测绘外文翻译外文文献英文文献水准尺和水准仪.docx
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测绘外文翻译外文文献英文文献水准尺和水准仪.docx
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LevelRodsandLenels
Therearemanykindsoflenelrodsareinonepieceandothers(foreaseoftransporting)areeithertelescopingorrodsareusuallymadeofwoodandaregraduatedfromzeroatthemaybeeitherselfreadingrodsthatarereaddirectlythroughthetelescopeortargetrodswheretherodmansetsaslidingtargetontherodandtakesthereadingdirectly.Mostrodsserveaseitherself-readingorastargetrods.
AmongtheseveraltypesoflevelrodsavailablearethePhiladelphiarod,theChicagorod,andtheFloridarod.ThePhiladelphiarod,themostcommonone,ismadeintwosections.Ithasarearsectionthatslidesonthefrontsection.Forreadingsbetween0and7ft,therearsectionisnotextended;forreadingbetween7and13ft,itisnecessarytoextendedtherod.Whentherodisextended,itiscalledahighrod.ThePhiladelphiarodisdistinctlydividedintofeet,tenths,andhundredthsbymeansofalternatingblackandwhitespacespaintedontherod.
TheChicagorodis12ftlongandisgraduatedinthesamewayasthePhiladelphiarod,butitconsistsofthreeslidingsection.TheFloridarodis10ftlongandisgraduatedinwhiteanredstripes,eachstripebeingftwide.Alsoavailableforeaseoftransportationaretapesorribbonsofwaterproofedfabricwhicharemarkedinthesamewaythataregularlevelrodismarkedandwhichcanbeattachedtoordinarywoodstrips.Onceajobiscompleted,theribboncan,beremovedandrolledup.Thewoodstripcanbethrownaway.Theinstrumentmancanclearlyreadthesevariouslevelrodsthroughhistelescopefordistancesupto200or300ft,butforgreaterdistanceshemustuseatarget.Atargetisasmallredandwhitepieceofmetalattachedtotherod.Thetargethasavemierthatenablestherodmantotakeareadingtothenearestft.
Iftherodmanistakingthereadingswithatargetandifthelineofsightofthetelescopeisabovethe7-ftmark,itisobviousthathecannottakethereadingdirectlyinthenormalfashion.Therefore,thebackfaceoftherodisnumbereddownwardfrom7to13ft.Thetargetissetatacertainmarkonthefrontfaceoftherodandasthebacksectionispushedupward,itrunsunderanindexscaleandavernierwhichenablestherodmantotakethereadingonthefront.
Beforesettinguptheleveltheinstrumentmanshouldgivesomethoughtowherehemuststandinordetomakehissights.Inotherwords,hewillconsiderhowtoplacethetripodlegssothathecanstandcomfortablybetweenthemforthelay-outoftheworkthathehasinmind.
Thetripodisdesirablyplacedinsolidgroundwheretheinstrumentwillnotsettleasitmosecertainlywillinmuddyorswampyareas.Itmaybenecessarytoprovidesomespecialsupportfortheinstrument,suchasstakesoraplatform.Thetripodlegsshouldbewellspreadapartandadjustdesothatthefootplateunderthelevelingscrewsisapproximatelylevel.Theinsatrumentmanwalksaroundtheinstrumentandpusheseachlegfrimlyintotheground.Onhillsidesitisusuallyconvenienttoplaceongleguphillandtwodownhill.
Aftertheinstrumenthasbeenleveldeasmuchaspossiblebyadjustingthetripodlegs,thetelescopeisturnedoverapairofoppositelevelingscrewsifafour-screwinstrumentisbeingthebubbleisroughlycenteredbyturningthatpairofscrewinoppositedirectionstoeachother.Thebubblewillmoveinthedirectionoftheleftthumb.Next,thetelescopeisturnedovertheotherpairoflevelingscrewsandthebubbleisagainroughlycentered.Thetelescopeisturnedbackiverthefirstpairandthebubbleisagainroughlycentered,andsoon.Thisprocessisrepeatedafewmoretimeswithincreasingcareuntillthebubbleiscenteredwiththetelescopeturnedovereitherpairofscrews.Ifthelevelisproperlysdjusted,thebubbleshouldremaincenteredwhenthetelescopeisturuedinanydirection.Itistobeexpectedthattherewillbeaslightmaladjustmentoftheinstrumentthatwillresultinaslightmovementofthebubble;however,theprecisionoftheworkshouldnotbeadverselyaffectedifthebubbleiscenteredeachtimearodreadingistaken.
Thefirststepinlevelingathree-screwinstrumentistoturnthetelescopeuntillthebubbletubeisparalleltotwoofthescrews.Thebubbleiscenteredbyturningthesetwoscrewsinoppositedirections.
Next,thetelescopeisturnedsothatthebubbletubeisperpendiculartoalinethroughscrews.Thebubbleiscenteredbyturningscrew.
Thesestepsarerepeateduntillthebubblestayscenteredwhenthetelescopeisturnedbackandforth.
ElectronicDistanceMeasurements
Amajoradvanceinsurveyinginrecentyearshasbeenthedevelopmentofelectronicdistance-measuringinstruments(ED-MIs).Thesedevicesdeterminelengthsbasedonphasechangesthatoccuraseletromagneticenergyofknownwavelengthtravelsfromoneendofalinetotheotherandreturns.
ThefirstEDMinstrumentwasintronducedin1948bySwedishphysicistErikBergstrand.Hisdevice,calledthegeodimeter(anacronymforgeodeticdistancemeter),resultedfromattemptstoimprovemethodsformeasuringthevelocityoflight.Theinstrumenttransmettedvisiblelightandwascapableofaccuratelymeasuringdistancesuptoabout25mi(40km)atnight.In1957asecondEDMapparatus.thetellurometer.DesignedbyandintroducedinSouthAfrica,transmittedinvisiblemicrowavesandwascapableofmeasuringdistancesupto50mi(80km)orornight.
ThepotentialvalueoftheseearlyEDMmodelstotheSurveyingprofessionwasimmediatelyrecognized:
houever,theywereexpensiveandnotreadilyportableforfieldoperations.Furthermore,measuringprocedureswerelengthyandmathematicalreductionstoobtaindistancesfromobservedvaluesweredifficultandtime-consuming.Inaddition.Therangeofoperationofthefirstgeodimeterwaslimitedindaytimeuse.Continuedresearchanddevelopmenthaveovercomeallthesedeficiencies.
Thechiefadvantagesofelectronicsurveyingarethespeedandaccuracywithwhichdistancescanbemeasured.Ifalineofsightisavailable,longorshortlengthscanbemeasuredoverbodiesofwaterorterrainthatisinaccessiblefortaping.WithmodernEDMequipment,distanceareautomaticallydisplayedindigitalforminfeetormeters,andmanyhavebuilt-inmicrocomputersthatgiveresultsinternallyreducedtohorizontalandverticalcomponents.Theirmanysignificantadvantageshaverevolutionizedsurveyingproceduresandgainedworldwideacceptance.Thelong-distancemeasurementspossiblewithEDMequipmentmakeuseofradiosforcommunication,whichisanabsolutenecessityinmodernpractice.
OnesyetemforclassifyingEDMIsisbywavelengthoftransmittedelectromagneticenergy;thefollowingcategoriesexist:
Electro-opticalinstrumentsWhichtransmiteithermodulatedlaserorinfraredlighthavingwavelengthswithinorslightlybeyondthevisibleregionofthespectrum.
MicrowaveequipmentsWhichtransmitsmicrowaveswithfrequenciesintherangeof3to35GHzcorrespondingtowavelengthsofabouttomm.
AnotherclassificationsystemforEDMIsisbyoperationalrange.Itisrathersubjective,butingeneraltwodivisionsfitintothissystem:
shortandmediumrange.Theshort-rangegroupincludesthosedeviceswhosemacimummeasuringcapabilitydoesnotexceedabout5km.Mostequipmentinthisdivisionistheelectro–opticaltypeandusesinfraredlight.Theseinstrumentsaresmall,portable,easytooperate,suitableforawidevarietyoffieldsurveyingwork,andusedbymanypractitioners.
Instrumentsinthemedium-rangegrouphavemeasuringcapabilitiesextendingtoabout100kmandareeithertheelectro-optical(usinglaserlight)ormicrowavetype.Althoughfrequentlyusedinprecisegeodetictheyarealsosuitableforlandandengineeringsurveys.Longer-rangedevicealsoavailablecanmeasurelineslongerthan100km,buttheyarenitgenerallyusedinordinarysurveyingwork.Mostoperatebytrasmittinglongradiowaves,butsomeemploymicrowaves.Theyareusedprimarilyinoceanogaraphicandhydrograpgicsurvingandnavigation.
Ingeneral,EDMequimentmeasuresdistancesbycomparingalineofunkownlengthtotheknownwavelengthofmodulatedelectromagneticenergy.Thisissimilartorelatinganeededdistancetothecalibratedlengthofasteeltape.
Electromagneticenergypropagatesthroughtheatmosphereinaccordanceswiththefollowingequation:
V=fλ
(1)
WhereVisthevelocityofelectromaneticenergy,inmeterspersecond;fthemodulatedfrequencyoftheenergy,inhertz,andλthewavelenth,inmeteres.
WithEDMIsfrequencycanbepreciselycontrolledbutvelocityvarieswithatmopheretemperature,pressure,andhumidity.ThuswavelengthandfrequencymustvaryinconformancewithEQ.
(1).Foraccurateelectronicdistancemeasuement,therefor.,theatmospheremustbesampledandcorrctiosmadeaccordingly.
Thegeneralizedprocedureofmeasuringdistanceelectronicallyisdepictedin.anedmdevice,centeredbymeansofaplumbboboropticalplummitoverstatonA,trasmitsacarriersignalofelectromagneticenergyuponwhichareferencefrequencyhasbeensuperimposedormodulated.ThesignalisreturnedfromstaionBtotherevevier,soitstrvelpathisdoubletheslopedistanceAB.In,themodulatedelectromagneticenergyisrepresentedbyaseriesofsinewaveshavingwave-lengthλ.Anypositionalongagivenjwavecanbespecifiedbyitsphaseangle,whichis0°atitsbeginning,180°atthemidpoint,and360°atitsend.
EDMdevicesusedinsurveyingoperatebymeasuringphaseshift.Inthisprocedure,thereturnedenergyundergoesacomplete360°phasechangeforeachevenmultipleofexactlyone-halfthewavelengthseparatingtheline-sendpo
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