91MAGNETICCOMPASSADJUSTMENTTotal Curve(A&B)East(+)ADeg-Dev.BNWest-()90180"270S360Compass Heading-DegreesFigure 612b.A and B deviation.For example,on090°or270°compass headings,thethe 090°heading would then be 4°E due to Bundercorrec-tion. To eliminate this endlessly iterative process andonlvdeviationwhichiseffectiveisthatduetoB.Thisiso-lation,and thefactthattheB effectisgreateston thesetwocorrect theBerrortothebestpossibleflatcurve,splitthis4°Edifference,leaving2Edeviation on eachoppositeheadings,maketheseheadingsconvenientforBcorrection.Correction oftheBdeviation on a 090°heading will correctheading. This would, in effect correct the B error, leavingonlytheAerror of 2°Ewhichmustbecorrected byothertheBdeviationon the270°headingbythe same amountbutintheoppositedirectionandnaturally,itwillnotchangethemeans.It isforthis reason that,(1)splitting is donebetweendeviations onthe 000°and 180°headings, exceptwhereBthe errors noted on opposite headings, and (2)good adjust-errors are large.However, the total deviation on all the in-ments entail checking on all headings rather than on thetercardinal headings will be shifted in the samedirection asfundamentalthree.theadjacent 090°or 270°deviation correction,but only byseven-tenths(0.7)of that amount,since the sine of 45°613.Adjustment ProceduresAtSeaequals 0.707.The same convenient isolation of effects andcorrections of C error will also change the deviations on allBefore proceeding with the adjustment at sea the fol-the intercardinal headings by the seven-tenths rulelowingprecautions should beobserved:Note that only after correcting the B and C errors on thecardinal headings, and consequently theirproportional val-1. Secure all effective magnetic gear in the normalues of the total curve on the intercardinal headings,can theseagoingpositionD error be observed separately on any of the intercardinalheadings.TheDerror maythenbecorrected by useof the2. Make sure the degaussing coils are secured, usingspheres on any intercardinal heading.Correcting D errorthe reversal sequence, if necessary (See sectionwill.asarule.changethedeviationsonthe intercardina643),headings only,and not on the cardinal headings.Onlywhenthe D error is excessive, the spheres are magnetized, or theThe adjustments aremade with the ship on an evenpermanent magnet correctors are so close as to create ex-keel, swinging from heading to heading slowly,and aftercessive induction in the spheres will there bea change in thesteadying on each heading for at least 2minutes to avoiddeviations on cardinal headings as aresultofsphere adjust-Gaussin error.ments.Although sphere correction does not generallyMostadjustments can bemadebytrial and error,orbycorrect deviations on cardinal headings,itdoes improveroutine procedure such as the one presented in section 601compass stability on theseheadings.However,theprocedurespresented belowprovide analyti-Ifit werenotforthe occasional A orEerrors,adjustingcal methods inwhichtheadjusterisalwaysawareof theobserveddeviations to zero on two adjacentcardinal head-errors magnitude on all headings as a result of his move-ings and then on the intermediate intercardinal headingmentofthedifferent correctors.would be sufficient.However, Figure 612b, showingaAnalysis Method.A complete deviation curve can becombination ofA and Berrors,illustrates why the adjustingtakenfor anygiven condition,and an estimatemadeof allproceduremust include correctingdeviations onmorethantheapproximate coefficients.See section615.Fromthis es-the three essential headings.timate,theapproximatecoefficientsareestablished andtheAssuming no A error existed in the curve illustrated inappropriate correctionsaremadewithreasonableaccuracyFigure 612b, and the total deviation of 6° E on the 0900on aminimumnumber ofheadings.Iftheoriginal deviationheadingwerecorrected withBmagnets,theerroronthecurve has deviations greater than 20o,rough adjustments270°heading would be 4°Edue toBovercorrection.If thisshould bemade on twoadjacent cardinal headings before4°Eerror weretakenouton the270°heading,the erroronrecording curve datafor suchanalysis.Themechanics of
MAGNETIC COMPASS ADJUSTMENT 91 For example, on 090° or 270° compass headings, the only deviation which is effective is that due to B. This isolation, and the fact that the B effect is greatest on these two headings, make these headings convenient for B correction. Correction of the B deviation on a 090° heading will correct the B deviation on the 270° heading by the same amount but in the opposite direction and naturally, it will not change the deviations on the 000° and 180° headings, except where B errors are large. However, the total deviation on all the intercardinal headings will be shifted in the same direction as the adjacent 090° or 270° deviation correction, but only by seven-tenths (0.7) of that amount, since the sine of 45° equals 0.707. The same convenient isolation of effects and corrections of C error will also change the deviations on all the intercardinal headings by the seven-tenths rule. Note that only after correcting the B and C errors on the cardinal headings, and consequently their proportional values of the total curve on the intercardinal headings, can the D error be observed separately on any of the intercardinal headings. The D error may then be corrected by use of the spheres on any intercardinal heading. Correcting D error will, as a rule, change the deviations on the intercardinal headings only, and not on the cardinal headings. Only when the D error is excessive, the spheres are magnetized, or the permanent magnet correctors are so close as to create excessive induction in the spheres will there be a change in the deviations on cardinal headings as a result of sphere adjustments. Although sphere correction does not generally correct deviations on cardinal headings, it does improve compass stability on these headings. If it were not for the occasional A or E errors, adjusting observed deviations to zero on two adjacent cardinal headings and then on the intermediate intercardinal heading would be sufficient. However, Figure 612b, showing a combination of A and B errors, illustrates why the adjusting procedure must include correcting deviations on more than the three essential headings. Assuming no A error existed in the curve illustrated in Figure 612b, and the total deviation of 6° E on the 090° heading were corrected with B magnets, the error on the 270° heading would be 4° E due to B overcorrection. If this 4° E error were taken out on the 270° heading, the error on the 090° heading would then be 4° E due to B undercorrection. To eliminate this endlessly iterative process and correct the B error to the best possible flat curve, split this 4° E difference, leaving 2° E deviation on each opposite heading. This would, in effect correct the B error, leaving only the A error of 2° E which must be corrected by other means. It is for this reason that, (1) splitting is done between the errors noted on opposite headings, and (2) good adjustments entail checking on all headings rather than on the fundamental three. 613. Adjustment Procedures At Sea Before proceeding with the adjustment at sea the following precautions should be observed: 1. Secure all effective magnetic gear in the normal seagoing position. 2. Make sure the degaussing coils are secured, using the reversal sequence, if necessary (See section 643). The adjustments are made with the ship on an even keel, swinging from heading to heading slowly, and after steadying on each heading for at least 2 minutes to avoid Gaussin error. Most adjustments can be made by trial and error, or by routine procedure such as the one presented in section 601. However, the procedures presented below provide analytical methods in which the adjuster is always aware of the errors’ magnitude on all headings as a result of his movement of the different correctors. Analysis Method. A complete deviation curve can be taken for any given condition, and an estimate made of all the approximate coefficients. See section 615. From this estimate, the approximate coefficients are established and the appropriate corrections are made with reasonable accuracy on a minimum number of headings. If the original deviation curve has deviations greater than 20°, rough adjustments should be made on two adjacent cardinal headings before recording curve data for such analysis. The mechanics of Figure 612b. A and B deviation
92MAGNETICCOMPASSADJUSTMENT21465AnticipatedAnticipatedAnticipatedAnticipatedAnticipatedOriginalcurve aftercurve afftercurveaftercurve aftercurveafterHeading bydeviationfirstnextnextnextnextcompasscorrectingcorrectingcorrectingcorrectingcorrectingcurveA=1.0°EB= 12.0°EC= 8.0°ED=5.0°EE=1.5°EDegeesDegreesDegreesDegreesDegreesDegreesDegrees0.000010.5 E.9.5E.9.5E1.5 E.1.5 E.0.004520.0E.19.0E10.6E5.0E.0.0090115E.10.5 E.1.5 W.1.5 W.,0.01.5 W.0.01350.01.2 w.2.2 W.10.6 W.5.0W.1805.5W.6.5W.6.5W.1.5E1.5 E.0.02258.0W.9.0w.5.0E0.00.00.6W.58012.5 W.13.5 W.1.5 W.1.5 W.1.5 W.6.8 W.7.8 W.0.6E.5.0 W.0.0Figure 613a.Tabulating anticipated deviations.record of deviations is available at all times during theapplyingcorrectors arepresented inFigure601.Amethodof tabulating the anticipated deviations after each correc-swing.Arrows indicate where each change is madetionisillustratedinFigure613a.ThedeviationcurveusedSince the B error is generally greatest, it is correctedforillustrationistheonewhichisanalyzedinsection615first.Therefore,on a 090°heading the 11.5E deviationAnalysisrevealedthesecoefficients:is corrected to approximately zero byusingfore-and-aftBmagnets.A lot of time need not be spent trying to re-duce this deviation to exactly zero since the B coefficientA=1.0°Emaynotbeexactly11.5E, and some splitting mightbeB=12.0°Edesirable later. After correcting on the 090° heading, thec=8.0°Eswingwouldthenbecontinuedto135°wherea9.2°WD=5.0°Eerror wouldbe observed.This deviation is recorded, butnocorrection is made becausethequadranterror isbestE=1.5°Ecorrected after the deviations on all four cardinal head-ings have been corrected.The deviation on the 1800heading would be observed as 5.5°W.Since this devia-One-Swing Method. More often it is desirable totion is not too large and splitting may be necessary later,beginadjustmentimmediately,eliminatingtheoriginalit need not be corrected at this time. Continuing theswingfor deviations andthe estimate ofapproximate co-swingto225°a0.0°deviationwouldbeobservedandre-efficients.In this case the above problem would becorded.Onthe270°headingtheobserved error would besolved by tabulating data and anticipating deviation1.0oW, which is compared with 0.0deviation on thechanges as the corrections are made. Figure 613b illus-opposite 090oheading.This could be split, leavingtrates thisprocedure.Notethat a new columnof values0.5°W deviation 0n both 090°and 270°,but since this isis started after each change is made.This method of tab-sosmall itmaybeleftuncorrected.On315°theobservedulation enables the adjuster to calculate the new residualdeviation would be 1.2°E.At 000°a deviation of 10.50deviations each time a corrector is changed, so that aObservedAnticipatedAnticipatedAnticipatedAnticipateddeviationsdeviationsdeviationsdeviationsdeviationsFirst obser-HeadingafterafterafterafteraftervationcorrectingcorrectingcorrectingcorrectingcorrectingD=5.0PEE=1.5°EB=11.5°EA=1.0°EC=8.0°EDegeesDegreesDegreesDegreesDegresDegreesDegrees0002.5E2.5E.1.5 E.0.010.5 E,0456.4E1.4E0.4E.0.4E0.00900.011.5E0.01.0 w.0.5E.1359.2 W.3.6 W.1.4E.0.4E.0.4E.心1805.5 W.2.5E.2.5E.1.5E0.0..5550.05.6 E.0.6 E.0.4W.0.4 W.1.0W.1.0w.380.5W.1.0 W.4.4 W.0.6 E.0.4 W.1.2 E.Figure 613b.Tabulating anticipated deviations by the one-swing
92 MAGNETIC COMPASS ADJUSTMENT applying correctors are presented in Figure 601. A method of tabulating the anticipated deviations after each correction is illustrated in Figure 613a. The deviation curve used for illustration is the one which is analyzed in section 615. Analysis revealed these coefficients: One-Swing Method. More often it is desirable to begin adjustment immediately, eliminating the original swing for deviations and the estimate of approximate coefficients. In this case the above problem would be solved by tabulating data and anticipating deviation changes as the corrections are made. Figure 613b illustrates this procedure. Note that a new column of values is started after each change is made. This method of tabulation enables the adjuster to calculate the new residual deviations each time a corrector is changed, so that a record of deviations is available at all times during the swing. Arrows indicate where each change is made. Since the B error is generally greatest, it is corrected first. Therefore, on a 090° heading the 11.5° E deviation is corrected to approximately zero by using fore-and-aft B magnets. A lot of time need not be spent trying to reduce this deviation to exactly zero since the B coefficient may not be exactly 11.5° E, and some splitting might be desirable later. After correcting on the 090° heading, the swing would then be continued to 135° where a 9.2° W error would be observed. This deviation is recorded, but no correction is made because the quadrant error is best corrected after the deviations on all four cardinal headings have been corrected. The deviation on the 180° heading would be observed as 5.5° W. Since this deviation is not too large and splitting may be necessary later, it need not be corrected at this time. Continuing the swing to 225° a 0.0° deviation would be observed and recorded. On the 270° heading the observed error would be 1.0° W, which is compared with 0.0° deviation on the opposite 090° heading. This could be split, leaving 0.5° W deviation on both 090° and 270°, but since this is so small it may be left uncorrected. On 315˚ the observed deviation would be 1.2° E. At 000° a deviation of 10.5° 123456 Heading by compass Original deviation curve Anticipated curve after first correcting A = 1.0° E Anticipated curve after next correcting B = 12.0° E Anticipated curve after next correcting C = 8.0° E Anticipated curve after next correcting D = 5.0° E Anticipated curve after next correcting E = 1.5° E Degrees Degrees Degrees Degrees Degrees Degrees Degrees 000 10.5 E. 9.5 E. 9.5 E. 1.5 E. 1.5 E. 0.0 045 20.0 E. 19.0 E. 10.6 E. 5.0 E. 0.0 0.0 090 11.5 E. 10.5 E. 1.5 W. 1.5 W. 1.5 W. 0.0 135 1.2 W. 2.2 W. 10.6 W. 5.0 W. 0.0 0.0 180 5.5 W. 6.5 W. 6.5 W. 1.5 E. 1.5 E. 0.0 225 8.0 W. 9.0 W. 0.6 W. 5.0 E. 0.0 0.0 270 12.5 W. 13.5 W. 1.5 W. 1.5 W. 1.5 W. 0.0 315 6.8 W. 7.8 W. 0.6 E. 5.0 W. 0.0 0.0 Figure 613a. Tabulating anticipated deviations. A = 1.0˚ E B = 12.0˚ E C = 8.0˚ E D = 5.0˚ E E = 1.5˚ E Heading First observation Observed deviations after correcting B = 11.5° E Anticipated deviations after correcting C = 8.0° E Anticipated deviations after correcting D =5.0° E Anticipated deviations after correcting A = 1.0° E Anticipated deviations after correcting E = 1.5° E Degrees Degrees Degrees Degrees Degrees Degrees Degrees 000 . 10.5 E.→ 2.5 E. 2.5 E. 1.5 E. 0.0 045 . . 6.4 E.→ 1.4 E.→ 0.4 E. 0.4 E. 090 11.5 E.→ 0.0 0.0 0.0 1.0 W.→ 0.5 E. 135 . 9.2 W. 3.6 W. 1.4 E. 0.4 E. 0.4 E. 180 . 5.5 W. 2.5 E. 2.5 E. 1.5 E. 0.0 225 . 0.0 5.6 E. 0.6 E. 0.4 W. 0.4 W. 270 . 1.0 W. 1.0 W. 1.0 W. 2.0 W. 0.5 W. 315 . 1.2 E. 4.4 W. 0.6 E. 0.4 W. 0.4 W. Figure 613b. Tabulating anticipated deviations by the one-swing