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Day Skipper - Part 3 - Compass
variation
Every chart is printed with one compass rose or more, which indicates
true north on the outside ring and magnetic north on the inside ring.
The difference between them is called variation. Currently, in the UK,
there is around a 3-5° offset to the left or the west. In other parts
of the world there could be up to 90° difference!
So it is important not only to appreciate this phenomenon, but to understand
how it can introduce errors into navigation if not taken into account.
The amount that variation changes is also shown on the compass rose in
the format: 12°00'W 1982 (10'E).
This means that in 1982 there were 12 degrees 0 minutes of westerly
variation but this was changing 10 minutes east (ie: reducing) every year.
As there are 60 minutes in each degree, it can be calculated that in
1983 it would have moved east by ten minutes resulting in a variation
of 11°50'W; and by 1988 over a period of six years
it reduces to 11°00'W.
By applying variation you change true courses to magnetic courses.
Compass deviation
Deviation is the compass error caused by electrical equipment and metal
objects on the boat, and unlike variation, changes according to the way
the craft is pointing.
It is usually at this point that would-be navigators become totally
bemused and wish they were somewhere else!
Deviation can be measured by swinging the compass using a distant point
whose bearing is known.
By moving the boat slowly in a circle and comparing the ship's heading,
say every 22.5°, with the changing bearing of the point, you can
find out the deviation of the compass.
This job is best done by professional compass adjusters, who will then
set the steering compass to minimise the deviation (or error) from magnetic
north. This can usually be reduced to between 1° and 2° in most
cases.
Once completed, a deviation table can be constructed, and from this
a graphical display of the deviation can be drawn. (See diagrams.)
This is known as the deviation curve and is only valid for the vessel
in which the swing was done; it is likely to become invalid if additional
electrical equipment is installed in the area of the compass after the
swing has been completed.
It is also important that all electrical equipment in the area is turned
on during the swing.
By applying deviation you change magnetic courses to compass courses.
True and magnetic courses 
Because of the possible confusion between true, magnetic and compass
courses, it is recommended that navigation is calculated in true bearings
until the last moment, when allowances for variation and deviation are
made.
True to magnetic, or magnetic to compass add west or subtract
east. West is best (add) east is least (subtract).
If you are in any doubt, look at any compass rose on a chart and compare
the line through north on the true bearing ring with the magnetic bearing
ring. If the magnetic circle reads greater than north, ie: 003°,
then the variation must be west because it is greater than north.
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