2.5 The Magnetic Compass: Variation & Deviation

The Magnetic Compass: Variation & Deviation

Quick Answer: Your magnetic compass points to magnetic north, not true north, and it is thrown off by two separate errors. Variation is the difference between true and magnetic north at your location, the same for every boat there. Deviation is error from your own vessel's steel and electronics, and it changes with your heading. You must account for both to convert between compass, magnetic, and true directions.

Understanding why a compass is wrong is the foundation for the TVMDC conversions in the next section and for every accurate course you plot.

Two North Poles

The Earth has a geographic (true) North Pole, the top of the axis it spins on, and a separate magnetic north pole, where the planet's magnetic field points down. A magnetic compass aligns with the magnetic field, so it points toward magnetic north — which in most places is not the same direction as true north. (A gyrocompass, found on larger vessels, seeks true north mechanically and so has neither variation nor deviation, only a small "gyro error"; most six-pack vessels rely on the simple, power-independent magnetic compass, which is why understanding its two errors matters.)

Variation

Variation (also called declination) is the angular difference between true north and magnetic north at a given location. It is measured in degrees East or West of true north. If magnetic north lies to the east of true north, variation is easterly; if to the west, westerly.

Key facts about variation:

• It is the same for every vessel in the same area — it depends only on where you are, not on your boat or heading.
• It is printed inside the compass rose on the chart, along with the year measured and an annual rate of change, for example: "VAR 14 degrees 30 minutes W (2020), annual increase 8 minutes."
• It changes gradually as you move geographically — a passage of a few hundred miles can change variation by several degrees.

Updating variation to the current year: take the printed value and apply the annual change for the number of years elapsed. From the rose above, variation of 14 degrees 30 minutes West measured in 2020, increasing 8 minutes per year, becomes, six years later in 2026: 6 times 8 minutes equals 48 minutes, so 14 degrees 30 minutes plus 48 minutes equals 15 degrees 18 minutes West. Small, but it accumulates over the life of a chart.

Deviation

Deviation is compass error caused by the vessel's own magnetic influences: the steel in the hull and engine, electrical wiring, radios, speakers, and even a knife or phone left near the compass. Because the boat's magnetic field is fixed relative to the boat but the boat rotates relative to Earth's field, deviation changes with the vessel's heading. On one heading the compass might read 2 degrees east of magnetic; on another, 3 degrees west.

Key facts about deviation:

• It is unique to each vessel and to each heading — it is not the same for boats nearby, unlike variation.
• It is recorded on a deviation card (or deviation table) that lists the error for a series of headings, typically every 15 degrees, each labeled East or West.
• It can be reduced by professionally adjusting (swinging) the compass — placing small correcting magnets — and by keeping magnetic and electrical gear away from the binnacle. It usually cannot be eliminated entirely.

A Sample Deviation Card

To use it, you enter with the vessel's heading, not with the bearing you happen to be taking, and read off the deviation for that heading. One practical way to check deviation is to steady on a charted range and compare the bearing your compass gives with the range's known charted direction; the difference (after allowing for variation) is your deviation on that heading.

Combining the Two: Compass Error

The total compass error is variation and deviation combined, respecting their East/West labels. Same labels add; opposite labels subtract, and the result keeps the label of the larger.

Worked example 1: Variation is 15 degrees West and deviation (for your heading) is 3 degrees East. Opposite labels subtract: 15 W minus 3 E gives 12 degrees West total compass error.

Worked example 2: Variation is 6 degrees East and deviation is 4 degrees East. Same labels add: 10 degrees East total error.

Why it matters: your compass reads in compass degrees, but the chart is drawn in true degrees. Compass error is the total correction between them. If you ignored a 12-degree error over a 30-mile leg, you would end up roughly six miles off track — enough to run aground or miss a harbor entrance. The next section turns these labeled errors into a step-by-step conversion between true, magnetic, and compass directions.