Menu

Variation and Deviation

Truth lies within a little and certain compass, but error is immense.

Browse Our Courses

Variation

This is the difference between true North and magnetic North, this changes with:

  • Location
  • Time

How do I work out the variation?

  • You can’t just say that variation is x degrees and apply it. You must know the amount of variation and whether to add or subtract it. There are several different ways of calculating variation. Most charts have a compass rose (or several) on the chart and inside the rose there will be an arrow offset giving the number of degrees and minutes and the date. This is a good starting point. The process is to work out how much the variation has changed since the chart was produced, then add or subtract his amount from a true course to find a magnetic course.

 

 

 

 

 

Day Skipper theory

A magnetic North arrow on the compass rose shows the variation at your location and the date this recorded with the rate of annual change.

In this example we can see that the variation in 5 degrees West in 2002 and is decreasing 7 minutes annually.

To find variation:

Work out how much it has changed since published and add/subtract that amount.

2002-2019 = 17 years

17 x 7 minutes = 119 minutes. We can round this up to 120 minutes which is 2 degrees.

(We always round up or down to the nearest whole degree).

As it is decreasing we subtract.

The variation in 2019 for this area will be 5 degrees minus 2 degrees = 3 degrees.

So how is it done?

The diagram above shows how to apply variation, if you are going from true to compass add WEST, subtract EAST. If you are going from compass to true, subtract WEST and add EAST.

There is a whole section on this subject with examples in the RYA Day Skipper theory course.

How to calculate variation

If the variation given is West and the annual Variation correction to apply is East then you subtract.
Conversely if variation was East and the annual deviation correction to apply is West then you again subtract.
If Variation given is West and the annual Variation correction to apply is also West then you ADD.
If Variation given is East and the annual Variation correction to apply is also East then you ADD.

Deviation

Deviation

This is the effect of the vessel on the magnetic compass. If you stick a great big lump of metal (an engine for example) or electrical cables near a magnetic compass then chances are it will deviate from it’s original “pointing to magnetic North” position. Of course, this is exactly the situation on board a boat! You will need to account for this when navigating.

Each boat has a deviation table written for it and lists the deviation that can’t be adjusted out by a compass adjuster, so you can apply the corrections yourself when calculating compass courses.

School Principal - Stuart Cooper

Pitfalls and common mistakes

Forgetting to add or subtract magnetic variation and deviation when working out an estimated position or course to steer will put your calculations out and could put you into danger. Always check your workings, if it looks wrong, the chances are it is wrong. Check and recheck before you proceed with your voyage.

True to compass crib sheet. This is self-explanatory and there is an example of how to use it on the page. Feel free to email me if you have any questions about this.

RYA Navigation Theory courses

We offer Navigation courses both online and classroom based. You might like to taking Day Skipper theory and practical in one combined course.