02COMPASS

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Chapter 2
Marine Magnetic Compass
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Marine Magnetic Compass

• What you will learn in this lesson
• The anatomy of a compass
• Compass types
• Compass deviation and its measurement
• TVMDC calculations
• Compass error

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Marine Magnetic Compass

• Brief history
• Parts of the compass
• Compass dial design
• Top reading compass
• Front reading compass
• Telltale compass
• Brief advice on compass selection

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Marine Magnetic Compass

• Compass mounting
• Principle of operation deviation
• Swinging ship
• Direct observation using a range
• Plotting the results
• Use of a Pelorus
• Spot checks

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Marine Magnetic Compass

• Deviation on intermediate headings
• Use of the deviation table
• Compass calculations
• TVMDC sequence
• Additional pointers on the compass
• Local magnetic disturbances
• The fluxgate compass

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Make A Compass
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Top Reading or Flat Card
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Front Reading or Direct Reading
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Fluxgate
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Digital
Analog
Fluxgate
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Hockeypuck
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Datascope
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Compass Selection
Bigger Is Better
Buy Quality
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Lubbers Line
Lubbers Line
Compass
Compass
Keel Line
Keel Line
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Oh2-1
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Oh2-2
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21 E variation
10 W variation
0 variation
Isogonic Lines
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One Degree Compass Error Results In One Mile
Distance Error For Every Sixty Miles Distance
Traveled
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Oh2-7
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EASTERLY VARIATION
EASTERLY VARIATION
EASTERLY AND WESTERLYVARIATION
EASTERLY VARIATION
GEOGRAPHIC NORTH POLE
MAGNETIC NORTH POLE
WESTERLY VARIATION
ANGULAR DIFFERENCE BETWEEN TRUE NORTH AND
MAGNETIC NORTH
WESTERLY VARIATION
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VARIATION
MAGNETIC MERIDIANS
ALIGN WITH THE MAGNETIC POLES--NOT SAME AS
GEOGRAPHIC NORTH/SOUTH POLES
WHEN NOT PARALLEL WITH THE LINES OF LONGITUDE--
THE DIFFERENCE IS VARIATION
LONGITUDINAL MERIDIANS
ALIGN WITH TRUE NORTH AND SOUTH POLES
EASTERLY VARIATION
WESTERLY VARIATION
MAGNETIC MERIDIANS ALIGN WITH MAGNETIC POLES
THE COMPASS ROSE ON THE CHART SHOWS THE MAGNETIC
VARIATION
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VARIATION
TRUE NORTH
MAGNETIC MERIDIAN POINTS TO MAGNETIC NORTH
LONGITUDINAL MERIDIAN
EASTERLY VARIATION
N
MAGNETIC NORTH
000
POINTS TO TRUE NORTH
270
W
VARIATION AND THE COMPASS
E
090
THE COMPASS
S
180
THE COMPASS
ALIGNS ITSELF WITH THE NET MAGNETIC MERIDIAN WHEN
THERE IS NO OTHER STRAY MAGNETIC FIELD
HAS INTERNAL MAGNETS TO COMPENSATE FOR ERROR
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COMPASS ROSE
USED TO DETERMINE DIRECTION ON THE CHART
THE STAR REPRESENTS THE NORTH STAR (POLARIS)
WHICH IS ABOVE THE TRUE GEOGRAPHIC NORTH POLE TO
REMIND YOU THAT --
THE OUTER RING IS ALIGNED WITH TRUE NORTH
THE INNER RING SHOWS THE MAGNETIC DIRECTION AT
THE CENTER IS THE MAGNETIC VARIATION
THE ANNUAL INCREASE INDICATES HOW MUCH THE
MAGNETIC NORTH HAS MOVED IN RELATION TO
THIS COMPASS ROSE
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DEVIATION AND THE COMPASS
TRUE NORTH
MAGNETIC NORTH
EASTERLY VARIATION
COMPASS NORTH
TRUE NORTH
000 N
270
MAGNETIC NORTH
W
EASTERLY DEVIATION
DIRECTION AND MAGNITUDE OF MAGNETIC FLUX ON BOAT
DUE TO METALLIC EQUIPMENT AND ELECTRIC CURRENTS
S
THE COMPASS
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A VECTOR IS A GEOMETRIC REPRESENTATION OF A
QUANTITY WHICH HAS A MAGNITUDE AND A GIVEN
DIRECTION
THE COMPASS HAS INTERNAL MAGNETS TO
MINIMIZE ERRORS
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DEVIATION AND THE COMPASS
TRUE NORTH
DEVIATION AND THE COMPASS
MAGNETIC NORTH
EASTERLY VARIATION
MAGNETIC NORTH
COMPASS NORTH
EASTERLY DEVIATION
TRUE NORTH
270
000 N
W
COMPASS NORTH
EASTERLY DEVIATION HAS INCREASED
S
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DIRECTION AND MAGNITUDE OF MAGNETIC FLUX ON BOAT
DUE TO METALLIC EQUIPMENT AND ELECTRIC CURRENTS
DOES NOT CHANGE
THE COMPASS
THE COMPASS HAS INTERNAL MAGNETS TO
MINIMIZE ERRORS
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DEVIATION AND THE COMPASS
MAGNETIC NORTH
TRUE NORTH
DEVIATION AND THE COMPASS
EASTERLY VARIATION
MAGNETIC NORTH
COMPASS NORTH
000 N
TRUE NORTH
ZERO DEVIATION
COMPASS NORTH
ZERO DEVIATION
S
DIRECTION AND MAGNITUDE OF MAGNETIC FLUX ON BOAT
DUE TO METALLIC EQUIPMENT AND ELECTRIC CURRENTS
DOES NOT CHANGE
180
THE COMPASS
THE COMPASS HAS INTERNAL MAGNETS TO
MINIMIZE ERRORS
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DEVIATION AND THE COMPASS
TRUE NORTH
MAGNETIC NORTH
DEVIATION AND THE COMPASS
EASTERLY VARIATION
MAGNETIC NORTH
TRUE NORTH
000 N
COMPASS NORTH
WESTERLY DEVIATION CANCELS OUT EASTERLY VARIATION
WESTERLY DEVIATION
COMPASS NORTH
S
DIRECTION AND MAGNITUDE OF MAGNETIC FLUX ON BOAT
DUE TO METALLIC EQUIPMENT AND ELECTRIC CURRENTS
DOES NOT CHANGE
180
THE COMPASS
THE COMPASS HAS INTERNAL MAGNETS TO
MINIMIZE ERRORS
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WORKING WITH DEVIATION AND VARIATION
T
TRUE HEADING
TELE-
V
VARIATION
VISION
M
DOWN ADD WEST
MAKES
MAGNETIC HEADING
D
DULL
DEVIATION
C
CHILDREN
COMPASS HEADING
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DEVIATION TABLE
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DEVIATION TABLE
EASTERLY DEVIATION
3
2.5
2
1.66
1.66 E
3.34
1
2.5 E
7.5
0
0 1 2 3 4 5 6 7 8 9 10 11 12
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MAGNETIC DIRECTION
PERFORM OUR TVMDC
animation
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WORKING WITH DEVIATION AND VARIATION
T
TRUE HEADING
TINY
V
VARIATION
VERMIN
M
DOWN ADD WEST
MAKE
MAGNETIC HEADING
D
DISASTROUS
DEVIATION
C
COMPANY
COMPASS HEADING
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WORKING WITH DEVIATION AND VARIATION (UNCORRECTING
)
T
TRUE DIRECTION IS 080
080
T
V
MAGNETIC VARIATION IS 6 W
USING
IF
6W
V
DOWN ADD WEST
M
COMPASS DEVIATION IS 4 EAST
D
M
?
C
WHAT IS THE COMPASS HEADING TO STEER?
D
-4E
080 6 W ___ 086 DEGREES MAGNETIC M
C
?
86 -4 E ___ 82 DEGREES MAGNETIC COMPASS HEADINGC
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WORKING WITH DEVIATION AND VARIATION (CORRECTING)
T
COMPASS DIRECTION IS 230
T
?
GOING UP YOU ADD EAST AND SUBTRACT WEST
V
COMPASS DEVIATION IS 7 WEST
USING
IF
V
12E
M
MAGNETIC VARIATION IS 12 EAST
D
M
?
C
WHAT IS THE TRUE HEADING?
D
7W
230 - 7 W ___ 223 DEGREES MAGNETICM
C
230
223 12 E ___ 235 DEGREES TRUE HEADING T
DOWN ADD WEST AND SUBTRACT EAST
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USE OF A RANGE TO COMPUTE A DEVIATION TABLE
FOR YOUR BOAT
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Plantation Creek Range
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Rear Range Mark
Front Range Mark
Shelter Island Range, San Diego
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A Natural Range
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Pelorus or Dumb Compass
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RELATIVE BEARINGS
000 degrees relative
270 degrees relative
A PELORUS MEASURES RELATIVE BEARINGS
TAKEN CLOCKWISE FROM THE BOW OF THE BOAT
090 degrees relative
180 degrees relative
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DEVIATION FROM BEARING OF RANGE AS READ
OVER COMPASS
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COMPASS BEARING OF RANGE AS READ OVER COMPASS
T 310 V 15W M 325 D ? C 340
340
DOWN ADD WEST
DEVIATION MUST EQUAL 325 ? 340 D 15 W
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AT A VESSEL HEADING OF 340, THE DEVIATION IS 15W
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DEVIATION FROM BEARING OF RANGE AS READ
OVER COMPASS
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COMPASS BEARING OF RANGE AS READ OVER COMPASS
T 310 V 15W M 325 D ? C 338
060
338
DOWN ADD WEST
DEVIATION MUST EQUAL 325 ? 338 D 13 W
158
240
AT A VESSEL HEADING OF 060, THE DEVIATION IS 13W
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DEVIATION FROM BEARING OF RANGE AS READ
OVER COMPASS
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COMPASS BEARING OF RANGE AS READ OVER COMPASS
T 310 V 15W M 325 D ? C 310
060
310
DOWN ADD WEST
DEVIATION MUST EQUAL 325 ? 310 D 15E
130
240
AT A VESSEL HEADING OF 240, THE DEVIATION IS 15E
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DEVIATION FROM RELATIVE BEARINGS
310
ADD 070R TO COMPASS HEADING
060
240 070R 310 COMPASS
DOWN ADD WEST
070R
T 310 V 15W M 325 D ? C 310
DEVIATION MUST EQUAL 325 ? 310 D 15E
240
AT A VESSEL HEADING OF 240, AND A RELATIVE
BEARING OF RANGE 070R, THE DEVIATION IS 15E
LUBBERS LINE
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Typical Deviation 1E to 1W
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