterrainmodeling,andthetechnicalresourcesavailable.Generallyspeaking,groundsurveymethodsaremostsuitableforlargescaleterrainmodelingforengineeringandminingapplications.Atsmallerscalescoveringlargergeographicareas,photogrammetricmethodsarealwaysused.Currentlyterraininformationmaybeacquiredbyremotesensorsonairborneplatforms.However,asavastamountofterraindataarealreadyinexistenceintopographicmaps,manynationalmappingagenciestendtoacquiredigitalterraindatabydigitizingexistingmaps.Digitalterraindatasetsobtainedinthiswayareusuallyofasmallscaleandhaveanationalorregionalcoverage.

DataProcessingandAnalysis

Thecoreactivitiesofdigitalterrainmodelinginvolvethreetypicalphases:

processingterraindatatoensurethattheyareoptimizedforstorageandapplication;performinganalysistoconverttopographicattributes（elevation,slope,aspect,profilescurvature,andcatchments（area）derivedfromDEMsorTINsintousefulterraininformation;andpresentingtheterraininformationtotheuserinaneasilyunderstandablemanner.MeshsimplificationistheprocessbywhichaTINmodelisconstructedfromDEMdata.TheobjectiveistoextractfromaDEMthetopographicallyimportantelevationpointstoformaTINwiththeminimumnumberofpointspossible,whileatthesametimepreservingthemaximumamountofinformationaboutterrainstructure.ThisisanessentialfunctionindigitalterraindataprocessingbecauseitallowstheusertotakeadvantageofboththeDEMandTINmodels.TheDEMapproachismoresuitablethantheTINapproachforautomaticdigitalterraindatasampling.Withmeshsimplificationtechniquesandpowerfulcomputers,itisnowpossibletoacquiredigitalterraindatawithDEMapproach.ThedataarethenprocessedtoformTINstooptimizestorageandmodelingefficiency.

Interpolationistheprocessbywhichelevationvaluesofoneormorepointsingeographicspaceareusedtoproduceestimatedvaluesforpositionswhereelevationinformationisrequired.ItisusedforcontouringandforthegenerationofDEMsfromselectivelyorrandomlysampledelevationpoints.Asurface-fittingalgorithmiscommonlyusedtoimprovetheresultofterrainmodeling.Itmaybelinearornonlinear,dependingontheorderofthepolynomialequationsusedforestimatingtheelevationoftherequiredpoint.

DigitalTerrainVisualization

TheultimateaimofDTMistopresentrelevantterraininformationaboutagivengeographicspacethatresultsfromtheanalysisofthecharacteristicsofitstopographyandrelatedspatialphenomena.VisualizationisthereforeanintegralcomponentofDTM,formstheperspectiveofbothprocessandtechnology.

Therearenumerouswelldevelopedtechniquesfordigitalterrainvisualization.Accordingtothedimensionofthegraphicaldisplay,digitalterrainvisualizationcanbeclassifiedas:

twodimensional,twoandahalfdimensional,three-dimensional,andmultidimensional.Contourlinesarethemostconventional,andprobablystillthemostcommonlyusedmethodofdigitalterrainvisualization.Usingcontourlineisaquantitativewayofrepresentingthree-dimensionalterrainintwodimensionsbecausenumericalmeasurementsofelevationmaybereadilymadeonthedisplay.Atwo-and-a-halfdimensionaldisplayisbasicallyanisometricmodel.Insuchamodel,theZattributeassociatedwithanX,YlocationisprojectedontoanX,Y,Zcoordinatesreferencesystem.ThistransformationsthemapofZattributesforanX,YpositionsothateachZattributedefinesapositionontheZaxis,creatingasurfacethatisperceivedasthreedimensional.Athree-dimensionalterrainmodelisasolidmodelinwhichmanyX,Y,Zdatapointsareusedtoformasolidstructurethatmaybevisualizedinaperspectiveview.Unlikeatwo-and-a-half-dimensionalview,whichpresentsonlyapseudoperspectiveoftheterrain,athree-dimensionalterrainmodelisananalogforthephysicalspaceinnatureasperceivedbyanobserver.Three-dimensionalterrain-modelingallowsthefullspecificationofthree-dimensionaloperationsontheobjectsandphenomenawithintheconstraintsofthegeometricalmodelused.

ItrepresentsoneofthemostexcitingdevelopmentsinGIStechnology.

ApplicationofDTM

OnceaDTMhasbeencreated,contours,profiles,volumesbetweensurfacesandthree-dimensionaldisplaysareavailable.Inthelastseveralyears,therehasbeenatremendousgrowthintheapplicationofDTMs,notonlyinthetraditionalfieldsofgeography,surveyingandmapping,andearthandenvironmentalsciences,butalsoinlandscapedesign,biodiversityanalysis,environmentalimpactanalysisandsiteselectionfortelecommunicationfacilities.

Unit26ApplicationsofGIS

Duetoincreasingcomplexityoftherealworldsituations,morechallengesemergeinknowingaboutthepreciousearthandalsoinplanninganddecisionmakingprocesses.GISisnowadaysconsideredasanimportanttoolinplanninganddecisionmaking.Ithasbeenfoundappliedinmanyfields,suchascadastralmapping,landuseplanning,forestry,wildlifemanagement,infrastructureplanning,zoning,military,environmentalmonitoring,networkplanning,facilityselecting,archaeology,agriculture,businessmarketingandsales,banking,healthandhumanservices,landscapearchitecture,librariesandmuseums,marine,coast,andoceans,media,miningandearthsciences,petroleum,retailbusiness,stateandlocalgovernment,transportation.Someoftheadvancedapplicationsatpresentinvolveairtrafficmonitoring,roadnavigation,crimeanalysis,andsoon.Atpresent,GIShasbecometheacceptedandstandardmeansofutilizingspatialdata.Likewise,theuseofspatial