Unlock the Mystery of Magnetic Variation

Before heading out to sea, it is essential that you are able to navigate even when your electronics go awry. Captain John of www.skippertips.com offers us tips on how to use magnetic variation to ensure you find your way efficiently and safely.

Boost your sailing navigation safety when you know two little-known ways to calculate a magnetic compass course anywhere in the world. That way, if your chart plotter or nautical GPS goes “on the blink”, you’ll be ready to take command with confidence. Follow these sailing tips to keep safe and sound wherever you sail!

True North and True Course Direction
Imagine that you point one arm toward the North Pole and another in the direction you want to go. If you could measure the angle from one arm to the other, and you would have true direction.

On the nautical chart, you use a protractor or course plotter to plot courses. Draw a line on the chart from your position to where you want to go. Measure the angle between any vertical line to the course to find true direction to your destination.

Magnetic North and Magnetic Course Direction
Most cruising sailboats use a magnetic compass for steering. The magnetic compass points to the Magnetic North pole, located in northern Canada.

Imagine that you point one arm toward the Magnetic North pole and the other toward your destination. Most of the time, the magnetic angle and true angle “vary”, by several degrees. Navigators call this difference “variation”.

If your compass read true direction, you could steer the true course you plotted on the chart. But because you use a magnetic compass, you must apply variation to true course to know the magnetic course.

Variation changes when your location changes. For instance, on a sailing route from Miami to Bermuda, variation changes by over 10 degrees. If you forget to use variation along the way, you could miss Bermuda by more than 100 miles!

So, how do you know how much variation to use? Scientists calculate this factor for you and place it in one of two convenient places on your navigation chart.

How to Find Variation with a Compass Rose

Every chart has at least one compass rose overprinted onto it, and most have three or four. Printed in magenta (a shade of purple used on nautical charts that shows up well under red night-lights), most roses will have two concentric rings; the outer one shows true directions and the inner shows magnetic directions.

Locate the closest compass rose to your location, and just above its center crosshairs you will find a label noting the local variation (as of the stated year) and its name–east or west. Just below the crosshairs, you will find a note of the annual increase or decrease in variation.

Take the difference between the year shown and the current year. Multiply this difference by the minutes of increase or decrease. If increasing, add the difference to variation. If decreasing, subtract the difference. Round your answer to the closest whole degree of variation.

Use the compass rose illustration to work through this example:

Variation: 4º 15′ west (1985). Annual decrease: 8′. The current year is 2012; what is variation at this location?

1. Find the difference in years between 2012 and 1985. 2012 – 1985 = 27 (years).
2. Multiply 27 (years) X 8′ (decrease per year) = 216′ or 3º 36′.
3. Apply this to the variation shown. In this example, we subtract: 4º 15′ W – 3º 36′ = 0º 39′ W.
4. Round off the final number to the closest whole degree. Use 1º W variation at this location.

Sometimes the annual change in variation is noted as east or west, rather than increasing or decreasing. If the annual change has the same name (east or west) as the variation, variation is increasing. If the names are opposite, variation is decreasing.

For example: If your compass rose says 12°14′W (10′E), this indicates a decrease of 10 minutes a year because the names, W and E, are opposite. If it says 12°14′W (10′W), that would indicate an increase by 10 minutes a year because the names are the same.

How to Find Variation with Isogonic Lines

Offshore charts rarely show more than one or two compass roses and only show true direction. Look for magenta colored, diagonal dashed lines running across the chart. Scan along the line and find the variation. Use the variation indicated on the isogonic line closest to your position.

Notice how tough it can be to spot these isogonic lines and the variation written on top. We've highlighted them with blue dashed-lines. Highlight your charts in a similar way. Use the variation on the isogonic line that lies closest to your present position.

How to Find Your Magnetic Course Anywhere on Earth

1. Plot the true course on your chart and measure the direction.
2. Find the variation by one of the two methods shown above.
3. Add westerly variation to true course; subtract easterly variation from true course.

Example #1:
True course to Shelter Cove is 135ºT The closest compass rose indicates a variation of 17ºE. Find the magnetic course to steer. 135ºT -17ºE = 118ºM

Example #2:
True course to Safety Harbor is 317ºT The closest compass rose indicates a variation of 6ºW. Find the magnetic course to steer. 317ºT + 6ºW = 323ºM


Now you understand how the magnetic north pole creates a challenge for navigators worldwide. Use the closest compass rose or isogonic line for safe nautical navigation anywhere you choose to sail.

This entry was posted in Boat Rat's Tip of the Week and tagged , . Bookmark the permalink.

Leave a Reply

Your email address will not be published. Required fields are marked *


You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <strike> <strong>