NSCC SEAMAN MODULE TWO COMPASS 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

U. S. Naval Sea Cadet CorpsCOMPASS :: Enabling Objectives Explain the operation of the gyrocompass and magnetic compass Explain magnetic compass error, including variations and deviations

U. S. Naval Sea Cadet CorpsCOMPASS :: 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

U. S. Naval Sea Cadet CorpsCOMPASS :: Compass Card (Example)

U. S. Naval Sea Cadet CorpsCOMPASS :: 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

U. S. Naval Sea Cadet CorpsCOMPASS :: 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!

U. S. Naval Sea Cadet CorpsCOMPASS :: 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

U. S. Naval Sea Cadet CorpsCOMPASS :: 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

U. S. Naval Sea Cadet CorpsCOMPASS :: 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

U. S. Naval Sea Cadet CorpsCOMPASS :: 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

U. S. Naval Sea Cadet CorpsCOMPASS :: 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

U. S. Naval Sea Cadet CorpsCOMPASS :: 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

U. S. Naval Sea Cadet CorpsCOMPASS :: 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

U. S. Naval Sea Cadet CorpsCOMPASS :: 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