What is the difference between magnetic variation and deviation?

Variation and deviation are the two separate errors that stand between a magnetic compass reading and true north, and every navigator must be able to name, find and apply both. Variation comes from the Earth: the magnetic pole the needle seeks does not coincide with the geographic pole, so at any spot on the globe there is an angle between magnetic north and true north. Variation depends on where the ship is, not on which way it is heading. Its value is printed on the compass rose of the chart for that area together with the annual rate of change, so the navigator updates the printed figure to the current year before using it.

Deviation comes from the ship itself: hull steel, machinery, cargo and electrical equipment create magnetic fields that pull the needle away from magnetic north. Deviation belongs to that particular ship and compass position, and, critically, it changes with the ship's heading, because the ship's magnetism rotates with the ship while the needle does not. A compass adjuster periodically swings the ship, measuring the error on successive headings and placing corrector magnets to cancel most of it; what remains is recorded on the deviation card, which should show only a few degrees on any heading.

To convert between compass and true directions the navigator applies both errors in sequence: compass to magnetic using the deviation for the current heading, then magnetic to true using variation, remembering that easterly and westerly errors carry opposite signs. The combined figure is the total compass error, and checking it is watchkeeping routine: compare the magnetic compass against the gyro regularly, and verify the error independently by an azimuth of the sun or another celestial body, or by a transit of two charted objects, logging the result each watch.

Be alert for deviation changing during service. Loading steel or magnetic cargo, structural work and welding, a lightning strike, or someone stowing a tool box, loudspeaker or phone near the binnacle can all alter it, and an unexplained difference between the observed compass error and the deviation card is the warning sign. If deviation grows large or erratic, have the compass swung and adjusted again. The gyrocompass points true and carries neither error, which is why it is the primary heading reference, but the magnetic compass needs no power and survives blackouts, so keeping its errors known and small is what makes it a genuine backup rather than a decoration.