True meridian: A great Circle that passes through both the North and South Geographic Poles on the Earth. This may be considered as a straight line running True North/South through any observer.
Magnetic meridian: A great Circle that passes through both the North and South Magnetic Poles on the Earth. This may be considered as a straight line running Magnetic North/South through any observer and is the direction a compass needle would point in, but only when under the sole influence of the Earth’s magnetic field.
Compass meridian: A line passing through the centre of the ships magnetic compass drawn in the direction in which the compass needle points under the combined influence of the Earth’s magnetic field and the magnetic forces acting on the compass due to the ship itself.
The Magnetic Variation is the angle between the True meridian and the Magnetic meridian.
The Variation is named EAST when the angle between the two lies to the right of the true meridian and WEST when the angle between the two lies to the left of the true meridian.
Therefore in the diagrams below the Variation would be named WEST in the left hand diagram and EAST in the right hand diagram.
This is defined as the angle between the Magnetic meridian and the Compass meridian. It is named EAST if the angle lies to the right of the magnetic meridian and WEST if the angle lies to the left of the magnetic meridian.
The diagrams below show each case
Changes in Variation:
From the diagram on page 1 it can be seen that the actual value of Magnetic variation differs from one place to another, due to the angle between the meridians.
However the variation at one particular place will also vary because the Magnetic poles are not fixed points on the earth’s surface.
The actual value of variation at any point can be found from an admiralty variation chart. This type of chart has curves of equal variation printed on it and there are charts for all the world’s oceans.
These are known as isogonic lines, those for Easterly variation are coloured red and those for Westerly variation are blue.
The chart also shows lines indicating the equal annual rates of change of variation, which are known as isalloginic lines. Where the rate of change is Easterly the lines are red and where the rate of change is Westerly the lines are blue.
The isalloginic lines are lighter in colour than the isogonic lines.
Ordinary Admiralty charts also give an indication of the variation in the area covered by the chart. These can be found on the compass roses, which are printed on the chart. The variation for a given year will be found on the rose and the annual rate of change will be found in brackets, named East or West.
To calculate the variation for any given time, we simply multiply the annual rate of change by the number of years since the one stated on the compass rose. This figure is then applied to the value given on the rose, taking note of whether the annual change is to the east or the west.
Changes in Deviation:
Deviation is caused by magnetic fields generated with in the steelwork of the ship which act on the compass needle. The amount of deviation caused by these magnetic fields depends on a number of factors, including those stated below.
Place the ship was built
Heading the ship was built on
Distribution of steel around the compass position
Type of cargo
However the main cause for changes in deviation once the ship is at sea is a change in the heading of the vessel.
Every ship will have a deviation card on which is drawn a curve showing how the deviation changes with compass heading. It may also be in the form of a table with compass headings from 0o to 360o , 10 degree steps, and opposite the associated deviation for each of the headings.
From these values it is possible to interpolate to find the deviation for any compass heading.
The deviation of the compass should be checked whenever possible and at least once a watch, as it can change with time and also with position on the earth’s surface.
Compass Error:
This is the angle between the true meridian and the compass meridian.
It is an algebraic combination of the variation and the deviation.
e.g. Variation = 6o W
Deviation = 10o E
Compass Error = 4o E
Remember Error East Compass Least; Error West Compass Best
Note the deviation changes only with the ships heading therefore the compass error will also change with ships heading. However if a series of compass bearings are taken whilst the ship is on a particular heading the compass error to be applied to each of the bearings to convert them to true bearings will be the same for each one.
Course:
The course of a vessel is the direction the vessel is heading in , measured from either the True, Magnetic or Compass meridians. In all cases it is measured clockwise from the meridian from 000o to 360o.
When laying off a course on a chart the TRUE COURSE must always be used.
Consider the diagram below
OD is the vessels course
<TOD is the vessels True Course
<MOD is the vessels Magnetic Course
<COD is the vessels Compass Course
If we assume a variation of 10 W and a deviation of 7 W and a True Course of 45 degrees then we have
True Course 045 (T)
Variation 10 W (added to True Course)
Magnetic Course 055 (M)
Deviation 7 W (added to Magnetic Course)
Compass Course 062 (C)
In other word the Magnetic and Compass courses both have greater values than the True course because the Variation and Deviation are both west.
You should satisfy yourself that had the Variation and Deviation both been East then the Magnetic and Compass course would both have been less than the True Course.
The order in which the Variation and Deviation are applied are always the same.
If going from Compass to True the they are applied as follows
Compass Deviation Magnetic Variation True
And if going from True to Compass
True Variation Magnetic Deviation Compass
(The more challenging conversion)
There are two useful mnemonics for memorising the order when going either way
Compass to True
Cadets Don’t Meet Virgins Twice
True to Compass
Timid Virgins Make Dull Companions
To remember which direction to apply variation and deviation if going from Compass to True corrections to the East are always added and from True to Compass corrections to the East are always subtracted. The opposite is correct for corrections to the west.Another way of remembering how the two quantities of variation and deviation are applied, when going from True to Compass courses, is as follows:
Variation WEST MAGNETIC BEST
Variation EAST MAGNETIC LEAST
Deviation WEST COMPASS BEST
Deviation EAST COMPASS LEAST
Example:
A vessel is on a course of 220 (T). The variation is 10 W and the deviation is 20 E. Find the magnetic, Compass courses and the compass error.
True Course 220 (T)
Variation 10 W
(If West Magnetic Best so add to True)
Magnetic Course 230 (M)
Deviation 20 E
(If East Compass Least so subtract from Magnetic)
Compass Course 210 (C)
Compass Error = Variation ± Deviation
= 10 W – 20 E
= 10 E
Lesson 8.doc Compass DGR1999
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dariusz.lipinski