What are errors of magnetic compass?

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Written by Amit Sharma
Errors  of magnetic compass:-
  1. VARIATION

The true North Pole and the magnetic north pole are not located at the same spot. This variation causes a magnetic compass needle to point more or less away from true north. The amount the needle is offset is called variation because the amount varies at different points on Earth’s surface. Even in the same locality variation usually does not remain constant, but increases or decreases at a certain known rate annually.

The variation for any given locality, together with the amount of annual increase or decrease, is shown on the compass rose of the chart for that particular locality.

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Remember:  If the annual variation is an increase, you add; if it is a decrease, you subtract

How to calculate variation from compass rose.

To find the amount of variation in this locality in1995, count the number of years since 1990 (in this case 5); Multiply that by the amount of annual increase;(which here gives you 5 X 1′, or 5); add that to the variation in 1990 and you have a 1995 variation of14°50′  W
Variation remains the same for any heading of the ship at a given locality. No matter which way the ship is heading,  the  magnetic  compass,  if  affected  only  by variation, points steadily in the general direction of the magnetic  north  pole.

  1. DEVIATION

The amount a magnetic compass needle is deflected by magnetic  material  in  the  ship  is  called  deviation.Although deviation remains a constant for any given compass heading, it is not the same on all headings.  Deviation  gradually  inc
reases,  decreases, increases,  and  decreases  again  as  the  ship  goes  through an entire 360° of swing.

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The magnetic steering compass is located in the pilothouse, where it  is  affected  considerably  by deviation.  Usually the standard compass  is  topside, where the magnetic forces producing deviation are not as strong. Courses and bearings by these compasses must be carefully differentiated by the abbreviations PSC (per  standard  compass),  PSTGC (per  steering compass), and PGC (per gyrocompass). The standard compass provides a means for checking the steering compass  and  the  gyrocompass.

  1. Turning Error

A turn from the north lags or indicates a turn in the opposite direction. So to roll out on the correct heading one must roll out of the turn, past the correct heading.A turn from the south leads. So to roll out on the correct heading one must roll out of the turn, before reaching the correct heading.

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REMEMBER:  THE SOUTH LEADS AND THE NORTH LAGS and THERE IS NO ACCELERATION / DECELERATION ERROR ON A NORTH OR SOUTH HEADING.

  1. Acceleration and Deceleration Error

When on an east or west heading, any increase in airspeed (Acceleration) will cause the magnetic compass to indicate a false turn toward the north, and any decrease in airspeed (deceleration) will cause the magnetic compass to indicate a false turn toward the south.

REMEMBER:  ACCELERATE NORTH & DECELERATE SOUTH and THERE IS NO TURNING ERROR ON A EAST OR WEST HEADING.

  1. Oscillation Error

This error is caused by turbulence or rough control movements and results in erratic movement of the compass card.
Oscillation is a combination of all of the other errors, and it results in the compass card swinging back and forth around the heading being flown. When setting the gyroscopic heading indicator to agree with the magnetic compass, use the average indication between the swings.

  1. Magnetic dip Error

Magnetic dip is the tendency of the compass needles to point down as well as to the magnetic pole. Dip is greatest near the poles and least near the Magnetic Equator. The compass card is designed to operate in the horizontal, therefore, any movement from the horizontal plane introduces dip error.
The needle of your magnetic compass will be parallel with Earth’s surface at the Magnetic Equator, but will point increasing downward as it is moved closer to the Magnetic Pole.
Northerly turning error is due to the mounting of the compass. Since the card is balanced in fluid, when the aircraft turns, the card is also banked as a result of centrifugal force. While the card is banked, the vertical component of the Earth’s magnetic field causes the north-seeking ends of the compass to dip to the low side of the turn. When making a turn from a northerly heading, the compass briefly gives an indication of a turn in the opposite direction. When making a turn from the south, it gives an indication of a turn in the correct direction but at a faster rate.

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Acceleration error is also due to the dip of the Earth’s magnetic field. Because of the way the compass card is mounted, the aft end of the compass card is tilted upward when accelerating, and downward when decelerating during airspeed changes. This error is most pronounced on an east / west heading. When accelerating on an east or west heading, the error indicates a turn to the north. When decelerating on an east or west heading the error is toward the south.

To see summary of magnetic compass Error, Click here.
INDERJEET SINGH
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About the author

Amit Sharma

Graduated from M.E.R.I. Mumbai (Mumbai University), After a brief sailing founded this website with the idea to bring the maritime education online which must be free and available for all at all times and to find basic solutions that are of extreme importance to a seafarer by our innovative ideas.