HoMCA.pdf

HANDBOOK OF MAGNETIC

COMPASS ADJUSTMENT

NATIONAL

GEOSPATIAL-INTELLIGENCE AGENCY

BETHESDA, MD

2004

(Formerly Pub.No. 226)

AS ORIGINALLY PUBLISHED BY

DEFENSE MAPPING AGENCY

HYDROGRAPHIC/TOPOGRAPHIC CENTER

WASHINGTON, D.C.

1980

INTRODUCTION

This document has been prepared in order to present all pertinent information regarding the practical procedures of

magnetic compass adjustment in one text. As such, it treats of the basic principles of compass deviations and their correction,

and not of the details of particular compass equipment.

Although this text is presented as a systematic treatise on compass adjustment, ship's personnel who are inexperienced with

compass correction will find sufficient information in Chapters I and XIV to eliminate compass errors satisfactorily without

intensive study of the entire text. Reference should also be made to figure 318 for condensed information regarding the

various compass errors and their correction.

In this handbook, the term compass adjustment refers to any changes of permanent magnet of soft iron correctors whereby

normal compass errors are reduced. The term compass compensation refers to any change in the current supplied to compass

compensating coils whereby the errors due to degaussing are reduced.

The basic text is the outgrowth of lecture notes prepared by Nye S. Spencer and George F. Kucera while presenting courses

of instruction in adjustment and compensation during World War II at the Magnetic Compass Demonstration Station, Naval

Operating Base, Norfolk, Virginia.

CHAPTER I

PROCEDURES FOR MAGNETIC COMPASS ADJUSTMENT (CHECK-OFF LIST)

NOTE: If the magnetic adjustment necessitates (a) movement of degaussing compensating coils, or (b) a change of Flinders

bar length, the coil compensation must be checked. Refer to Chapter XIV.

101. Dockside tests and adjustments

1. Physical checks on the compass and binnacle.

(a) Remove any bubbles in compass bowl (article 402).

(b) Test for moment and sensibility of compass needles (article 403).

(d) Magnetization check of spheres and Flinders bar (article 404).

(e) Alignment of compass with fore-and-aft line of ship (article 405).

(f) Alignment of magnets in binnacle.

(g) Alignment of heeling magnet tube under pivot point of compass.

(h) See that corrector magnets are available.

2. Physical checks of gyro, azimuth circle, and peloruses.

(a) Alignment of all gyro repeater peloruses or dial peloruses with fore-and-aft line of ship (article 405).

(b) Synchronize gyro repeaters with master gyro.

3. Necessary data.

(a) Past history or log data which might establish length of Flinders bar (articles 407 and 607)

(b) Azimuths for given date and observer's position (Chapter VIII).

(d) Correct variation (local charts).

(e) Degaussing coil current settings for swing for residual deviations after adjustment and compensation (ship's

Degaussing Folder).

4. Precautions.

(a) Determine transient deviations of compass from gyro repeaters, doors, guns, etc. (Chapter X).

(b) Secure all effective magnetic gear in normal seagoing position before beginning adjustments.

(d) Whenever possible, correctors should be placed symmetrically with respect to the compass (articles 318

and 613).

5. Adjustments.

(a) Place Flinders bar according to best available information (articles 407, 608 and 609).

(b) Set spheres at midposition, or as indicated by last deviation table.

* Applies when system other than gyro is used as heading reference

102. Adjustments at sea. These adjustments are made with the ship on an even keel and after steadying on each heading.

When using the gyro, swing from heading to heading slowly and check gyro error by sun's azimuth or ranges on each heading

if desired to ensure a greater degree of accuracy (article 706). Be sure gyro is set for the mean speed and latitude of the

vessel. Note all precautions in article 101(4) above. "OSCAR QUEBEC" international code signal should be flown to

indicate such work is in progress. Chapter VII discusses methods for placing the ship on desired headings.

1. Adjust the heeling magnet, while the ship is rolling, on north and south magnetic heading until the oscillations

of the compass card have been reduced to an average minimum. ( This step is not required if prior adjustment

has been made using a dip needle to indicate proper placement of the heeling magnet.)

2. Come to an east (090°) cardinal magnetic heading. Insert fore-and-aft B magnets, or move the existing B

magnets, in such a manner as to remove all deviation.

3. Come to a south (180°) magnetic heading. Insert athwartship C magnets, or move the existing C magnets, in

such a manner as to remove all deviation.

4. Come to a west (270°) magnetic heading. Correct half of any observed deviation by moving the B magnets.

5. Come to a north (000°) magnetic heading. Correct half of any observed deviation by moving the C magnets.

(The cardinal heading adjustments should now be complete.)

6. Come to any intercardinal magnetic heading, e.g. northeast (045°). Correct any observed deviation by moving

the spheres in or out.

7. Come to the next intercardinal magnetic heading, e.g. southeast (135°). Correct half of any observed deviation

by moving the spheres. (The intercardinal heading adjustments should now be complete, although more

accurate results might be obtained by correcting the D error determined from the deviations on all four

intercardinal heading, as discussed in article 501.)

8. Secure all correctors before swinging for residual deviations.

9. Swing for residual undegaussed deviations on as many headings as desired, although the eight cardinal and

intercardinal headings should be sufficient.

10. Should there still be any large deviations, analyze the deviation curve to determine the necessary corrections

and repeat as necessary steps 1 through 9 above (Chapter V).

11. Record deviations and the details of corrector positions on standard Navy Form NAVSEA 3120/4 and in the

Magnetic Compass Record NAVSEA 3120/3 (article 901).

12. Swing for residual degaussed deviation s with the degaussing circuits properly energized (Chapter XIV).

13. Record deviations for degaussed conditions on standard Navy Form NAVSEA 3120/4.

103. The above check-off list describes a simplified method of adjusting compasses, designed to serve as a simple workable

outline for the novice who chooses to follow a step-by-step procedure. The "Dockside Tests and Adjustments" are essential

as a foundation for the "Adjustments at Sea", and if neglected may lead to spurious results or needless repetition of the

procedure at sea. Hence, it is strongly recommended that careful considerations be given these dockside checks prior to

making the final adjustment so as to allow time to repair or replace faulty compasses, anneal or replace magnetized spheres or

Flinders bar, realign binnacle, move gyro repeater if it is affecting the compass, or to make any other necessary preliminary

repairs. It is further stressed that expeditious compass adjustment is dependent upon the application of the various correctors

in a logical sequence so as to achieve the final adjustment with a minimum number of steps. This sequence is incorporated in

the above check-off list and better results will be obtained if it is adhered to closely. Figure 318 presents the various compass

errors and their correction in condensed form. The table in figure 103 will further clarify the mechanics of placing the

corrector magnets, spheres, and Flinders bar. Chapter IV discusses the more efficient and scientific methods of adjusting

compasses, in addition to a more elaborate treatment of the items mentioned in the check-off list. Frequent, careful

observations should be made to determine the constancy of deviations and results should be systematically recorded.

Significant changes in deviation will indicate the need for readjustment.

To avoid Gaussin error (article 1003) when adjusting and swinging ship for residuals, the ship should be steady on the

desired heading for at least 2 minutes prior to observing the deviation.

Fore-and-aft and athwartship magnets

Quadrantal spheres

Flinders bar

Deviation

Easterly on east

and westerly on

west.

(+ B error)

Westerly on east

and easterly on

west.

(– B error)

Deviation

E on NE'ly,

W on SE'ly,

E on SW'ly,

and

W on NW'ly.

(+ D error)

W on NE'ly,

E on SE'ly,

W on SW'ly,

and

E on NW'ly.

(– D error)

Deviation change

with change in

latitude

E on E'ly and W

on W'ly when sail-

ing toward equator

from N latitude or

away from equator

to S latitude

W on E'ly and E

on W'ly when sail-

ing toward equator

from N latitude or

away from equator

to S latitude

Magnets

Spheres

Bar

No fore and aft

magnets in

binnacle.

Place magnets red

forward.

Place magnets red

aft.

No spheres on

binnacle.

Place spheres

athwartship.

Place spheres fore

and aft.

No bar in holder.

Place required

amount of bar

forward.

Place required

amount of bar aft.

Fore and aft

magnets red

forward.

Raise magnets.

Lower magnets.

Spheres at

athwartship

position.

Move spheres

toward compass or

use larger spheres.

Move spheres

outward or

remove.

Bar forward of

binnacle.

Increase amount

of bar forward.

Decrease amount

of bar forward.

Fore and aft

magnets red aft.

Lower magnets.

Raise magnets.

Spheres at fore

and aft position.

Move spheres

outward or

remove.

Move spheres

toward compass or

use larger spheres.

Bar aft of

binnacle.

Decrease amount

of bar forward.

Increase amount

of bar forward.

Deviation

Easterly on north

and westerly on

south.

(+ C error)

Westerly on north

and easterly on

south.

(– C error)

Deviation

E on N'ly,

W on E'ly,

E on S'ly,

and

W on W'ly

(+ E error)

W on N'ly,

E on E'ly,

W on S'ly,

and

E on W'ly

(– E error)

Bar

W on E'ly and E

on W'ly when sail-

ing toward equator

from S latitude or

away from equator

to N latitude

E on E'ly and W

on W'ly when sail-

ing toward equator

from S latitude or

away from equator

to N latitude

Magnets

Spheres

Deviation change

with change in

latitude

No athwartship

magnets in

binnacle

Place athwartship

magnets red

starboard

Place athwartship

magnets red port

No spheres on

binnacle

Place spheres at

port forward and

starboard aft inter-

cardinal positions.

Place spheres at

starboard forward

and port aft inter-

cardinal positions.

Heeling magnet

(Adjust with changes in magnetic latitude)

Athwartship

magnets red

starboard

Raise magnets

Lower magnets

Spheres at

athwartship

position

Slew spheres

clockwise through

required angle.

Slew spheres

counter-clockwise

through required

angle.

If compass north is attracted to high side of ship when rolling,

raise the heeling magnet if red end is up or lower the heeling

magnet if blue end is up.

Athwartship

magnets red port

Lower magnets

Raise magnets

Spheres at fore

and aft position

Slew spheres

counter-clockwise

through required

angle.

Slew spheres

clockwise through

required angle.

If compass north is attracted to low side of ship when rolling,

lower the heeling magnet if red end is up or raise the heeling

magnet if blue end is up.

NOTE: Any change in placement of the heeling magnet will

affect the deviations on all headings.

Figure 103 – Mechanics of magnetic compass adjustment

CHAPTER II

MAGNETISM

201. The magnetic compass. The principle of the present day magnetic compass is in no way different from that of the

compass used by the ancients. It consists of a magnetized needle, or array of needles, pivoted so that rotation is in a

horizontal plane. The superiority of the present day compass results from a better knowledge of the laws of magnetism,

which govern the behavior of the compass, and from greater precision in construction.

202. Magnetism. Any piece of metal on becoming magnetized, that is, acquiring the property of attracting small particles of

iron or steel, will assume regions of concentrated magnetism, called poles. Any such magnet will have at least two poles, of

unlike polarity. Magnetic lines of force (flux) connect one pole of such a magnet with the other pole as indicated in figure

202. The number of such lines per unit area represents the intensity of the magnetic field in that area. If two such magnetic

bars or magnets are placed side by side, the like poles will repel each other and the unlike poles will attract each other.

Figure 202 – Lines of magnetic force about a magnet

203. Magnetism is in general of two types, permanent and induced. A bar having permanent magnetism will retain its

magnetism when it is removed from the magnetizing field. A bar having induced magnetism will lose its magnetism when

removed from the magnetizing field. Whether or not a bar will retain its magnetism on removal from the magnetizing field

will depend on the strength of that field, the degree of hardness of the iron (retentivity), and also upon the amount of physical

stress applied to the bar while in the magnetizing field. The harder the iron the more permanent will be the magnetism

acquired.

204. Terrestrial magnetism. The accepted theory of terrestrial magnetism considers the earth as a huge magnet surrounded

by lines of magnetic force that connect its two magnetic poles. These magnetic poles are near, but not coincidental, with the

geographic poles of the earth. Since the north-seeking end of a compass needle is conventionally called a red pole, north pole,

or positive pole, it must therefore be attracted to a pole of opposite polarity, or to a blue pole, south pole, or negative pole.

The magnetic pole near the north geographic pole is therefore a blue pole, south pole, or negative pole; and the magnetic pole

near the south geographic pole is a red pole, north pole, or positive pole.

205. Figure 205 illustrates the earth and its surrounding magnetic field. The flux lines enter the surface of the earth at

different angles to the horizontal, at different magnetic latitudes. This angle is called the angle of magnetic dip, θ , and

increases from zero, at the magnetic equator, to 90° at the magnetic poles. The total magnetic field is generally considered as

having two components, namely H , the horizontal component, and Z , the vertical component. These components change as

the angle θ changes such that H is maximum at the magnetic equator and decreases in the direction of either pole; Z is zero at

the magnetic equator and increases in the direction of either pole.

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