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NSCC SEAMAN MODULE TWO COMPASS TYPES AND COMPASS ERRORS

Enabling Objectives. Explain the operation of the gyrocompass and magnetic compassExplain magnetic compass error, including variations and deviations. 1. Compasses. Instrument that tells you the direction you are headingTells you where north isTwo different kinds of compassesGyrocompassWorks o

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NSCC SEAMAN MODULE TWO COMPASS TYPES AND COMPASS ERRORS

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    1. NSCC SEAMAN MODULE TWO COMPASS TYPES AND COMPASS ERRORS By ENS MATTHEW LANDRY, NSCC USS JOSEPH P KENNEDY, JR (DD 850) DIVISION This presentation is developed from Chapter 2 (Pages 2-1 to 2-5) of Seaman Training Manual (NAVEDTRA 14067), and is designed for use either with the course, or independently from it. This presentation is developed from Chapter 2 (Pages 2-1 to 2-5) of Seaman Training Manual (NAVEDTRA 14067), and is designed for use either with the course, or independently from it.

    2. Enabling Objectives Explain the operation of the gyrocompass and magnetic compass Explain magnetic compass error, including variations and deviations

    3. 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 Earth’s magnetic field

    4. Compass Card (Example)

    5. 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

    6. 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 doesn’t move, the ship does!

    7. 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

    8. 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

    9. Circular Measurement Circumference always contains 360 degrees Each degree contains 60 minutes Each minute contains 30 seconds Example: 30°45’55” is, thirty degrees, forty-five minutes and fifty-five seconds

    10. 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

    11. 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

    12. Deviation Deviation = The amount a magnetic compass is deflected by magnetic material in the ship Deviation number changes during a ship’s 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

    13. 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 ship’s heading

    14. 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

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