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Navigation 201

Compass Errors

This is the second half to the STCW assessement.

Compass Errors
Variation can be observed on any magnetic compass, whether or land away from any magnetic or on a ship at sea.

When Magnetic North is to the East of True North, the Angle of Variation will be labeled East.

It is caused by the Earth's magnetic field and the fact that the North Magnetic Pole is not in the same location on the Earth as the Geographic North Pole.
It is the difference between the angle of the magnetic meridian and the true meridian.
It is the difference between True (Geographic) North and Magnetic North, measured by the angular difference.
Variation is designated East or West depending on the way the compass needle is deflected from True North.
Variation changes with location.
At any given point on the Earth, the Variation is somewhat constant. The Earth's Magnetic Poles are always changing location, slightly. This slight change causes an Annual Change in variation that can be predicted.
Variation is printed on Charts on the Compass Rose or Isogonic Lines on small-scale charts. It is normally stated in Degrees and minutes to the closest quarter of a degree.
The Annual Change is also printed on the charts. 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. It may state an Increase/Decrease or it may give the change in direction (East or West.)
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. 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. The ship's magnetic field will also react to the Earth's magnetic field.
The heading on a magnetic compass cannot point to the Earth's Magnetic Pole because of the distortion caused by the Ship's magnetic field.
Deviation can range from 000 to 180. A compass having deviation greater than 5 is often corrected by adjusting the position of the magnets and correction devices (flinders bar, heeling magnet & quadrantal spheres.)
Deviation can be East or West depending on whether Compass North lies to the East or West of Magnetic North.

When Compass North is to the East of Magnetic North, the Angle of Deviation will be labeled East.

Variation changes with geographic location and deviation changes with the heading of the vessel.

Compass Error - the algebraic sum of Variation and Deviation. It is the difference between True North and Compass North. If Variation and Deviation are the same direction (lets say, both West), by adding together the angles of Variation & Deviation, you will determine the Compass Error (also labeled West.) If the Variation and Deviation are in different direction, assume the small number is a negative and add the 2 together, the direction will be that of the larger angle.
Example:
Deviation = 1.5E
Variation = 5.0W
(-1.5) + 5.0 = 3.5
Compass Error = 3.5W

Figuring out Compass Error
Your ship is steering course 054 True, 060 psc (psc is per standard compass)
What is the Compass Error?
60 - 54 = 6 but is it West or East?

Since True Heading is to the West of Compass Heading, the Angle of Compass Error will be labeled West. In order for the Compass to read what the true heading of the vessel is, the needle would have to be pointing a little to the West of where it is pointing now.

Remember the saying, Compass is Best, Error is West. Compass is least, Error is East.
Or
True Less West

Once we have the Compass Error and Variation can be found on the chart, we can now determine the deviation for the vessel on that heading.

Same example:
True Heading 054
Compass Heading 060
Compass Error 6W
We find that the variation for the area is 4W.
What is the deviation?
6W - 4W = 2W.
This is just the opposite of how we found the Compass error when we knew the variation and deviation.


Gyro Error - A gyrocompass is a machine that is full of gears, which help it run properly. Machines are not perfect, because they are man-made. The gears are not 100% free of friction so the gyro cannot align itself perfectly with True North. The difference between what the Gyrocompass reads as North & Geographic North is known as Gyro Error. The wear & tear of the equipment, dirt in the equipment and improper start-up of the equipment can also cause gyro Error.

Gyro Error and Compass Error are basically the same thing, but on different types of compasses.

Whenever going away from True, you add West Errors and subtract East Errors.
Going toward True - add East & subtract West

Memory Aid
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True Virgins Make Dull Companions at weddings
GET Gyro + East = True

Compass Record Book - The purpose of this book is to keep track of the vessel's deviation on various headings as well has monitor gyro error. You will learn more about this in Celestial Class.

Swing the Ship - To correct a compass we first need to know how far off it is from the Earth's Magnetic Field. We swing the ship and take bearing of a fixed object on shore using the magnetic & gyro headings of the ship when it is pointing at the object or it has the object dead astern. By determining the different headings, you correct for gyro error and variation to determine the deviation on the different headings. The results are calculated and written out on a form known as a Deviation Card. If the compass has considerable error, the magnets must be moved to be re-aligned with the Earth's magnetic field.