Is Your Compass Lying - Part II
Brought to you by coastalnavigation.com
In the last issue you saw that the steering compass, the one fixed to the boat, probably tells lies - by giving you improper readings for courses and headings. That's because the boat's metal objects, and electronics such as wiring, have their own magnetic fields and cause the compass to be deflected. This is deviation: its deflection can be a little or a lot: if the north point of the compass points eastward of magnetic north, the deviation is East. If it points westward of magnetic north, the deviation is West.
Finding your boat's unique deviation - by making a deviation table - is the only answer, and it is important for determining your magnetic course, heading or bearing. So, correcting for deviation will give you accurate magnetic readings. Is that enough? No, now you have to correct for variation, the difference between magnetic and True North, and variation changes from East to West as you move across the country, and in magnitude too. But first, a word about "the two norths", and variation.
True,
or geographic North, is the North Pole at the top of the planet. Magnetic
north is a spot located in Canada's high arctic, about 700 miles south of
the North Pole, roughly in the direction of the Manitoba-Saskatchewan border.
From Ottawa the north-seeking point of a hand-held compass shows you where
magnetic north is, so long as you keep it away from any ferrous metal, of
course. This type of compass gives you magnetic readings. The variation is
14° as the diagram shows, but is it east or west? Well, it's west because
that's where magnetic north is found, in fact 14° west of the line to True
North from Ottawa. Call it "14°W". If you were near Vancouver, the hand-held
compass would point to the same magnetic north, but that line would be to
the east of another line going to True North. It would make an angle 23° between
the two lines, and would represent a variation of 23° east, or "23°E".
Both deviation and variation cause a deflection of the compass, and together they are called compass error, despite not being errors at all. They can be added algebraically to convert a reading from compass to true. While your boat's steering compass differs from true by compass error, a magnetic compass differs from your boat's steering compass by deviation. And of course true readings differ from magnetic readings by variation.
There are many ways to convert steering compass degrees to magnetic and to true readings, and back again. There is the intuitive, common sense way of figuring it, out as shown by the illustration, and there are easy-to-remember jingles to jog you your memory. All work. Check them out in www.coastalnavigation.com, an on-line coastal navigation course that will help you separate lies your compass tells you, from facts your head tells you, as you hoist your sails, chart your course and put your boat confidently on an accurate heading.
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