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Understanding Compass Variation & Deviation

Variation is the difference between true north and magnetic north, which can be found by looking at the compass rose on nautical charts. Deviation is the deflection of a compass from magnetic north caused by ferrous metals and electrical equipment on a boat. To convert a compass reading to true north, you add any easterly variation and deviation values or subtract any westerly values. The reverse is done to convert a true north reading to compass - subtract easterly values and add westerly values.

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142 views5 pages

Understanding Compass Variation & Deviation

Variation is the difference between true north and magnetic north, which can be found by looking at the compass rose on nautical charts. Deviation is the deflection of a compass from magnetic north caused by ferrous metals and electrical equipment on a boat. To convert a compass reading to true north, you add any easterly variation and deviation values or subtract any westerly values. The reverse is done to convert a true north reading to compass - subtract easterly values and add westerly values.

Uploaded by

nyshauna strachan
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© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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VARIATION

VARIATION is the difference between True North and Magnetic North.  The Variation for your local area
is found by looking at the Compass Rose on your chart (see the diagram below). It shows a Magnetic
North Arrow, the local variation and the annual rate of change. There are usually several Compass Roses
on the chart, look at the one nearest your position.

DEVIATION
How Deviation changes depending on the boat’s heading

DEVIATION – Ferrous metal (engines etc.) and electrical equipment can deflect the compass from
magnetic north.  A compass corrector can adjust your steering compass to minimise errors and then
produce a Deviation Card showing the remaining deviation.  You can also ‘swing’ your own compass and
produce your own deviation card.

When we are converting from Compass to True the word CADET reminds us that we ADD any
Eastery variation and deviation. Any Westerly variation or deviation would, of course, be
subtracted.

When converting from True to Compass the reverse is correct. We SUBTRACT any Eastery
variation or deviation and add any Westerly variation or deviation.
Converting True to Compass in steps
Converting Compass to True in Steps

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