Earth Magnetism — Page 62, Lesson 69

Let's start with the heart of this: the difference between True, Magnetic, and Compass directions, and how variation and deviation move you between them. First, a quick foundation. True North is the geographic North Pole — the top of the Earth's axis of rotation. Magnetic North is where a compass needle actually points, which is not the same place. The angle between True North and Magnetic North is called variation. When Magnetic North and True North line up exactly, variation is nil — zero. Otherwise, Magnetic North lies either to the West or to the East of True North. Let me give you the two worked examples from the figures. In the first case, the aircraft's direction relative to True North is 105°. Relative to Magnetic North it's 122°. So Magnetic North is to the West of True North, and the variation is 17° West. In the second case, the aircraft's direction relative to True North is again 105°, but relative to Magnetic North it's 088°. Here Magnetic North is to the East of True North, and the variation is 17° East. Now, the direction in which the fore and aft axis of the aircraft is pointing is called the aircraft's heading. So we build a table. Heading True 105°, variation 17°W, gives Heading Magnetic 122°. And Heading True 105°, variation 17°E, gives Heading Magnetic 088°. Notice the arithmetic: with Westerly variation you add to get from True to Magnetic; with Easterly variation you subtract. That gives us the rule you must remember: Variation West — Magnetic Best. And Variation East — Magnetic Least. "Best" means the Magnetic heading is numerically larger than the True heading; "Least" means it's smaller. West makes it bigger, East makes it smaller. Now, deviation. Deviation is the angle between Magnetic North and Compass North, measured in degrees East or West from Magnetic North. This is caused by the aircraft's own magnetic fields — the metal structure, electrical systems — pulling the compass away from Magnetic North. So we now have a third reference: Compass North, what the compass actually reads. Look at the next pair of examples. In the first, the aircraft's direction relative to True North is 100°, relative to Magnetic North is 125°, and the compass indicates 135°. So variation is 25° West, and deviation is 10° West. In the second, True is 100°, Magnetic is 125°, but the compass reads 115°. Variation is still 25° West, but deviation is 10° East. The table for the first: Heading True 100°, variation 25°W, gives Heading Magnetic 125°, then deviation 10°W, gives Heading Compass 135°. For the second: Heading True 100°, variation 25°W, Heading Magnetic 125°, deviation 10°E, Heading Compass 115°. And here's the second rule you must remember: Deviation East — Compass Least. Deviation West — Compass Best. Same logic as before: Easterly deviation makes the Compass heading smaller than Magnetic; Westerly deviation makes it larger. Now, sometimes instead of writing East or West, we use plus and minus signs as prefixes. Plus means East, minus means West. The rule then is: apply the deviation as stated to the Compass Heading to get the Magnetic Heading, and conversely apply the deviation with the sign changed to the Magnetic Heading to get the Compass Heading. Let me walk you through the example. Deviation is 3° East, or +3°. Heading 263° Magnetic. To convert from Magnetic to Compass, we change the sign and apply it to the Magnetic heading. So 263 minus 3 equals 260° Compass. That matches the rule: Deviation East, Compass Least — the compass reading is smaller. Conversely, starting from Heading 260° Compass, we apply the deviation as stated, plus 3, to get 263° Magnetic. There's an equivalent rule that's also used: From Compass to Magnetic, the signs are true — East is plus, West is minus. So when you go from Compass to Magnetic, you add Easterly deviation and subtract Westerly deviation. When you go the other way, from Magnetic to Compass, you reverse the sign. So the full picture: True to Magnetic uses variation; Magnetic to Compass uses deviation. And the two memory rules — Variation West Magnetic Best, Deviation East Compass Least — keep you straight on which way the numbers move.