Did you know that the location of the magnetic north pole affects every course you plot on your navigational chart? Are you familiar with the two ways to correct your course in order to steer a safe, accurate magnetic course?
Use this easy guide to gain a good understanding of true and magnetic direction. Then you’ll have the confidence to expand your horizons for cruising wherever you wish to go.
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 on 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 12 degrees. If you forget to use variation along the way, you could miss Bermuda by over 120 miles!
Two Ways to Find Magnetic Variation.
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:
1. The Nautical Compass Rose
Cartographers place large, concentric circles, or compass roses, on coastal or inland charts. The outer ring indicates true direction. Do not use the inner magnetic ring. It’s difficult to read and doesn’t show degrees like the outer ring does.
Look at the center of the compass rose to find the variation. Write down the variation along with name shown: East or West.
2. Isogonic Lines
Offshore charts rarely show more than one or two compass roses, and they 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.
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.
True course to Shelter Cove is 135T
The closest compass rose indicates a variation of 17 E.
135T -17E = 128M
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.