If you’re considering a significant voyage this coming boating season, now is a good time to examine and update your backup paper charts—especially those printed before 2020. Look on a paper chart at the two concentric compass roses. The outer true rose circle never varies. The inner magnetic rose varies by place and over time.
Inside the rose circles, you’ll find the latest printing date. There’s a statement of annual variation—how much the printed inner magnetic rose must be mentally rotated, or corrected, in the years since that latest printing date.
If you have a 4-year-old chart, you can simply multiply the annual variation by four to get the new “V” in T-V-M (true variation magnetic) calculations. Right? This should be a close-enough approximation. It’s been this way for years. In fact, for the past 200 years, the magnetic pole has moved closer to the North Pole at a relatively constant speed and direction—approximately 25 miles per year.
However, in 2020, the magnetic pole shifted almost 50 miles. In some locations, this means the annual variation could be nearly double. That’s not printed on your 4-year-old 2018 chart.
For example, the -15°00’(W) variation line that went through Plainfield, Connecticut, (the fictitious location of the Bowditch Bay training chart), is now over Harpswell, Maine, as of 2022. An old 1972 chart for the Harpswell, Maine, area shows -17°15’(W) and No Annual Change. (Yes, some people have paper charts that old. Mine is a wall decoration—not on the boat.) That old chart said “no annual change” as the apparent shift in the magnetic pole was directly away from the chart. It’s now 2 degrees different.
This 2-degree change can affect your navigation results. That’s in New England. However, this change can be much greater for boaters in the Great Lakes and the Pacific Northwest. The pole is moving away from New England, but those other boaters see this movement from the side. See Figure 1.
What to do?
To get a new updated chart, you can download the most current (within a week) corrected raster navigation chart from NOAA. The present annual variation is accounted for in the inner magnetic rose’s degree of rotation. Also, the statement of annual variation is updated to this coming year’s predicted correction. You can download any chart image from NOAA for free. Buying a print-on-demand chart from an approved NOAA chart agent or having the RNC printed locally costs between $30 and $36. Either way, you get up-to-date information on your new paper chart.
Correct an old chart from the web
You can update that 4-year-old paper chart yourself by visiting a World Magnetic Model website. If you are looking for results for a particular location, visit magnetic-declination.com.
Click on a location that’s near (or within) a compass rose on your chart. Write the current variation and year somewhere in the middle of that compass rose. Don’t cover or destroy the old information.
If you use the United States Power Squadrons 4 x 15 plotter (which always measures against a chart’s true north chart grid), the number you wrote is your current “V” in T-V-M calculations.
If you are using parallel rulers against the inner magnetic rose, you now must mentally rotate the rose more or less by the difference between the current value you wrote and the original variation stated at the last printing. You must physically rotate your placement of the parallel ruler on the inner magnetic rose by that amount. It’s simpler to use the USPS plotter and the “V” you wrote in the compass rose circle.
Correct an old chart from your chart plotter
On most electronic chart plotters, a magnetic course or bearing is automatically computed from a built-in World Magnetic Model. The WMM accounts for the magnetic pole movement over time and will compute this for every location.
If you’re using OpenCPN’s WMM plug-in, first make sure OpenCPN is up to date. Then click on the compass rose (WMM) icon, just above the life preserver icon in the left toolbar. Other chart plotters should behave similarly. See Figure 2.
Moral of the story
Use a chart plotter with the WMM built in for day-to-day navigation. For paper backup, update your charts from a WMM website or your chart plotter. Use the USPS 4 x 15 plotter with paper charts to avoid complicated gymnastics on the magnetic rose using parallel rulers.
