Variation and Deviation


The steering compass on any vessel remains essential to navigation.  It will almost certainly be a magnetic compass and as such will be subject to errors of Variation and Deviation. We must be aware of these and take into them into account for safe navigation.

Two main errors affect the magnetic compasses on any boat. Variation and Deviation.

Image showing how Magnetic Variation will differ depending on your position.
Magnetic Variation will differ depending on your position


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.

Image showing how Variation is shown on a typical Admiralty Chart.
How Variation is shown on Charts

The diagram at the top of the page shows that Variation varies with your location. Have a look at the examples below.

In the United Kingdom Variation is currently (2020) very small, with the East Coast of England approximately zero degrees 45minutes East. Bantry Bay (South West Ireland) is about 3 degrees 30 minutes West. Limassol (Cyprus) is about 5 degrees East. Nantucket (USA) is about 11 degrees West. Maine (USA) is about 20 degrees West. Tokyo is about 7 degrees 40 minutes West.


Image showing how Deviation changes depending on the heading of the vessel.
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.


Image showing an example of a Deviation card.
Example of a Deviation Card



To convert a course to steer from True (T) to a course to steer to Compass (C) we must allow for both Variation (V) and Deviation (D). A simple way to remember the order of work is by using the mnemonic ‘True Virgins Make Dull Company’ and the word CADET (Compass to True Add East) to remind us if we are adding or subtracting.

Image showing the order in which to convert True to Compass or vice versa.
Converting Compass to True

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.

Image showing the workings of converting from True to Compass.
Converting True to Compass in steps
Image showing the workings of converting Compass to True.
Converting Compass to True in Steps

Using this step by step method of working between True and Compass greatly reduces the chances of making mistakes.

For information on our Cockpit Card series have a look here.

Learn More

Secondary Port Calculations

Tidal Curves

Tidal Streams and Tidal Diamonds

Rule of Twelfths

How to Read a Tide Table

Visit our shop

Cockpit Cards

Cockpit Card Complete Set product image
Cockpit Cards – Complete SetCockpit Cards Complete Set