Preparation
There is no pre-reading before this section of the course.
Why is this important?
The needle on our compass does not point to the north pole but the north on our chart does. We need to allow for this error.
True North
On the Day Skipper course we will introduce you to the idea that the lines that go vertically up our charts (lines of longitude), if continued would run directly up to the North Pole. This direction is known a True North.
Therefore, in order to use the chart effectively we need a device that tells us the direction of true north. You might think that a magnetic compass does this, but it is not quite that simple.
Magnetic Bearings & Variation
A magnetic compass does not point to the North Pole it points to the Magnetic North Pole which slowly moves around the globe. The magnetic pole is currently in the far north of Canada and has been for there approximately for 200 year.
This all means that is you are sitting in the UK a magnetic compass points approximately 3-5 degrees to the west of true north. This difference is known a Variation.
Good News
– Variation is relatively predictable and is marked on the chart.
Bad News
– Variation slowly changes over the years;
– Variation is different depending on where you are standing on the earth’s surface;
– The rate of change varies from place to place.
So for example, if you are standing in Cornwall the variation will be different to someone standing in Norfolk and could be very different to that experienced by someone in Sydney, Australia.
The variation you experience today my also be different to the variation you experience in 20 years time in the same location.
Finding your local variation
We can find the variation by looking at the compass rose on our charts. The bearings on the rose refer to true north, however the arrow inside the rose indicates the direction of magnetic north. You should always use the compass rose closest to your position.
Reading the arrow
You need to look at the writing along the arrow. It will say something like:
2o15’ W 2011 (8’E)
This decodes to read:
– In 2011 the variation is predicted to be 2o15’ west of True North.
– It is predicted to change by 8′ east per year.
If we wanted to know the predicted variation in 2014 we would:
– see that there is a 3 year difference between 2014 and 2011.
– calculate the change over 3 years ie. 3 x 8′ = 24′
– subtract 24′ from 2o15’ W (we subtract because the variation is west but the yearly change is moving east).
– The answer is 1o51’ W (note there are 60′ in a degree not 100)
We can only ever measure bearings to the nearest degree (at best) so once calculated we round to the nearest degree ie. 2o in this case.
As we are dealing with predictions we cannot endlessly keep applying these corrections. At some point you need to buy a new chart with an updated prediction on.
Converting between True and Magnetic
Now we know how to find the variation we need to use this information. There are three popular approaches, simply find the one that works for you.
Method 1 – Practical
At the top of the compass scale on your Portland plotter you will find a ‘Error Scale’. This is a scale printed onto the base of the plotter with degrees going either way from zero.
Using a soft pencil mark your variation with a arrow (see the 7oWest marked on the picture).
For all chart work done in your current location use the new mark as the zero point. This will ensure you automatically convert between true and magnetic north without problems.
Method 2 – Calculation
We can make the same adjustment by doing a simple calculation. There are many ways of remembering how to do it, here is one.
First remember the following saying:
“Variation West, Compass Best – Variation East, Compass Least”
Translated this means, when the variation is west the magnetic (compass) bearing is biggest (best); when the variation is east the magnetic (compass) bearing is smallest (least).
So for example: If the magnetic bearing is 340o and the variation is 3oW then in this case the magnetic bearing is the largest (best) so we subtract 3o so we get a true bearing of 337o.
Now if the magnetic bearing is 056 o and the variation is 5 oE then in this case the magnetic bearing is the smallest (least) so we add 5 o so we get a true bearing of 061 o.
Method 3 – CADET
Some people use the word CADET to help convert between true and compass bearings.
“CADET – Compass ADd East True”
Translated this means, if you have the magnetic (compass) bearing you should add east variation to get the true bearing.
Therfore you can reverse the above statement so that for an east variation you subtract the variation to translate from true to compass.
For a west variation, all of the above is reversed.
So for example: If the magnetic bearing is 056 o and the variation is 5 oE then in this case we use CADET in the order it is written. We therefore, add 5 o so we get a true bearing of 061 o.
Now, if the magnetic (compass) bearing is 340o and the variation is 3oW then this is CADET in the order it is written but with a west and not east variation. We therefore subtract 3o so we get a true bearing of 337o.
Links
Sail Train
Variation & Deviation – An explanation of Variation (and Deviation) with questions to practice.
Check your understanding
Check your knowledge of the basics using this quick self-marking test.
Open the quiz
Progression
Now that you understand these topics you are ready to navigate, whether calculating a Fix, a Dead Reckoning position or a Course to Steer.
If you are doing the Yachtmaster course you will also need to know about Compass Bearings.
Feedback
The content of these pages is put together in good faith and is constantly evolving. It is possible that errors exist within this content. If you spot an error or would like to add anything to these pages please contact use via email.
Reading the content of these pages is not a substitute for completing a RYA Shorebased course or similar.
