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测绘工程专业英语翻译

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MethodsofElevationDeterminationAnelevationisaverticaldistanceaboveorbelowareferencedatum.Althoughverticaldistancecanbereferencedtoanydatum,insurveying,thereferencedatumthatisuniversallyemployedisthatofmeansealevel(MSL.MSLisassignedaverticalvalue(elevationof0.000ftor0.000m.Allotherpointsontheearthcanbedescribedbytheelevationsaboveorbelowzero.
Permanentpointswhoseelevationshavebeenpreciselydetermined(benchmarksareavailableinmostareasforsurveyuse.InChina,7yearsofobservationsattidalstationsinQingdaofrom1950to1956werereducedandadjustedtoprovidetheHuanghaiverticaldatumof1956.Inthe1987,thisdatumwasfurtherrefinedtoreflectlongperiodicaloceantidechangetoprovideanewnationalverticaldatumof1985,accordingtotheobservationsattidalstationsfrom1952to1979.Although,strictlyspeaking,thenationalverticaldatummaynotpreciselyagreewiththeMSLatspecificpointsontheearth’ssurface,thetermMSLisgenerallyusedtodescribethedatum.MSLisassignedaverticalvalue(elevationof0.000ftor0.000m.Differenceinelevationmaybemeasuredbythefollowingmethods:
1.Directorspiritleveling,bymeasuringverticaldistancesdirectly.Directlevelingismostprecisemethodofdeterminingelevationsandtheonecommonlyused.
2.Indirectortrigonometricleveling,bymeasuringverticalanglesand

horizontalorslopedistances.3.Stadialeveling,inwhichverticaldistancesaredeterminedbytacheometryusingengineer’stransitandlevelrod;plane-tableandalidadeandlevelrod;orself-reducingtacheometerandlevelrod.4.Barometricleveling,bymeasuringthedifferencesinatmosphericpressureatvariousstationsbymeansofabarometer.
5.Gravimetricleveling,bymeasuringthedifferencesingravityatvariousstationsbymeansofagravimeterforgeodeticpurposes.
6.Inertialpositioningsystem,inwhichaninertialplatformhastreemutuallyperpendicularaxes,oneofwhichis“up”,sothatthesystemyieldselevationasoneoftheoutputs.Verticalaccuraciesfrom15to50cmindistancesof60and100km,respectively,havebeenreported.Theequipmentcostisextremelyhighandapplicationsarerestrictedtoverylargeprojectswhereterrain,weather,time,andaccessimposespecialconstraintsontraditionalmethods.7.GPSsurveyelevationsarereferencedtotheellipsoidbutcanbecorrectedtothedatumifasufficientnumberofpointswithdatumelevationsarelocatedintheregionsurveyed.Standarddeviationsinelevationdifferencesof0.053to0.094marepossibleundertheseconditions.Spiritleveling

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