Magnetic variation

Magnetic variation is the term used on water; on land it is referred to as magnetic declination.

Magnetic variation is the difference in angle between magnetic north and true north. Marine charts have superimposed compass roses that show the variation for the areas they cover. The chart may also have isogonic lines that show locations with the same magnetic veriation.

The compass rose will show the variation for a specific year and an annual rate of change. In order to determine the current variation, take the annual rate and multiply it by the number of years that have passed since the referenced year. Add or subtract this from the specified variation.

Example 1
A compass rose shows that the magnetic variation is   010°30'W 2000 (15'W).

This means that the variation is 10.5° west in year 2000 and is increasing (change is in the same direction) by 15 minutes (one quarter degree) per year.

If it is currently 2006, then 6 years have passed since the variation indicated. Therefore, the current variation is:

10.5 + 6*0.25 = 10.5 + 1.5 = 12.0°

Example 2
Magnetic variation is 021°W 1995 (18'E).

Note that the change in variation is opposite in direction from the variation. It is therefore reducing (negative change) each year. If it is currently 2006, then 11 years have passed since the variation indicated. Therefore, the current variation is:

21°00' - 11*18 = 21°00' - 198' = 21° - 3.30° = 17.7°

Magnetic variation is usually fairly constant in any area. However, it is important to know that there are places where magnetic variation can change dramatically over relatively short paddling distances. These are caused by local magnetic anomalies and will be noted on a chart.