测绘工程专业英语翻译第三章.docx

测绘工程专业英语翻译第三章.docx

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测绘工程专业英语翻译第三章

DistanceMeasurement

Oneofthefundamentalsofsurveyingistheneedtomeasuredistance.Distancesarenotnecessarilylinear,especiallyiftheyoccuronthesphericalearth.InthissubjectwewilldealwithdistancesinEuclideanspace,whichwecanconsiderastraightlinefromonepointorfeaturetoanother.Distancebetweentwopointscanbehorizontal,slope,orvertical.

Horizontalandslopedistancescanbemeasuredwithlotsoftechniquesofmeasurementdependingonthedesiredqualityoftheresult.Ifthepointsareatdifferentelevations,thenthedistanceisthehorizontallengthbetweenplumblinesatthepoints.Heregivesabriefsummaryofrelevanttechniquesandtheirrespectiveaccuracies:

PacingandOdometer

Pacingisaveryusefulformofmeasurementthoughitisnotprecise,especiallywhensurveyorsarelookingforsurveymarksinthefield.Pacingcanbeperformedatanaccuracylevelof1/100~1/500whenperformedonhorizontalland,whiletheaccuracyofpacingcan’tberelieduponwhenpacingupordownsteephills.Theodometerisasimpledevicethatcanbeattachedtoanyvehicleanddirectlyregistersthenumberofrevolutionsofawheel.Withthecircumferenceofthewheelknown,therelationbetweenrevolutionsanddistanceisfixed.

OrdinaryTapingandPreciseTaping

Tapingisaverycommontechniqueformeasuringhorizontaldistancebetweentwopoints.Ordinarytapingreferstotheverycommontapesthatwecanbuytheminstores,suchastheplastictapesorpolytapes.Suchtapeshavelowprecisionindistancemeasurementswithabout1/3000~1/5000.Theprecisetapingreferstothesteeltapesandwhicharemuchmoreexpensivethantheplastictapeandhavehigherprecisionof1/10000~1/30000.Invartapesarecomposed35%nickeland65%steel.Thisalloyhasaverylowcoefficientofthermalexpansion,makingthetapesusefulinprecisedistancemeasurement.Manytapesarenowgraduatedwithfootunitsononesideandmetricunitsonthereverseside.Metricunitsareinmeters,centimeterandminimeterwiththetotallengthof20m,30m,50mand100m.

IfwewanttomeasurethehorizontaldistancebetweenthetwopointsAandB,wecandolikethis:

WithzeroofthetapetothehigherpointBandtapegoingalongthepointA,wecanmeasurethehorizontaldistancebyusingtheplumbbobwithpumplineenteringtothepointA.Tojudgetheexacthorizontalline,weshouldmovethetapeupanddownalongthepumplineandwewillfindthechangesofreadinginthetape.Theshortestreadingofthetapeisthehorizontaldistance.Ifthedistanceislongerthanthelengthoftape,thenwecandividethelongdistanceintoseveralsegmentsandgetthetotaldistancebypluseachsegmenttogether.Sincedifferenttapeshavedifferentstartsofzeroofthetapes,itisveryimportanttojudgewherethezeroofthetapebegins.

TacheometryandStadia

Tacheometryisanopticalsolutiontothemeasurementofdistance.ThewordisderivedfromtheGreekTacns,meaning“swift”,andmetrot,meaning“ameasure”.Tacheometryinvolvesthemeasurementofarelateddistanceparametereitherbymeansofafixed-angleintercept.Theodolitetacheometryisanexampleofstadiasystem.Thetheodoliteisdirectedatthelevelstaffwherethestaffisheldverticallyandthelineofsightofthetelescopeishorizontal.ByreadingthetopandbottomstadiahairsonthetelescopeviewandthenthehorizontaldistancefromcenterofinstrumenttorodcanbeobtainedbymultiplyingthestadiaintervalfactorKbythestadiaintervalandplusthedistanceCwhichisfromthecenterofinstrumenttoprincipalfocus,i.e.D=Ks+C.UsuallythenominalstadiaintervalfactorKequals100whichisaconstantforaparticularinstrumentaslongasconditionsremainunchanged,butitmaybedeterminedbyobservationinpractice.ThevalueofCisdeterminedbythemanufacturerandstatedontheinsideoftheinstrumentbox.Forexternal-focusingtelescopes,underordinarycondition,Cmaybeconsideredas1ftwithouterrorofconsequence.Internal-focusingtelescopesaresoconstructedthatCis0ornearlyso;thisisanadvantageofinternal-focustelescopesforstadiawork.Mostinstrumentsnowusedforstadiaareequippedwithinternal-focusingtelescopes.

Applicationsoftacheometryincludetraversingandlevelingforthetopographicsurveys,locationofdetailsurveys,levelingandfieldcompletionsurveysforthetopographicmapping,andhydrographicmapping.Therelativeprecisionis1:

1000to1:

5000.

Stadiaisaformoftacheometrythatusesatelescopiccross-hairconfigurationtoassistindeterminingdistances.

Aseriesofrodreadingsistakenwithatheodoliteandtheresultantintervalsareusedtodeterminedistances.

ElectronicDistanceMeasurement（EDM）

TheElectronicDistanceMeasurement（EDM）wasfirstintroducedin1950sbythefoundersofGeodimeterInc.TheadventofEDMinstrumenthascompletelyrevolutionizedallsurveyingprocedures,resultinginachangeofemphasisandtechniques.Distancecannowbemeasuredeasily,quicklyandwithgreataccuracy,regardlessofterrainconditions.

EDMinstrumentsrefertothedistancemeasurementequipmentsusinglightandradiowaves.Bothlightwavesandradiowavesareelectromagnetic.

Theyhaveidenticalvelocitiesinavacuum（orspace）to299,792.458±0.001km/sec.

Thesevelocities,whichareaffectedbytheair’sdensity,arereducedandneedtoberecalculatedintheatmosphere.ThebasicprincipleofEDMinstrumentsisthatdistanceequalstimemultipliedbyvelocity.Thusifthevelocityofaradioorlightwaveandtimerequiredforittogofromonepointtoanotherareknown,thedistancebetweenthetwopointscanbecalculated.

TheEDMinstrumentsmaybeclassifiedaccordingtothetypeandwavelengthoftheelectromagneticenergygeneratedoraccordingtotheiroperationalrange.

EDMinstrumentsusethreedifferentwavelengthbands:

（1）Microwavesystemswithrangeupto150km,wavelength3cm,notlimitedtolineofsightandunaffectedbyvisibility;

（2）Lightwavesystemswithrangeupto5km（forsmallmachines）,visiblelight,lasersanddistancereducedbyvisibility;（3）Infraredsystemswithrangeupto3km,limitedtolineofsightandlimitedbyrain,fog,otherairborneparticles.

AlthoughthereisawidevarietyofEDMinstrumentsavailablewithdifferentwavelengths,therearebasicallyonlytwomethodsofmeasurementemployedwhichmaydividetheinstrumentsintotwoclassificationaselectro-optical（lightwaves）andmicrowaves（radiowaves）instruments.Thesetwobasicmethodsarenamelythepulsemethodandmorepopularphasedifferentmethod.Theyfunctionbysendinglightwavesormicrowavesalongthepathtobemeasuredandmeasuringthetimedifferencesbetweentransmittedandreceivedsignals,orinmeasuringthephasedifferencesbetweentransmittedandreceivedsignalsinreturningthereflectinglightwavetosource.

ModernEDMinstrumentsarefullyautomatictosuchanextentthat,aftertheinstruments,setupononestation,emitsamodulatedlightbeamtoapassivereflectorsetupontheotherendofthelinetobemeasured.Theoperatorneedonlydepressabutton,andtheslopedistanceisautomaticallydisplayed.MorecompleteEDMinstrumentsalsohavethecapabilityofmeasuringhorizontalandverticalorzenithanglesaswellastheslopedistance.Theseinstrumentsreferredtoastotalstationinstruments.

AngleandDirectionMeasurement

Horizontalandverticalanglesarefundamentalmeasurementsinsurveying.Itisnecessarytobefamiliarwiththemeaningsofcertainbasictermsbeforedescribingangleanddirectionmeasurement.Thetermsdiscussedherehavereferencetotheactualfigureoftheearth.

BasicTerms

Averticallineatanypointontheearth’ssurfaceisthelinethatfollowsthedirectionofgravityatthatpoint.Itisthedirectionthatastringwillassumeifaweightisattachedatthatpointandthestringissuspendedfreelyatthepoint.Atagivenpointthereisonlyoneverticalline.

Ahorizontallineatapointisanylinethatisperpendiculartotheverticallineatthepoint.

Atanypointthereareanunlimitednumberofhorizontallines.

Ahorizontalplaneatapointistheplanethatisperpendiculartotheverticallineatthepoint.Thereisonlyonehorizontalplanethroughagivenpoint.

Averticalplaneatapointisanyplanethatcontainstheverticallineatthepoint.Thereareanunlimitednumberofverticalplanesatagivenpoint.

HorizontalAngleandVerticalAngle

Ahorizontalangleistheangleformedinahorizontalplanebytwointersectingverticalplanes,orahorizontalanglebetweentwolinesistheanglebetweentheprojectionsofthelinesontoahorizontalplane.Forexample,observationstodifferentelevationpointsBandCfromAwillgivethehorizontalangle∠bacwhichistheanglebetweentheprojectionsoftwolines（ABandAC）ontothehorizontalplane.Itfollowsthat,althoughthepointsobservedareatdifferentelevations,itisalwaysthehorizontalangleandnotthespaceanglethatismeasured（Figure1）.Thehorizontalangleisusedprimarilytoobtainrelativedirectiontoasurveycontrolpoint,ortopographicdetailpoints,ortopointstobesetout.

Averticalangleisananglemeasuredinaverticalplanewhichisreferencedtoahorizontallinebyplus（up）orminus（down）angles,ortoaverticallinefromthezenithdirection.Plusandminusverticalanglesaresometimesreferredtoaselevationordepressionangles,respectively.Averticalanglethusliesbetween0°and±90°.Zenithisthetermdescribingpointsonacelestialspherethatisasphereofinfinitelylargeradiuswithitscenteratthecenteroftheearth.

Thezenithisana