NSCC SEAMAN MODULE TWO COMPASS TYPES AND COMPASS ERRORS

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NSCC SEAMAN MODULE TWOCOMPASS TYPES AND COMPASS
ERRORS

• By ENS MATTHEW LANDRY, NSCC
• USS JOSEPH P KENNEDY, JR (DD 850) DIVISION

FOR USE WITH CHAPTER 2, PAGES 2-1 TO 2-5 OF
NAVEDTRA 14067
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Enabling Objectives

• Explain the operation of the gyrocompass and
  magnetic compass
• Explain magnetic compass error, including
  variations and deviations

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1. Compasses

• Instrument that tells you the direction you are
  heading
• Tells you where north is
• Two different kinds of compasses
• Gyrocompass
• Works on the gyro principle of a spinning wheel
• Magnetic compass
• Affected by Earths magnetic field

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Compass Card (Example)
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Gyrocompass

• Powered by electricity, subject to power failures
• Complicated and delicate mechanism
• Unaffected by magnetic influence
• Points consistently to true north, not magnetic
  north
• Standard shipboard installation
• One master gyrocompass whose indications are sent
  to repeaters throughout the ship

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Magnetic Compass

• Operates through the attraction exerted by Earth
• Unfailing power source (the magnetic field of
  Earth)
• Magnetic compass located in the pilothouse
• Consists of
• Magnetized compass needle attached to a circular
  compass card
• Set in a bronze bowl filled with fluid to allow
  the card to turn
• Remember - the card doesnt move, the ship does!

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True/Magnetic Course

• True course can be converted into magnetic course
  (for steering)
• Add or subtract variation for the area
• Add or subtract deviation for the compass on that
  heading
• Subtract easterly errors, add westerly errors

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Circular Measurement

• Measurements along a meridian, a perfect circle
  are expressed in degrees of arc
• Degrees may be transformed into linear
  measurement
• The compass card is the best example of circular
  measurement

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Circular Measurement

• Circumference always contains 360 degrees
• Each degree contains 60 minutes
• Each minute contains 30 seconds
• Example 304555 is, thirty degrees, forty-five
  minutes and fifty-five seconds

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2. Magnetic Compass Error

• The magnetic compass does not point directly
  north
• The difference can be up to several degrees
• The difference is expressed by variation and
  deviation

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Variation

• Variation the difference between the true
  North pole and magnetic North pole
• The variation number
• Is different for each area of the earth
• Is printed on the compass rose in each chart
• Changes each year, but the change is constant and
  can be easily calculated

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Deviation

• Deviation The amount a magnetic compass is
  deflected by magnetic material in the ship
• Deviation number changes during a ships 360
  swing
• Magnetic steering compass is located in the
  pilothouse, where it is affected by deviation
• When quoting bearings, use the abbreviations
• psc per standard compass
• pstgc per steering compass
• pgc per gyrocompass

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Gyrocompass Repeaters

• Located on the bridge wings
• Used to take bearings on objects outside the ship
• True bearing Direction of an object, measured
  clockwise from true north
• Compass bearing Direction of an object as
  indicated by magnetic compass
• Must convert to true heading by applying
  variation/deviation corrections
• Relative bearing Direction on an object measured
  clockwise from the ships heading

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Gyrocompass repeaters

• All bearings are assumed to be true
• Note relative bearings with a R.
• Example True and relative bearings of a
  lighthouse from a ship