Course to Steer: From True to Compass

You've plotted a course on the chart, but that line points to true north (the geographic pole). Your steering compass doesn't — it's pulled off by two errors you must correct before you can steer a reliable heading. Getting this wrong by a few degrees can put you onto rocks over a long leg.

The first error is variation: the angle between true north and magnetic north (where the Earth's magnetic field actually points). It depends on where you are on the planet, is printed inside the compass rose on your chart, and changes slowly each year. The second is deviation: error caused by your own boat's magnetism — engine, electronics, steel — which changes with the boat's heading and is read from a deviation card made for that vessel.

Work the correction in order: True → Variation → Magnetic → Deviation → Compass (remember it with True Virgins Make Dull Companions). The golden rule for going from true to compass: add West errors, subtract East — captured by "variation West, compass best" (compass number is larger). Example: true course 100°, variation 5°W, deviation 3°W → magnetic = 100 + 5 = 105°, compass = 105 + 3 = 108°. That 108° is the number you actually steer.

Had the errors been easterly, you'd subtract them instead — 100° − 5° − 3° = 92°. Same method, opposite sign.