測繪工程專業(yè)英語：Section 3 The Surveyors Level

Section3TheSurveyor’sLevelThreebasictypesoflevelcanbedistinguished.(1)DumpylevelInaDumpylevelthetelescopeandverticalspindlearecastasonepiece.Thelevellingheadshownconsistsessentiallyoftwoplates,thetelescopebeingmountedontheupperplatewhilethelowerplatescrewsdirectlyontoatripod.Thetwoplatesareheldapartbythreelevellingscrewsorfootscrews,andadjustmentstotheseenableaccuratelevellingoftheinstrumenttobecarriedout.Whenthishasbeeneffected,usingthebubbleattachedtothetelescope,theinstrumentshouldremainlevelnomatterinwhichdirectionitispointed(Fig1).(2)TiltinglevelInthislevelthetelescopeisnotrigidlyfixedtotheverticalspindle,butiscapableofaslighttiltintheverticalplaneaboutanaxisplacedimmediatelybelowthetelescope.Thismovementisgovernedbyafine-settingscrewattheeyepieceend,andthebubbleisbroughttothecenterofitsrunforeachreadingofthelevel.Thusthelineofcollimationneednotbeperpendiculartotheverticalaxis,asitmustintheDumpylevel.Tiltinglevelsarerobustandcapableofthehighestaccuracy,andmanymodernlevels,whetherforpreciseworkorforordinarylevelling,incorporatetheprinciple(Fig2).(3)AutomaticlevelThereisnowagreatrangeoflevellinginstrumentsavailablewithnoprecisebubbleattached.Thetelescopesoftheseinstrumentsneedonlybeapproximatelylevelledandthenacompensatingdevice,usuallybasedonapendulumsysteminsidethetelescope,correctsfortheresidualmislevelment.Theselevelsareverypopularbecauseoftheireaseofuse,buttheydosufferfrominstabilityasmentionedlater.ThetiltinglevelFigure3showsacurrentmodelofasimpletiltinglevelsuppliedbyHallandWattsLimited.Theupperpartoftheinstrumentconsistsofatelescopeandasensitivebubble.Aninclinedmirrorenablesthebubbletobeviewedfromtheeyepieceendofthetelescopeandthis,whenfoldeddown,protectsthebubble.Usingthefine-settingscrewmountedjustbelowtheeyepieceofthetelescope,thebubbleassemblycanbetiltedverticallytocentrethebubbleandsodefineahorizontallineofsight.Thetelescopemaybedirectedalonganyparticularlineofsightbymeansofaclampingscrew,slightlateralmovementstotheleftorrightofthislinebeingmadewiththe'slowmotion'or'tangent'screw.Asmallcircularbubblesetinthelowerpartcanbeusedtoapproximatelyleveltheinstrumentusingaquick-release'ballandsocket'base,andthisiswhytheinstrumentmaybereferredtoasa'quickset'level.Therearemanypossiblevariationsindesign.Figure4showsanotherWattslevel,mountedinamoreconventionalthree-screwbaseandprovidedwithaprismaticcoincidencereader.PrismaticcoincidencebubblereaderThisrefinement,designedtoeliminateexcessivemovementaroundtheinstrument,isthecoincidencebubblereader,andthedeviceisnowfittedtomany,ifnotmost,moderninstruments.Anarrangementofprismsreflectsanimageofbothendsofthebubbleintotheeyepiece.Theimageissplitdownthecentre,andone-halfofeachendisseennexttotheotherhalfinthebubblereadereyepiece.Asthetelescopeistilted,thetwohalvesappeartomoveinoppositedirections.Thismagnifiestheactualmovementtwofoldand,intheWattsmodelforexample,thisisfurthermagnifiedbytheopticalsystem,whichmakesveryexactsettingpossible.Consequently,bubblesoflowersensitivitythanwouldotherwisebenecessarycanbeemployed,andthisgreatlyimprovesstabilityandeaseofsetting.SettingupThefollowingsequenceofoperationsisrequiredtobringatiltinglevelreadyforuse.Screwthelowerplateoftheinstrumentontheheadofthetripod,whoselegshavebeenopenedandfirmlyfixedontheground.Thecircularbubbleshouldbebroughttoitscentralposition,usingthefootscrewsor'ballandsocket'assembly.Ifusinganinstrumentfittedwithfootscrews,thebestprocedureisasfollows.ReferringtotheFigure,byrotatingfootscrews1and2inoppositedirectionsatthesametimethebubblecanbesetontheline12.Now,byrotatingfootscrew3only,thebubblecanbecentredinthetargetring.Itisessentialthatparallaxbetweenthecross-hairsandtheimageofthelevellingstaffbeeliminated,forreasonsalreadyexplained.Thereisnodoubtthatfailuretodosoisresponsibleformucherrorinlevelling.Toeliminateparallax:(i)turnthetelescopetothesky,orholdapieceofwhitepaperinfrontoftheobjective,andfocustheeyepiecesothatthecross-hairsappearclearanddistinct.Thisisusuallyachievedbyturningtheeyepiece,whichisthreadedintothetelescopebarrel.Itmustberealizedthattheeyepiecesettingdependsonthecharacteristicsofthesurveyor'seye,sothatitwillvaryfromonepersontoanother;foronegivenoperator,thesettingwillnotvary;(ii)nowsightthelevellingstaffandfocusitsimagewiththefocusingscrewsothatwhentheeyeismovedslightlythereisnorelativemovementbetweentheimageandthecross-hairs.(d)Centerthesensitivebubbleusingthetiltingscrewbeforeeveryreading.(b),(c)and(d)canbereferredtoastemporaryadjustment.NewWordsandExpressionsLevel：n水平；水準(zhǔn)器，水準(zhǔn)儀；水平面，水平線；水準(zhǔn)測量；級，電平，能級；均衡；高度，程度；vt置平，抄平，使水平，瞄準(zhǔn)，對準(zhǔn)；vi變平，拉平Dumpylevel：定鏡水準(zhǔn)儀telescope：望遠(yuǎn)鏡verticalspindle：垂直軸，豎軸levellinghead：三角架頭plate：版，板，片；底片，感光板；刻圖片，印刷版，印張；整頁插圖；電鍍screw：螺旋，螺釘，螺絲tripod.：三角架levellingscrew：傾斜螺旋（水準(zhǔn)儀的），安平螺旋footscrew：腳螺旋adjustment：調(diào)整，調(diào)節(jié)，校正；平差carryout：完成，實現(xiàn)，貫徹，執(zhí)行bubble：氣泡，水準(zhǔn)器tiltinglevel：傾斜水準(zhǔn)儀rigidly：堅硬地，嚴(yán)格地tilt：（使）傾斜，（使）翹起verticalplane：鉛垂面，豎面fine-settingscrew：微調(diào)螺旋eyepiece：目鏡collimation：視準(zhǔn)，瞄準(zhǔn)，準(zhǔn)直，平行校正collimationaxis：視準(zhǔn)軸collimate：照準(zhǔn)，視準(zhǔn)，準(zhǔn)直，平行校正；確定象主點perpendicular：垂直的，正交的verticalaxis：豎軸，縱軸robust：精力充沛的，健壯的，穩(wěn)健的automaticlevel：自動安平水準(zhǔn)儀available：可利用的，可得到的compensate：償還，補償compensatingdevice：補償裝置pend