Annual change is expressed in minutes per year. To determine the Variation on old charts, the annual change must be applied. You must first multiply the minutes of annual change by the years since the Variation was determined on the chart. If the change is in direction, an East Annual change will be added to East Variation and subtracted from West Variation. Local Attraction or Local Magnetic Disturbance - An anomaly of the magnetic field of the Earth, extending over a relatively small area, due to local magnetic influences.
This changes the Variation for a small area.
Application of Error to a Compass
An extreme example would be Kingston, Canada. It is a small town on Lake Ontario that has a 45 degree difference in Variation from the surrounding area. Deviation Materials already magnetized, or even capable of being magnetized by the magnets of a compass, will cause the compass needle to deviate from its proper alignment with the magnetic meridian. Definition - It is the angle between the magnetic meridian and a line from the pivot through the North Point of the compass card.
The angle between the direction the compass would point if there were no deviating influences and the direction in which it actually points. The Magnetic Compass on a ship is effected not only by the Earth's Magnetic Field but also by the magnetic field of the ship. Vessels have their own magnetic field around them caused by the Permanent Magnetism and electric currents flowing on the ship.
Sometimes cargo can also impact the magnetic field of the ship. If you were to place a magnetic compass in a location that was only affected by the Earth's Magnetic Field, you could spin the compass around and the compass would always point to Magnetic North. If you put that compass on a ship and turn the ship around in the same location, the compass would move slightly due to the ship's magnetic field.
As a ship moves through the Earth's Magnetic Fields the Compass will react to the changing field. And is one way more correct than the others?
A Compass Error
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In the unlikely event that a vessel is constructed entirely from non magnetic materials and has no electronics close to the compass, variation is the only error which will need to be accounted for. When that vessel is heading towards magnetic north the effect of the pull towards the stern is to reduce the directive force at the compass but not to deflect it from magnetic north.
As the vessel turns onto easterly headings the compass needle is deflected towards the stern i. When the vessel heads west the compass needle is deflected to the east. There will be no deflection when the vessel heads south, but an increase in directive force. This deflection of the compass away from magnetic north is called deviation. A deviation card is produced by the compass adjuster when the vessel is first commissioned and at intervals throughout its life.
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It is displayed close to the compass position. An example of a deviation card can be found on Page The relationship between compass, magnetic and true courses and bearings is shown in the following diagram. Relationship between Compass, magnetic and truc courses and bearings. Rules for applying Variation and Deviation. To avoid drawing diagrams every time variation and deviation are applied, a number of memory aids have been developed to clarify the rules of application: Television Makes Dull Company T V M D C reminds us that to true we must apply the variation to find magnetic, and to this we apply deviation to arrive at compass course or bearing.
If we start with a compass bearing and wish to convert it to true so that it can be used on the chart the order of operation is reversed C D M V T. Having decided the correct order in which to apply variation and deviation, we need to know whether the correction should be added or subtracted. This may be decided using the word: This simply indicates that to get from compass to true the end points we Add East.
Given that we add east deviation or variation it follows that we must subtract west deviation or variation.
It also follows that if we add east to get from compass to true, we should add west when going from true to compass. A simple layout for applying these rules is shown on the next page. If we expect to take several bearings whilst steering the one steady course it makes good sense to arrive at a single correction to apply to all those bearings.
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Remembering that the variation remains effectively the same whilst operating in one area, and that deviation only changes when we change course, we can find the compass error for the course being steered and apply that to all bearings taken whilst on that course. Simple layout for applying variation and deviation. Deviation is taken from the deviation table at the end of this section.
Changing from compass to true.