Aux-N-ACNChap2.pdf

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CHAPTER 2

INTRODUCTION

This second chapter of theAdvanced Coastal Navigation

(ACN) Course explores the marinecompass and its use. The compassis one of the simplest and most use-ful navigation instruments to becarried aboard a vessel. Arguably, acompetent navigator, well-foundvessel, up-to-date charts, timepiece,and a good compass are the onlyreal requirements for a safe andefficient voyage. Columbus wasable to do this even without goodcharts or timepieces!

This chapter provides a briefhistory of the compass, a discussionof the types and parts of the moderncompass, an exposition of the prin-ciple of operation of the compass,and finally, a discussion of possiblecompass errors and their measure-ment so that the mariner can com-pensate for these errors and steercorrect courses. In particular, thischapter reviews so-called TVMDCcomputations, named for thesequence True-Variation-Magnetic-Deviation-Compass that is used to

determine compass courses fromtrue courses.

Compass adjustment refers tothe process of adjusting small mag-nets contained in the compass toremove as much error as possible.Many modern textbooks devoteconsiderable space to compassadjustment, and the reader maywonder why this topic is coveredonly briefly here. The reason issimple. Although compass adjust-ment is not impossibly complex, itis not trivial and needs to be doneright! Professional compassadjusters are available at reasonablecost to perform this service and,unless the mariner is willing todevote a substantial amount of timeand intellectual effort, this is a jobbest left to experts. In addition toadjusting the compass, a profes-sional can provide good advice onthe placement of the compass, ship-board electronics, and other gearthat may affect the compass. Forthose who disagree with this assess-ment and have the time and inclina-tion to master the intricacies ofcompass adjustment, a brief

THE MARINE MAGNETIC COMPASS

“Truth lies within a little and certain compass,but error is immense.”

–Henry St. John, Viscount Bolingbroke,Reflections upon Exile (1716)

description is included. Moreover,the references included at the end ofthis chapter provide a useful start-ing point for home study.

The material in this chapter isnot difficult. But teaching experi-ence indicates that considerablepractice is required in order torapidly and reliably solve the prob-lems discussed in this chapter.

BRIEF HISTORYThe exact origin of the compass

is lost in antiquity. Although someaccounts claim that the compasswas invented well before the birthof Christ (Hewson, 1983), docu-mentary evidence of its use inEurope and China dates back onlyto approximately 1100 AD (Aczel,2001; Bowditch, 1995; Collinder,1955; Jerchow, 1987). (Incidental-ly, by convention the early Chinesecompasses were said to point south,as this was considered a more nobleaspect.) The modern compass card(as opposed to needles used on theearliest compasses) apparently orig-inated with Flavio Gioia of Amalfiin southern Italy sometime around1300 AD (Collinder, 1955),although this is questioned by some(Aczel, 2001).

By the time of Columbus, thecompass was well developed andthere is evidence (from the diariesof Columbus) that the phenomenonof magnetic variation was at leastpartially understood. By the early1700s, charts showing the locationsof lines of equal variation (isogoniclines) were available. Likewise,compass deviation, an importantsubject discussed below, was under-stood in qualitative terms at aboutthis same time, although practicalmeans for compensating for devia-tion were not developed until 1801by Captain Matthew Flinders (fromwhich the Flinders bar used in com-pass adjustment takes its name).

The modern liquid-filled com-pass, similar to those used on yachtstoday, dates back to the period 1850to 1860 when it was developed andpatented by E. S. Ritchie of Boston,Massachusetts. (The companyfounded by Ritchie is still in busi-ness today.) Since that time, therehave been evolutionary rather thanrevolutionary developments in themagnetic (mechanical) compass.For example, new lightweightmaterials are used for compasscards, improved magnets are avail-able, and many other incrementalimprovements have been made toincrease the accuracy, stability, andutility of the magnetic compass.

Elmer Sperry, an American, andAnschutz-Kampfe, a German, dur-ing the early part of the 20th centu-ry developed the modern gyrocom-pass, an instrument capable of indi-cating true rather than magneticnorth. Gyroscopes were widelyused in naval and merchant shipssince the end of World War I.Heretofore, gyroscopes have beenelectromechanical devices, but lasergyros are now in development that

may revolutionize this field. (Gyro-scopes are not discussed in this text,as these are not presently availableat reasonable cost to the typicalboater.)

During the mid-1920s, an elec-tronic compass—termed a fluxgatecompass—was developed for air-craft to provide better directionalinformation in turns and duringmaneuvers. In recent years, thistechnology has become available ata reasonable cost to the mariner,and for this reason is given passingmention.

The “electronics revolution,” aphrase used frequently in this text,also includes directional systems.Outputs from a fluxgate compasscan be “processed” by a wide vari-ety of computer systems and usedfor automated steering (autopilots),and navigational computers (e.g., tocompute current set and drift, asdiscussed in Chapter 7). Yet moresophisticated developments arelikely in the near future.

For all these newer develop-ments, the traditional magneticcompass remains one of the mostimportant navigational instruments,as evidenced by the fact that eventhe most sophisticated ship or air-craft in service today still has atleast one magnetic compass aboard.Its relative simplicity, reliability,and lack of dependence on electricalpower sources will probably ensureits survival well into the future.

PARTS OF THE COMPASSOver the years, the marine mag-

netic compass has evolved into afunctional, easy-to-read, conve-nient, and relatively inexpensivenavigational instrument. The damp-ing system of a modern, spherical,

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2 The Marine Magnetic Compass

WHAT YOU WILLLEARN IN THISCHAPTER

❏ The “anatomy” of a compass

❏ Compass types

❏ Compass deviation and its measurement

❏ TVMDC calculations

❏ Compass errors

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liquid-filled marine magnetic com-pass is shown in Figure 2-1. In thiscompass, a lightweight dial or com-pass rose is graduated in degreesincreasing in a clockwise directionfrom 000 degrees to 359 degrees toindicate the compass heading. Theincrements shown on the compassdial can be 1 degree, 2 degrees, or,more typically for compasses usedon small vessels, 5 degrees. (Stud-ies conducted just prior to WorldWar II indicated that graduationsevery 5 degrees were significantlyeasier to read than finer graduationsand, in practical terms, nearly asaccurate.) Numbers are typicallyspaced every 30 degrees, and thecardinal points (north, south, east,and west, or abbreviated N, S, E,

and W) are also indicated on thedial. Arrows or other marks aresometimes used to designate theintercardinal points (e.g., NE, SE,SW, and NW). Older compasseswere traditionally graduated in themariner’s “point” system, men-tioned in Chapter 1, in which thecircle was divided into 32 compasspoints, each of 11.25 degrees.These are named, in clockwiseorder from north: north, north byeast, north-northeast, northeast bynorth, northeast, northeast by east,east-northeast, east by north, east,etc. Naming these points, termed“boxing the compass,” was anunpleasant and confusing task usedhistorically in hazing rituals formidshipmen and other would-be

mariners. Fortunately, marinershave rediscovered the joy of num-bers and the older point system isnow only of historical interest. (Ifyou have such a compass, mount itin your den, not on your boat!)

Attached to the dial are the“north-seeking” compass magnets.The dial is supported on a jeweledbearing, which turns on a pivot. Inturn, the pivot is mounted in a gim-bal system, designed to keep thedial level with the horizon if thevessel pitches or rolls. Fastened tothe gimbal is one (or more) lubber’sline(s). The lubber’s line (alsotermed lubber line [Moody, 1980])is the index mark against which thedial graduations are read to deter-mine the direction of the vessel rel-

2The Marine Magnetic Compass

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▲

FIG. 2-1–Damping System of Modern Compass

RICTHIE POWERDAMP® SYSTEM…DETAIL

High VisibilityUltra-LightAluminum Dial

PowerDampStabilizer

DirectiveForce®

Magnets

BrassCounterbalance

Hardened Steel andSapphire Jewel Pivot

Triple CupSapphireJewel Pivot

HardenedSteel Pivot

PHOTO COURTESY OF RITCHIE NAVIGATION

ative to that of the card. The lub-ber’s line (or principal lubber’s lineif there are more than one) shouldbe aligned with the fore-and-aftaxis of the vessel.

The gimbals, card, and magnetsare enclosed in a bowl with a clear,transparent, hemispherical glass (orplastic) top, within which the cardand gimbals are free to rotate inde-pendently of the attitude of the con-tainer. The top (dome) may beimpregnated with inhibitors toreduce any discoloration of the cardor fluid from ultraviolet radiationand may also magnify the readings,so that the apparent card size islarger. The bowl is filled with anonfreezing liquid to damp (slowdown) the motion of the dial forincreased stability and to supportmuch of the weight of the card and

the magnets, so asto reduce wear onthe pivot. The ultra-lightweight dials inuse can be dampedwith fluids that arenot viscous (thick),a combination thatprovides stabilityand accuracy with-out a tendency to“overshoot” andoscillate as the ves-sel is turned to anew heading. Thecompass also con-tains an expansiondiaphragm to allowfor the expansionand contraction ofthe damping fluid

with temperature or pressurechanges. A fill plug is used toreplace or “top off” the dampingfluid. (It is important that there areno air bubbles in the compassfluid.)

The bowl is supported by a caseor holder, generallycalled a binnacle.Somewhere near thebowl are found thecompensating mag-nets, used to adjustthe compass tocompensate for thevessel’s magneticenvironment.

Most compassesare lighted for nightuse. A low intensityred lamp is pre-ferred to avoid orminimize adverseeffects on the nightvision of the helms-man or crew. (Inci-

dentally, the wires to the compasslight should be twisted to minimizemagnetic effects.)

Many compasses come with ahood (adjustable on some models)to reduce glare and improve read-ability. Removable protective cov-ers are also recommended if thecompass is installed in a locationwhere it is exposed to the elements.

COMPASS DIAL DESIGNThere are two principal designs

for the compass rose or dial. Theseare discussed briefly below.

❏ The first design is termed a top-reading compass (also a flatcard compass by some manu-facturers). With this design, themariner reads the heading orbearing “across the card.” Thelubber’s line is located behindthe card. The numbers that indi-cate heading or bearing increasein a clockwise direction—a cor-rect geometric representation. Aheading of 030 degrees is to the

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▲

FIG. 2-2–Combination Front and TopReading Compass

▲

FIG. 2-3–Fluxgate Compass with Digital Readout

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right of a heading of 000degrees, for example, and thecompass provides the same rep-resentation. If the helmsman isasked to turn to 030 degreesfrom a heading of north, it isclear that this must be a turn tothe right. It is also relativelyeasy to read compass bearingsover this compass dial. Thecompass dial itself is unob-structed through 360 degrees,although its placement aboardthe vessel usually limits thisrange. A top-reading compass isinstalled forward of the steeringmechanism and beneath thehelmsman’s eye level.

❏ The second design is termed afront-reading compass (alsocalled a direct reading compassby some manufacturers). Thiscompass dial design is typical ofmost aircraft compasses and isalso used for marine compasses.With this design, the lubber’sline is in front of the dial andindicates the direction towardwhich the vessel is heading.However, the dial is graduated ina counterclockwise direction.Thus, for example, the 30-degreegraduation on the front-readingdial is located to the left of 000.This apparent reversal in direc-tion is made necessary becausethe lubber’s line is located infront of the dial. The mountingof a front reading compass usu-ally precludes its use for obtain-ing bearings. This is not a realdetriment, since a hand-bearingcompass, discussed in Chapter 4,is a ready substitute.Either design correctly shows

the vessel’s actual heading, but thefront-reading compass design is

slightly more con-fusing and requires abit more practicebefore familiarity isassured. An inexpe-rienced helmsman,asked to come to aheading of 030degrees from north,could glance at thefront-reading dialand see that thisheading is to the left,and, therefore, begina turn in the wrongdirection before dis-covering the error.From the perspectiveof ease of interpretation, the top-reading compass dial is greatly tobe preferred. But, it is also impor-tant to consider how the compasswill be viewed once installed. In thetypical powerboat installation (andin sailboats where the compass bin-nacle is integrated with the wheel),the compass is located on a panelimmediately in front of the helms-man, and so a top-reading compassis easy to see. However, in a typicallight aircraft the compass isinstalled at the top of the cockpit(where it is least likely to be affect-ed by magnetic interference fromradios or other electronics), at orabove the eye level of the pilot,necessitating a front-readingdesign. Similarly, in certain sail-boats the compass is mounted onthe outer cabin bulkhead—nearly ateye level for the helmsman seatedseveral feet away—and a front-reading compass is necessary.

Some compass designs, such asthat shown in Figure 2-2, combineboth types of compass displays inone unit. The model shown in Fig-ure 2-2 shows a front reading dial

graduated in 5-degree incrementsand a top-reading dial that omits anumerical display of degrees. Somecompasses also include inclinome-ters, to measure the angle of roll ofthe vessel.

Yet other compass displays, typ-ically those used with fluxgatecompasses, feature a direct digitalreadout of the heading. Figure 2-3shows the display unit of a fluxgatecompass with digital readouts. Dig-ital readouts are generally shown tothe nearest degree, rather than 5degrees as is common on conven-tional compasses. Although manyprefer digital displays, these alsohave limitations or disadvantages.(For example, it is impossible totake a bearing on any object that isnot aligned with the vessel’s head-ing.) Moreover, the digital displayprovides less “situational aware-ness” for the helmsman than does


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▲

FIG. 2-4–Fluxgate Compass with Digital Readout

and Conventional Displays

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the top-reading compass. This dis-advantage can be overcome if thefluxgate compass also incorporatesa conventional dial, as is shown inFigure 2-4, which provides preciseheading information and situationalawareness. The display shown inFigure 2-4 cannot be used to takebearings, however.

Finally, some compasses—called telltale compasses—shouldbe mounted in an upside down(overhead mount) position. Thetelltale compass is usually installedin the ceiling of the navigator’sberth, so that the navigator can readthe vessel’s heading when not onduty. Overhead-mount compassesare a favorite of “single handlers,”and can double as a backup com-pass. (Incidentally, the practice of“single-handing” (voyaging forlong distances with only one personaboard) is unsafe, a violation of thenavigation rules (a proper lookoutcannot be maintained by a sleepinghelmsman), and is strongly discour-aged.)

Whatever display is chosen, it isimportant that the numerals on thedial be large and easy to read.Ornate displays, such as were foundon older marine compasses, are lessreadable than the simple, cleandesigns of today.

BRIEF ADVICE ON COMPASS SELECTION

The best advice on compassselection is not to be miserly. Withcompasses, as with other items ofequipment, you generally get whatyou pay for. Because the compass issuch an important navigationalinstrument, it is essential that it beof high quality. Incidentally, thiscomment also applies to small ves-

sels. On average, small vessels aresignificantly more “lively” than

larger vessels. Larger and moreexpensive compasses have bettergimbals and have larger and easierto read dials. The saying “bigger isbetter” almost always applies to theselection of a compass. Finally, it isrecommended that a vessel beequipped with at least two com-passes as a precaution against com-pass failure. A handheld compass(discussed in Chapter 4) can serveas a backup.

COMPASS MOUNTINGIdeally, the compass should be

mounted where it can easily beread, is protected from the ele-ments, and is free of any magneticinfluences aboard the vessel (seebelow). The lubber’s line should beprecisely aligned with the fore-and-aft axis of the vessel. On larger ves-sels, these requirements are easy to

meet, but this is sometimes more ofa problem in smaller craft. Consulta compass adjuster and read theowner’s manual (or product litera-ture) for advice on this importanttopic.

There are several types of com-pass mounts, each with advantagesand disadvantages. Compassmounts include the bracket mount(fast and versatile installation—par-ticularly for angled surfaces), flushmount, deck or binnacle mount, andbulkhead/dash mount.

PRINCIPLE OF OPERATION:DEVIATION

The modem magnetic compassis highly sensitive and is able toalign itself with weak magneticfields, such as the earth’s magneticfield. The magnets underneath thecompass card will align with themagnetic field and indicate direc-tion relative to this field. But, themagnetic field aboard a ship is actu-ally a combination (resultant) ofmultiple magnetic fields—that ofthe earth and those of the vessel andits equipment.

Were the earth’s magnetic fieldacting alone, the compass wouldindicate direction in the magneticdirection system—that is, the com-pass would point in the direction ofmagnetic north. (Please refer againto Chapter 1.) Determination of thedirection with respect to true northwould involve nothing more thanadding or subtracting the local mag-netic variation from the indicatedcompass direction (more below).

However, the magnetic fieldaboard a vessel is not solely due tothe earth’s magnetic field. Ship-board electronics, windshield wipermotors, compressed-gas horns,

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PRACTICALTIP

Carry at least one spare com-

pass aboard—if only a hand-

bearing compass. This is

cheap insurance. According

to Morrison (1942), early

mariners carried plenty of

spare “needles” (compass

needles) aboard. Ferdinand

Magellan reportedly had

thirty-five spare needles on

his ship!

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tachometers, electrical motors, tele-vision sets, and other equipmentalso generate magnetic fields.Indeed, flashlights, camera lightmeters, tools, and even somekitchen utensils can also affect thecompass. (For skeptics, a simpleexperiment proves this and is high-ly instructive. For example, note thecompass reading, then place aflashlight near the compass andobserve how the reading changes.)The vessel itself—particularly steelvessels—may have magnetic fieldsoriented in a variety of ways. Thevessel’s magnetic field may evendepend upon the direction the ves-sel was facing when it was con-structed or last laid up for the win-ter (Kielhorn and Klimm, 1978).These additional fields also affectthe compass, with the result that thecompass heading of the vessel maydiffer from its magnetic heading.

The difference between these istermed deviation. There are actuallythree “norths” that the mariner needbe concerned with: true north, mag-netic north, and compass north.Simply put, deviation is the differ-ence between the direction that thecompass actually points and thedirection that it would point if therewere no local magnetic fieldsaboard the vessel. Although statis-tics on the deviation of uncompen-sated compasses aboard recreation-al boats are not available, thesedeviations could be quite large, say10 degrees to 15 degrees, and possi-bly even more.

It is precisely because of thedeviation caused by the vessel’smagnetic field that correcting mag-nets are found in all good compass-es. A skilled compass adjuster canmove the adjusting magnets so as toremove most of the deviation nor-mally caused by the vessel’s mag-netic field. (A good compassadjuster can also serve as a consul-tant on compass placement and canadvise the mariner how to stowother gear to minimize deviation inthe first place.)

Compass Adjustment—A Brief Digression

As noted above, the use of a pro-fessional compass adjuster is rec-ommended. This material is addedfor those interested in a do-it-your-self project. The material in thissection is adapted loosely from theformer AUXNAV specialty course(COMDTPUB P16798.16A) docu-mentation:

❏ First, carefully read the direc-tions that come with the vessel’scompass and ensure that thecompass is mounted in such a

way to minimize possiblesources of deviation.

❏ Second, follow the directionsgiven below to determine thecompass deviations on variousheadings. If these deviations are“acceptable” (a judgement call),then use the “For/Steer” table(see below) directly and do notundertake compass adjustment.If not, then either call a profes-sional compass adjuster or usethe following procedure. Readthe directions for compassadjustment (contained in theowner’s information suppliedwith the compass) again toensure that you are thoroughlyfamiliar with the procedure andthe location of the two adjustingscrews.

❏ Third, make the following work-ing tool. Take a sturdy card-board and a dowel (a pencil willdo). Make a hole for the dowelin the center of the cardboard


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DO NOT PLACE METAL OBJECTS NEAR COMPASS

Do not use the area near

the compass as a resting

place for metal objects,

such as flashlights, cameras,

kitchen utensils,

certain plotting instruments,

and even metal sunglasses.

These can affect the

accuracy of the compass

readings and cause serious

course errors!

W A R N I N G USEFUL TIP

It is important to emphasize

that deviation varies with the

vessel’s heading. When con-

verting a relative bearing to a

true or magnetic bearing,

novices often make the mis-

take of applying the devia-

tion appropriate to the rela-

tive bearing rather than the

vessel’s heading. Be careful

not to make this error!

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and draw a straight line acrossthe cardboard through the cen-ter. Select a calm, sunny day(with minimal traffic to avoid)for this evolution, in mid-morn-ing or afternoon when the sunwill cast a shadow on the dowel.Take the boat out and maintain aconstant heading of north asindicated by the compass. Placethe dowel in the hole and rotatethe board until the shadow of thedowel falls on the line. Nowturn the boat in the oppositedirection until the shadow fallson the other side of the line. Youwill have turned 180° (turn andsteady on the reciprocal coursepromptly because the sun movesabout 1° in 4 minutes). Read thecompass on this heading. Mostprobably, it will not read exactly180°. Now, use a stainless steelor brass screwdriver on theathwartships (N-S adjustment)adjusting screw; remove half ofthe difference between the com-pass reading and 180°. Forexample, if the compass were toread 170°, use the adjustingscrew to set the compass to175°. Turn back on the originalcourse until the shadow falls onthe other side and take out halfof the difference between thecompass reading and 000°.

❏ Fourth, repeat the process untilyou can’t remove any moreerror.

❏ Fifth, do the same thing on east-west headings. Head 090° bycompass, align the shadow withthe line, turn 180°, read thecompass, and take out half theerror with the other (E-W)adjusting screw. When no fur-ther improvements can be made,

make another compass deviationcard as described below.

For other perspectives, read throughappropriate sections of Brogden(1995), Eyges (1989), Denne(1979), and Kaufman (1978)included in the references at the endof this chapter.

It is seldom the case that all theeffects of this magnetic field can becompensated for by the adjustingmagnets, and usually a small resid-ual deviation (say 2 degrees to 4degrees, but sometimes more)remains after adjustment. Themariner has two options for dealingwith residual deviation. The first issimply to ignore any residual errorand effectively compensate for itspresence by fixing the vessel’s posi-tion more often. As a rough rule ofthumb, an unrecognized error of 1°means that a vessel would beapproximately 1 mile off course(termed cross-track error) if it traveled a distance of 60 miles.Table 2-1 shows the cross trackerror as a function of the distancetraveled and the angular error orresidual deviation. For short dis-tances, small angular errors arepractically insignificant and cansometimes be ignored. However,for longer distances or in conditionsof poor visibility (which would pre-vent detection and identification oflandmarks, fixed aids to navigation(ATONs), or buoys), simply ignor-ing deviation cannot be recom-mended.

The second, and generallypreferable, option is to measure thecompass deviation, and use thismeasured value to correct theobserved compass heading to amagnetic heading in the same man-ner as variation is used to “correct”

the magnetic heading to a trueheading. However, unlike variation,which depends solely on the ves-sel’s position, deviation varies withthe vessel’s heading. Therefore, it isnecessary to use the deviationappropriate to the vessel’s compassheading before it can be used toconvert to the correct magneticheading. Although, theoretically,this deviation could be different foreach possible heading, in practicethe deviation is determined for each15-degree or 30-degree headingincrement; then these values areinterpolated to estimate the devia-tion on intermediate headings. Thisprocess of determining the devia-tion on various headings is termedswinging ship or swinging the com-pass and is discussed below.

SWINGING SHIPNormally, professional compass

adjusters will swing ship as part oftheir services to compensate thecompass and provide a table ofdeviations to the mariner. In suchcases, the mariner will probablywish to spot-check this table peri-odically to verify its continuingaccuracy. However, the procedures(discussed below) are the samewhether the entire deviation table isbeing prepared or individual valuesare being spot-checked.

In brief, the procedure forswinging ship is to steady on aknown compass course and thentake bearings on a distant object orrange. The vessel is positioned sothat the magnetic bearing to theobject to be observed is known. Thecompass bearing is read directly, orconverted from a relative bearingobtained using a pelorus and com-pared with the object’s known mag-

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netic bearing, and the deviation iscalculated. Professional compassadjusters often use the sun forobservation, but most mariners areunfamiliar with celestial navigationand elect to use something simpler,

such as a prominent object or range.The object(s) selected for observa-tion should be a good distance away(e.g., 6 miles) to minimize parallaxerror in the calibration. It is impor-tant that swinging ship is done

when conditions are nearly ideal, incalm waters and in good visibility.The need for good visibility is obvi-ous. The reason why calm watersare preferred is to simplify steady-ing the vessel on a compass heading


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MILES 1.00 1.50 2.00 2.50 3.00 4.00 5.00 7.50 10.00

1.00 0.02 0.03 0.03 0.04 0.05 0.07 0.09 0.13 0.182.00 0.03 0.05 0.07 0.09 0.10 0.14 0.17 0.26 0.353.00 0.05 0.08 0.10 0.13 0.16 0.21 0.26 0.39 0.53

4.00 0.07 0.10 0.14 0.17 0.21 0.28 0.35 0.53 0.715.00 0.09 0.13 0.17 0.22 0.26 0.35 0.44 0.66 0.886.00 0.10 0.16 0.21 0.26 0.31 0.42 0.52 0.79 1.06

7.00 0.12 0.18 0.24 0.31 0.37 0.49 0.61 0.92 1.238.00 0.14 0.21 0.28 0.35 0.42 0.56 0.70 1.05 1.419.00 0.16 0.24 0.31 0.39 0.47 0.63 0.79 1.18 1.59

10.00 0.17 0.26 0.35 0.44 0.52 0.70 0.87 1.32 1.7612.50 0.22 0.33 0.44 0.55 0.66 0.87 1.09 1.65 2.2015.00 0.26 0.39 0.52 0.65 0.79 1.05 1.31 1.97 2.64

17.50 0.31 0.46 0.61 0.76 0.92 1.22 1.53 2.30 3.0920.00 0.35 0.52 0.70 0.87 1.05 1.40 1.75 2.63 3.5322.50 0.39 0.59 0.79 0.98 1.18 1.57 1.97 2.96 3.97

25.00 0.44 0.65 0.87 1.09 1.31 1.75 2.19 3.29 4.4127.50 0.48 0.72 0.96 1.20 1.44 1.92 2.41 3.62 4.8530.00 0.52 0.79 1.05 1.31 1.57 2.10 2.62 3.95 5.29

35.00 0.61 0.92 1.22 1.53 1.83 2.45 3.06 4.61 6.1740.00 0.70 1.05 1.40 1.75 2.10 2.80 3.50 5.27 7.0545.00 0.79 1.18 1.57 1.96 2.36 3.15 3.94 5.92 7.93

50.00 0.87 1.31 1.75 2.18 2.62 3.50 4.37 6.58 8.8260.00 1.05 1.57 2.10 2.62 3.14 4.20 5.25 7.90 10.5870.00 1.22 1.83 2.44 3.06 3.67 4.89 6.12 9.22 12.34

80.00 1.40 2.09 2.79 3.49 4.19 5.59 7.00 10.53 14.1190.00 1.57 2.36 3.14 3.93 4.72 6.29 7.87 11.85 15.87

100.00 1.75 2.62 3.49 4.37 5.24 6.99 8.75 13.17 17.63

ANGULAR ERROR (DEGREES)DISTANCE

TRAVELED

▲

TABLE 2-1–Cross-track error as a function of distance traveled and residual deviation or other angular helm error.

▲

FIG. 2-5–The range formed by the spire in New Bedford andthe Butler Flats light bears approximately 310 degrees.

310


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and reading the compass. Experi-ence shows it can take several hoursto swing ship, so the exerciseshould be planned with sufficienttime allowance to be completedwithin the daylight hours.

The procedure for swinging shipdepends upon the compass to beexamined and the ability to takebearings on objects not directlyaligned with the fore-and-aft axis ofthe vessel. If the compass is gradu-ated to the nearest degree, anddesigned and located so that anunobstructed view is possiblethroughout all 360 degrees, thenonly a compass is necessary. (Thisis likely to be the case for relativelyfew boats.) If, as is more common,the compass is graduated only in 5-

degree increments or bearings arenot easily read throughout 360degrees, then it is necessary to use apelorus as well (discussed below).

DIRECT OBSERVATIONUSING A RANGE

The easiest method for swingingship, if circumstances permit, is touse a range and read the compassdirectly. A range consists of twocharted objects that can be viewedand aligned from a distance. Forexample, consider the spire in NewBedford and the Butler Flats Lightlocated south and east of New Bed-ford shown in the l210-Tr chart andin Figure 2-5. Both of these objectsare likely to be prominent and rela-tively easy to identify. Approaching

from the south, these two objectsare exactly in line (one behind theother or in range) on a bearing of310 degrees true from the vessel tothe objects. The bearing can be readfrom the chart as discussed inChapter 3, but take the answer asgiven for the present. The variationin this area, read from the nearestcompass rose, is approximately 015degrees west; so the magnetic bear-ing of this range would be 310 +015 = 325 magnetic. (See below fora handy rule to remember whetherto add or subtract variation.)

Suppose that the vessel weresteadied on a compass course of000 degrees (compass north) whilethe vessel was somewhere south ofthe line drawn on the chart. (This

876543210-1-2-3-4-5-6-7-8

876543210-1-2-3-4-5-6-7-8

E

W

E

W

0 30 60 90 120 150 180 210 240 270 300 330 360

Deviation (Degrees)

Compass Heading (Degrees)

Vessel: Auxiliary Facility 273007, All Radiosand Electronics on, Data from 09-30-89 forCompass at Lower Helm Station.

▲

FIG. 2-6–A Plot of Compass Deviations

would, of course, be evident fromthe vessel because the Butler FlatsLight would be to the right of thespire. At the precise instant that thelight and the spire appeared to be inline, the magnetic bearing of therange would be 325 degrees fromthe vessel. At this same instant, sup-pose that the compass bearing ofthe range (read over the compass)was 323 degrees (while the com-pass heading was 000 degrees). Thedeviation on this heading is the dif-ference between the magnetic bear-ing, 325 degrees, and the bearingread from the compass, 323degrees, or 2 degrees. But, is it 2degrees east, or 2 degrees west? Itcan, of course, be worked out fromfirst principles (refer to Chapter 1),but it is easy to remember the sim-ple phrase, “compass least, erroreast.” That is, if the compass bear-ing is less than the magnetic bear-ing (as it is in this case, 323 degreesis less than 325 degrees), then thedeviation is “east.” (If not, then theerror would, of course, be “west.”)Thus, in this example, the estimatedcompass deviation on a heading of000 degrees is 002 degrees east. Toconfirm this result, the processmight be repeated and the averagedeviation noted.

It is convenient to use a work-sheet, such as is shown in Table 2-2to record the observations. Thisworksheet contains directions aswell, which makes it handy to use.The process is now repeated on acompass heading of 015 degrees,030 degrees, etc., until all observa-tions are recorded.

PLOTTING THE RESULTSThe results should be plotted on

a sheet of graph paper to see if there

are any “anom-alous” resultsthat do not fit thepattern. Overall,the line drawnthrough the mea-sured deviationsshould appear asa smooth curve(actually a mix-ture of trigono-metric functionsfor those techni-cally inclined)free of “bumps” or observationsthat appear discrepant. Such a curveis drawn in Figure 2-6 and appearsgenerally to confirm the adequacyof the measurements, although thedeviations on some headings, suchas 240 degrees, should berechecked. (In Figure 2-6 easterlydeviations are shown with a plussign, and westerly with a minussign.) Additionally, the deviationson some headings are relativelylarge (5 or 6 degrees), so the com-pensation is far from perfect. (Amore technical analysis, omittedhere, suggests that improved com-pensation is possible. However, theexample is continued for illustrativepurposes.)

USE OF A PELORUSAs noted, most marine compass

installations do not permit directreading of compass bearingsthrough a full 360 degrees. Addi-tionally, many compasses are grad-uated only to 2 degrees or 5degrees, rather than in 1-degreeincrements. If either of the state-ments is true, it is necessary tomodify the procedure given abovefor swinging ship. The most conve-nient solution is to use a pelorus,sometimes called a dumb compass.

A pelorus consists of a graduatedcompass-like dial (l-degree incre-ments) and sighting vanes that canbe rotated around the dial to takebearings. Unlike the “north-seek-ing” compass dial, however, theposition of the dial on the pelorusdoes not change with the vessel’sheading. The pelorus is mountedand the dial fixed so that the 000-degree mark on the dial is pointedso as to be parallel to the vessel’sbow, precisely aligned with thefore-and-aft axis of the vessel.(Consult the directions suppliedwith the pelorus for mountinginstructions and pay particularattention to the alignment proce-dure.) The pelorus should bemounted so that the navigator caneasily view objects throughout afull 360 degrees.

The bearings read from thepelorus are relative bearings (referto Chapter 1), rather than compassbearings. For this reason it is neces-sary to calculate the compass bear-ing from the simple equation:Ship’s heading + Object’s relativebearing = Object’s compass bear-ing. Figure 2-7 illustrates this equa-tion.

2-12


▲

FIG. 2-7–Relative Bearing


2-13

▲ TABL

E 2-

2–Po

rtion

of W

orks

heet

for D

eter

min

atio

n of

Dev

iatio

ns fr

om R

ange

DA

TE

:10

-15-

99N

AV

IGA

TO

R:

LD

MV

ESS

EL

:A

ltai

rR

AN

GE

:Sp

ire

in N

ew B

edfo

rd a

nd B

utle

r F

lats

Lig

htN

OT

ES:

Lor

an,R

adar

,and

VH

F R

adio

ON

TR

UE

BE

AR

ING

OF

RA

NG

E:

310

Det

erm

ined

fro

m N

autic

al C

hart

or

Oth

er S

ourc

eV

AR

IAT

ION

:01

5 W

est

Det

erm

ined

fro

m N

earb

y C

ompa

ss R

ose

MA

GN

ET

IC B

EA

RIN

G O

F R

AN

GE

:32

5 M

agne

tic

Tru

e +

/- V

aria

tion,

+W

est,

-Eas

t

Mag

neti

cD

irec

tion

Est

imat

edD

irec

tion

of R

ange

Dev

iati

onV

esse

l’s H

ead

ofP

eron

Thi

sE

ntry

Per

Com

pass

Ran

geC

ompa

ssH

eadi

ngN

otes

12

34

5W

here

Rea

d Fr

omTa

ken

From

Bea

ring

of

Dif

fere

nce

Bet

wee

nO

btai

ned

Lub

ber’

s L

ine

Abo

veR

ange

as

Col

umns

3 a

nd 4

Dir

ectly

Cal

cula

tion

Rea

d O

ver

If C

olum

n 4<

Com

pass

Col

umn

3,D

evia

tion

is“E

ast,”

Oth

erw

ise

Dev

iatio

n “W

est”

000

325

323

2E

015

325

322

3E

030

325

320

5E

045

325

319

6E

060

325

320

5E

075

325

320

5E

Thi

s w

orks

heet

isde

sign

ed to

be

used

for

dete

rmin

atio

n of

de

viat

ion

whe

re a

ran

geis

ava

ilabl

e an

d ca

n be

sigh

ted

over

the

com

-pa

ss. T

he tr

ue b

eari

ngof

the

rang

e ca

n be

dete

rmin

ed f

rom

a

naut

ical

cha

rt,L

ight

Lis

t,or

oth

er s

ourc

e.C

ompa

ss d

evia

tion

shou

ld b

e de

term

ined

ever

y 15

deg

rees

of

head

ing,

gene

rally

star

ting

at 0

00 d

egre

es.

The

“no

tes”

sect

ion

shou

ld id

entif

y th

eel

ectr

onic

s on

boa

rd,

and

oper

atin

g at

the

time

of th

e ca

libra

tion.

It is

sug

gest

ed th

at tw

ota

bles

be

prep

ared

,one

with

all

elec

tron

ics

on,

and

the

othe

r w

ith

elec

tron

ics

off.

This extra step requires a slightmodification to the worksheet givenin Table 2-2 to prepare a compassdeviation table. This modifiedworksheet is shown in Table 2-3. Toclarify by example, suppose that acompass deviation table is beingprepared using the same range asgiven in Table 2-2. While on a com-pass heading of 015 degrees, thenavigator sights over the vanes ofthe pelorus as the range is perfectlyaligned. The navigator calls “mark-mark-mark” to allow the helmsmanto make slight heading changes tobring the vessel back to theassigned 015-degree heading, andnotes the relative bearing, say 307degrees. Alternatively, the helms-man sings out the vessel’s headingon hearing “mark-mark-mark,” andthe navigator notes this heading.The compass bearing to the rangeis, therefore, 015 + 307 = 322degrees (360 degrees will have tobe subtracted from this total if thetotal exceeds 360). The deviationon a compass heading of 015degrees is, therefore, 3 degrees eastas in the earlier example (remem-ber, compass least, error east).

SPOT CHECKSTo spot-check a previously pre-

pared deviation table, all that is nec-essary is to take advantage ofranges that may appear near thevessel’s course. In this case, thehelm is put over briefly to align thevessel with the range, the compassheading noted, and the deviation onthis heading calculated as discussedabove. At a minimum, compassheadings should be recorded in alog (see sidebar) for later analysiswhen the vessel is anchored ordocked.

During a typical voyage thereare many opportunities for suchspot checks, and these should beused to advantage. Throughout thenavies of the world, it is commonpractice to check the compass atleast once daily and to report theresults to the captain. For the aver-age boater, it is not necessary tomake such frequent or formalchecks, but it is appropriate to

check deviation at least a few timesduring the boating season, andwhenever a major voyage isplanned. Deviation can changewhenever new electronic gear isbrought aboard (or moved), the vessel is laid up for the winter,or someone inadvertently leaves a flashlight or camera near the compass.

Lightning strikes near the vesselcan also affect deviation, and thecompass should be checked afterelectrical storms have passedthrough the area.

DEVIATION ON INTERMEDIATE HEADINGS

The deviation table consists ofentries spaced every 15 degrees orevery 30 degrees. Values for inter-mediate headings are obtained byinterpolation. For example, if thedeviation were 3 degrees east on acompass heading of 000 degreesand 0 degrees on a heading of 015degrees, it would be approximately2 degrees on a heading of 005degrees. Fancy formulas are notwarranted here; simply prorate thedeviation directly and round thecalculation to the nearest degree.

USE OF THE DEVIATION TABLE

The deviation table is used fortwo important purposes. First, it isused to calculate the vessel’s actualmagnetic heading when steering aknown compass heading. Second, itis used to calculate the correct com-pass heading to steer to make gooda desired magnetic heading.

The deviation table solves thefirst objective directly. Refer toTable 2-4, for example, based uponthe measurements discussed above.

2-14


PRACTICAL TIP:USE OF A

COMPASS IN REPEATABLE MODE

Get in the habit of entering

compass headings in a log on

your trips—particularly

when in good weather. If the

same trip is repeated in con-

ditions of reduced visibility,

these same compass head-

ings can be used—even if the

compass has not been

checked for deviation and

errors exist. This use of a

navigation instrument is

termed its use in repeatable

mode. If you return to the

same readings of the com-

pass (or other navigation

instrument), you compensate

for its error. For an interest-

ing discussion of this princi-

ple, refer to Brogden (1995).

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2-15

▲ TABL

E 2-

3–Po

rtion

of W

orks

heet

for D

eter

min

atio

n of

Dev

iatio

ns U

sing

Rel

ativ

e Be

arin

gs

DA

TE

:10

-15-

99N

AV

IGA

TO

R:

LD

MV

ESS

EL

:A

ltai

rR

AN

GE

:Sp

ire

in N

ew B

edfo

rd a

nd B

utle

r F

lats

Lig

htN

OT

ES:

Lor

an,R

adar

,and

VH

F R

adio

ON

TR

UE

BE

AR

ING

OF

RA

NG

E:

310

Det

erm

ined

fro

m N

autic

al C

hart

or

Oth

er S

ourc

eV

AR

IAT

ION

:01

5 W

est

Det

erm

ined

fro

m N

earb

y C

ompa

ss R

ose

MA

GN

ET

IC B

EA

RIN

G O

F R

AN

GE

:32

5 M

agne

tic

Tru

e +

/- V

aria

tion

(+W

est,

-Eas

t)

Dir

ecti

onM

agne

tic

Est

imat

edR

elat

ive

of R

ange

Dir

ecti

onD

evia

tion

Ves

sel’s

Hea

dB

eari

ng o

fP

erof

on T

his

Ent

ryP

er C

ompa

ssR

ange

Com

pass

Ran

geH

eadi

ng

12

34

56

Whe

reR

ead

From

Rea

dV

esse

l’s H

ead

(2)

Take

n Fr

omD

iffe

renc

e B

etw

een

Obt

aine

dL

ubbe

r’s

Lin

e Fr

omPl

us R

elat

ive

Abo

veC

olum

ns 4

and

5D

irec

tlyPe

loru

sB

eari

ng (

3)C

alcu

latio

nIf

Col

umn

4<(3

60°

may

hav

eC

olum

n 5,

to b

e su

bstr

acte

d)D

evia

tion

is“E

ast,”

Oth

erw

ise

Dev

iatio

n “W

est”

015

307

322

325

3E

Suppose that the vessel’s compassheading were 045 degrees. Fromthe first two columns of this table(headed by the phrase “compass tomagnetic”), the deviation corre-sponding to a compass heading of045 degrees is 6 degrees east. Themagnetic heading is the compassheading plus or minus the devia-tion. Converting from a compass

heading to a magnetic heading isoften termed “correcting,” becausethis process removes (corrects for)the deviation error. A simple rule toremember is “correcting add east,”meaning that in converting from acompass heading to a magneticheading, easterly deviation is added(westerly deviation is, therefore,subtracted). Using this rule, the cor-

rected or magnetic heading corre-sponding to a compass heading of045 would be 045 + 006 = 051degrees magnetic. Similarly, a com-pass heading of 030 degrees corre-sponds to a magnetic heading (referto Table 2-4) of 035 degrees.

Frequently, however, it is neces-sary to reverse the process. That is,to find the appropriate compass

2-16


▲

TABLE 2-4–Sample Deviation Table [All Values in Degrees(°)]

COMPASS TO MAGNETIC MAGNETIC TO COMPASS

(A) (B) (C) (D)Compass MagneticHeading Deviation Heading Deviation

000 2E 000 2E015 3E 015 3E030 5E 030 4E

045 6E 045 6E060 5E 060 5E075 5E 075 5E

090 3E 090 3E105 3E 105 3E120 2E 120 1E

135 1W 135 1W150 2W 150 2W165 3W 165 2W

180 4W 180 4W195 5W 195 5W210 6W 210 6W

225 5W 225 5W240 4W 240 4W255 5W 255 5W

270 4W 270 4W285 3W 285 3W300 2W 300 2W

315 1W 315 1W330 1E 330 1E345 2E 345 2E

360 2E 360 2E

course to steer to make good a par-ticular magnetic heading. For thistask it is necessary to reverse thelogic discussed above. For example,suppose that the mariner wants tomake good a course of 045 magnet-ic. What compass course should besteered? From the above discussion,note that on a magnetic heading of035 degrees the deviation is 5degrees east, whereas on a magneticheading of 051 degrees, the devia-tion is 6 degrees east. A simpleinterpolation (rounded to the nearestdegree) indicates that the deviation

on a 045 degree heading is approxi-mately 6 degrees (5 + 10(6-5)/16 =5.625, rounded to 6). Therefore, theapproximate deviation on a magnet-ic heading of 045 degrees is 6degrees east, as shown in Table 2-4.When making interpolations,remember that deviations are onlymeasured to the nearest degree. Acalculation is only as accurate as theleast accurate number, so round allinterpolated numbers to the nearestdegree.

Continuing the process leads tothe results shown in Table 2-4,

which completes the deviationtable. Use the left half of the tablewhen correcting from compass tomagnetic, and the right half when“uncorrecting” from magnetic tocompass. (Unless the deviations arequite large, the two halves of thetable are virtually identical and, inpractice, these differences are oftenneglected.) Incidentally, the datapresented in the right hand side(RHS) of Table 2-4 are often usedto make a simple “For/Steer” cor-rection card. Entries for this tablecan be computed as follows. Sup-


2-17

NAME BRIEF DESCRIPTION REMARKS

Northerly Turning Error Applies principally when vessel is on north- Of principal concern to aircraft, buterly or southerly headings and the compass of relevance to all fast boats. Arisescard is tilted with respect to the horizon. from magnetic dip. Phenomenon de-Effect is for compass to lag the turn, or scribed for northern hemisphere only.momentarily show a turn in the oppositedirection when turning from north. In turnsfrom south, the compass leads the turn, i.e.,shows the vessel turning more rapidly than itactually is. The effect is greatest in a rapid,steeply banked turn.

Acceleration Error Also due to dip, this error is greatest on head- Effect greatest with vessels capableings of east or west and zero on north or of large accelerations, e.g., speedsouth. If the vessel is accelerated on either of boats. Also seen with aircraft. Oftenthese headings, the compass will indicate an observed when boat butts into headapparent turn to the north. When decelerating, sea, or planes down a swell while onthe compass indicates a turn to the south. an east or west heading.A memory aid to remember the word “ANDS,”for Acceleration – North, Deceleration – South.

Oscillation Error Though listed as a separate error in sometexts, this is actually a combination of theabove errors. Results from erratic movementsof the compass card caused by rough seas orabrupt helm changes. Helmsman has to “av-erage’’ out oscillations mentally for precisesteering.

Heeling Error Of particular relevance to sailing vessels, this Adjusted for by heeling magnets onerror arises from change in the horizontal some compasses.Adjustment is par-component of the induced or permanent tially a function of the magneticmagnetic fields at the compass due to rolling latitude of the vessel.or pitching of the ship. To a lesser extentheeling errors may be affected by the angle ofplane of a powerboat.

▲

TABLE 2-5–Additional Compass Errors Which Arise if Vessel is not Straight and Level, and at Constant Speed

NOTE: See article by Kielhorn for details on some of these errors.

pose that the mariner wishes to fol-low a magnetic heading of 000.Reference to Table 2-4 shows thaton a magnetic heading of 000 thedeviation is 2° east. Therefore, thecompass heading in this case wouldbe 358. So, for a course of 000magnetic, the mariner should steer358. Under the heading “For” isplaced the magnetic course 000 andunder the heading “Steer” is placed358. The table is completed to cre-ate a For/Steer card that is normallyposted next to the compass.

COMPASS CALCULATIONSNavigators need to become

familiar with the calculations nec-essary for converting from true

courses to compass courses, andvice versa. Although these calcula-tions are quite simple, practice isnecessary to ensure familiarity. Forthis reason, the following text andexamples are given.

It is often necessary to convertfrom true to compass headings orbearings. As discussed in laterchapters, courses are laid out onnautical charts and, typically, mea-sured with respect to true north. Butto undertake the voyage, the naviga-tor needs to determine how to con-vert this true course to a compasscourse to steer. The overallsequence for this conversion is tostart with the true course, add orsubtract variation to calculate amagnetic course, and then againadd or subtract deviation to calcu-late a compass course: True, Varia-tion, Magnetic, Deviation, Com-pass, or as it is sometimes said,TVMDC. In addition to learningthe sequence of calculations, it isuseful to have a handy rule toremember whether to add or sub-tract variation and deviation.

Throughout history there havebeen a series of “salty” mnemonicsused to help remember the TVMDCsequence. The current politicallycorrect mnemonic is TeleVisionMakes Dull Children; Avoid Watch-ing. Decoded, it reveals thesequence of calculations TVMDCand the reminder (AW) to add westwhen converting from true to com-pass. For example, what is the com-pass heading to steer if the truecourse is 060, variation is 015 west,and the deviation table is as given inTable 2-4? The answer is calculat-ed as follows:

❏ First, start at the true course,060, and convert it to the mag-

netic course. Since the variationis west, it is added to the truecourse. The magnetic course is,therefore, 060 + 015 = 075.

❏ Second, from Table 2-4, thedeviation corresponding to amagnetic heading of 075degrees is 5 degrees east. Fromthe simple rule to add west inthis sequence, it follows that a 5-degree easterly deviationwould be subtracted.

❏ Third, from the above steps,the required compass course is075 - 005 = 070 degrees.The important points to remem-

ber are the sequence of calculationsand whether to add or subtract vari-ation and deviation. If all else fails,reference to the nearest compassrose on the nautical chart willenable you to figure out whether toadd or subtract variation (or devia-tion).

Sometimes it is necessary toreverse the process, and convertfrom compass to true. For example,a bearing on a distant object may betaken from the vessel’s compass,and it is necessary to convert thisbearing to true, before plotting on anautical chart. The sequence of cal-culations is just the opposite of thatdiscussed above, i.e., CDMVT. Aswell, the sign to apply to east orwest variation or deviation is alsoreversed. That is, east is added, andwest is subtracted. Although this issimple enough to remember, someprefer to use the additional memoryaid: Can Dead Men Vote Twice? AtElections! The first letters are thememory aid to the sequence: Com-pass, Deviation, Magnetic, Varia-tion, True, and Add East. (Some air-craft pilots were taught the phrase:Can Ducks Make Vertical Turns?)

2-18


VARIATION ANDDEVIATION

Variation is the angular

difference between true

and magnetic north—see

Chapter 1. Variation is a

function of the vessel’s loca-

tion on the earth. It can be

found on the nautical chart.

Deviation is the difference

between magnetic north and

compass north. It is particu-

lar to each vessel and is

a function of the heading of

the vessel. Although related,

these are distinct concepts.

DO NOT CONFUSETHESE TWO TERMS.

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To illustrate, suppose that thecompass heading of the vessel is065 degrees and an object is sightedbearing directly ahead per the ves-sel’s compass in an area where thevariation is 015 degrees west. Whatis the true bearing? Assuming thatthe deviation table is as given inTable 2-4, the deviation on a com-pass heading of 065 degrees is 5degrees east and, therefore, themagnetic heading of the vessel is065 + 005 = 070 degrees (add east).In turn, the true heading is the mag-netic heading plus or minus varia-tion. If east is to be added, thenwest should be subtracted for thiscalculation, so the true heading is070 – 015 = 055 true. That’s allthere is to it. Practice until you areproficient.

ADDITIONAL POINTERS ON THE COMPASS

It is important to remember thatcompass readings are most accurateonly when the vessel is level (asopposed to heeling), traveling at aconstant speed, and maintaining aconstant course. Otherwise, a seriesof additional compass errors is

introduced, as shown in Table 2-5.It is not a good idea to use compassreadings obtained while the vesselis heeling, turning, or accelerat-ing/decelerating. The effects ofthese errors are to make the com-pass difficult to read and/or to giveerroneous indications. These errorsare largest for vessels capable of

substantial acceleration (e.g.,speedboats), and substantial anglesof heel or bank. (Consult the refer-ences at the end of this chapter formore details.) Directional gyros orgyrocompasses are less prone tothese errors and sometimes favoredby mariners for this reason (amongothers).


2-19

LOCAL MAGNETIC DISTURBANCE

Notes indicating magnetic disturbance are

printed in magenta on U. S. charts. Where space

permits, these notes are printed in the specific area

of local magnetic disturbance. Here are some

examples of how these are shown:

LOCAL MAGNETIC DISTURBANCEDifferences from normal variation of as much

as 5° have been observed in Gastineau Channel

in the vicinity of Lat. 58° 15'.

LOCAL MAGNETIC DISTURBANCEDifferences of 12° or more from normal varia-

tion may be expected in X Channel in the vicin-

ity of Z Point.

Where limited by space, the full note is placed else-

where on the chart and the following reference note

shown (in magenta) in the area of the disturbance:

LOCAL MAGNETIC DISTURBANCE(See Note)Mariners should exercise particular vigilance

when operating in these areas.

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▲

FIG. 2-8–Fluxgate Digital Compass.Both the fluxgate sensor and the Liq-uid Crystal Display (LCD) are enclosedin one watertight unit.

PH

OTO

CO

UR

TE

SY

OF

KV

H IN

DU

ST

RIE

S,I

NC

.

LOCAL MAGNETIC DISTURBANCES

In some areas of the world, themeasured values of magnetic varia-tion differ from the expected (chart-ed) values by several degrees. Thesources of these discrepancies aretermed local magnetic distur-bances, local attractions, or mag-netic anomalies. Magnetic distur-bance notes identify such areas onU. S. nautical charts where errorsare greater than or equal to 2° (3° inAlaska). The note will indicate thelocation and magnitude of the dis-turbance. The indicated magnitudeshould not be considered as thelargest possible value that may beencountered. Large disturbancesare more frequently encountered inthe shallow water areas near land-masses (particularly mountains)than on the ocean. Fortunately, theeffect of a local magnetic distur-bance typically diminishes rapidlywith distance. However, in somelocations there are multiple sourcesof disturbances and the effects maybe distributed for many miles. Readthe nautical chart carefully to deter-

mine if there are areas of local mag-netic disturbance located alongyour proposed route. Exercise extravigilance when transiting theseareas. Do not rely entirely on thecompass; steer by reference to land-marks and/or ATONs, if possible,and fix your position frequently.Obviously, you should not attemptto calibrate a compass in an area ofknown local magnetic disturbance.(Local magnetic disturbances arenot a compass error, per se.Nonetheless, this magnetic phe-nomenon does affect the compassand should be noted.)

THE FLUXGATE COMPASSFinally, any modern discussion

of compasses would be incompletewithout, at least, a passing mentionof the fluxgate compass. The flux-gate compass senses the earth’smagnetic field electronically, ratherthan with magnets. (Readers wish-ing a more complete discussionshould refer to the bibliography.)The fluxgate compass consists of asensor and a display unit. (The sen-sor and the display unit may be in

the same, or different, units.) If sep-arate from the display unit, the sen-sor can be located remotely, in anarea of the vessel where magneticdisturbances are at a minimum. Thedisplay unit is small and can bemounted for optimum visibility.Most modern fluxgate systems areintegrated with microprocessors,which can perform many usefulfunctions. The model, pictured inFigure 2-8, automatically compen-sates for deviation (to within +/- 1degree, in most cases) by simplymaking a 360 degree turn with thevessel! Other models can also dis-play headings in either true or mag-netic (variation data are stored inthe microchip) and can furnish elec-tronic inputs to other navigationsystems such as the Global Posi-tioning System (GPS), Loran-C, orradar.

An electronic compass is veryconvenient. However, it does noteliminate the need for a magneticcompass, because electronics aredependent upon a reliable powersupply and are easily damaged inthe marine environment.

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Aczel A. D., (2001). The Riddle of the Compass, TheInvention That Changed the World, Harcourt, Inc.,New York, NY.

Anon., (1988). The Magnetic Compass, Principles,Selection, Navigation, E. S. Ritchie & Sons, Inc.,Pembroke, MA.

Bowditch, N. (1995). American Practical Navigator,An Epitome of Navigation, Pub. No. 9, DefenseMapping Agency Hydrographic/Topographic Center, Bethesda, MD. This publication is avail-able electronically from the National Imagery andMapping Agency (NIMA) Marine NavigationDepartment web site (http://www.nima.mil/).

Brogden, B., (1995). Boat Navigation for the Rest ofUs, Finding Your Way by Eye and Electronics,International Marine, Camden, ME.

Cohan, L. S., (1988). Compass Deviation Affected by Lightning, Ocean Navigator, No. 17,January/February.

Collinder, P. A., (1955). History of Marine Navigation,St. Martin’s Press, Inc., New York, NY.

Dahl, N., (1983). The Yacht Navigator’s Handbook,Hearst Books, New York, NY.

Defense Mapping Agency, Hydrographic/TopographicCenter, (1980). Handbook of Magnetic CompassAdjustment, Fourth Edition, Pub. No. 226,Washington, DC.

Denne, W., revised by Cockcroft, A. N., (1979). Magnetic Compass Deviation and Correction, AManual of the Theory of the Deviations andMechanical Correction of Magnetic Compasses inShips, Brown, Son & Ferguson, Ltd., Glasgow,Scotland.

Department of Transportation, Federal Aviation Administration, (1987). Instrument Flying Hand-book, AC61-27C, Washington, DC.

Eyges, L., (1989). The Practical Pilot, Coastal Navigation by Eye, Intuition and Common Sense,International Marine Publishing Co., Camden, ME.

Fuson, R. H. (Translator), (1987). The Log of Christopher Columbus, International Marine Publishing Company, Camden, ME.

Ganssle, J. G., (1989). Anatomy of a Fluxgate, OceanNavigator, September/October.

Hempstead, R. L., (1987). Magnetic Compasses in theSmall Boat Environment, Ocean Navigator, No. 12,March/April, pp. 21 et seq.

Hewson, J. B., (1983). A History of the Practice of Navigation, Second Edition, Brown, Son & Ferguson, Ltd. Publishers, Glasgow, Scotland, UK.

Jerchow, E., (1987). From Sextant to Satellite Navigation, 1837-1987, 150 Years, C. Plath, Ham-burg, Germany.

Kaufman, S., (1978). Compass Adjusting for SmallCraft, Surfside Harbor Associates, Surfside, FL.

Kielhorn, W. V., (1988). A Piloting Primer, privatelyprinted at Naples, FL.

Kielhorn, W. V., (1957). The Northerly Turning Error,The Rudder, Vol. 73, No. 2, pp. 40-42.

Kielhorn, W. V., and H. W. Klimm, (1978). A Preliminary Study of the Changes of MagneticStructure in a New Steel Trawler, Navigation,Journal of the Institute of Navigation, Vol. 6, No. 6,pp. 365 et seq.

Moody, A. B., (1980). Navigation Afloat: A Manual forthe Seaman, Van Nostrand Reinhold, New York,NY.

Morrison, S. E., (1942). Admiral of the Ocean Sea, ALife of Christopher Columbus, Little, Brown andCompany, Boston, MA.

Queeney, T. E., (1985). Beyond the Lodestone, OceanNavigator, No. 3, September/October, pp. 33 et seq.

Queeney, T. E., (1987). Future Heading Systems, AreLasers The Next Step, Ocean Navigator, No. 12,March/April, pp. 35 et seq.


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SELECTED REFERENCES

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Rousmaniere, J., (1989). The Annapolis Book of Seamanship, Simon and Schuster, New York, NY.

Saunders, A. E., (1987). Small Craft Piloting andCoastal Navigation, Alzarc Enterprises,carborough, Ontario, Canada.

Van Heyningen, M. A. K., (1987). The Evolution of theModern Electronic Compass, NMEA News,January/February.

SELECTED REFERENCES

Documents

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