Earth Magnetism — Page 62, Lesson 69

I want to walk you through the application of variation and deviation — how we actually use these corrections to convert between True, Magnetic, and Compass headings in real navigation. Let's start with variation. When magnetic direction is exactly the same as true direction, variation is nil — zero. But normally, Magnetic North lies either to the west or to the east of True North. If Magnetic North is to the west of True North, we call that westerly variation. If it's to the east, that's easterly variation. Let me give you a concrete example from the figures. Imagine an aircraft whose direction relative to True North is 105 degrees. If Magnetic North is west of True North, then the direction relative to Magnetic North becomes 122 degrees — that's a larger number. If Magnetic North is east of True North, the direction relative to Magnetic North becomes 088 degrees — a smaller number. In the first case, variation is 17 degrees west; in the second, variation is 17 degrees east. Now, the direction in which the fore-and-aft axis of the aircraft is pointing — that's the aircraft's heading. So we can build a simple conversion table. Starting from True heading, we apply variation to get Magnetic heading. From Figure 3.15, True heading 105 degrees plus variation 17 degrees west gives Magnetic heading 122 degrees. From Figure 3.16, True heading 105 degrees plus variation 17 degrees east gives Magnetic heading 088 degrees. From this we deduce a rule you absolutely must remember: Variation West — Magnetic Best. And Variation East — Magnetic Least. What does that mean? When variation is west, the Magnetic heading is a larger number — it's "best" in the sense of bigger. When variation is east, the Magnetic heading is a smaller number — it's "least." So if you have True heading and westerly variation, you add the variation to get Magnetic. If you have easterly variation, you subtract it. Now let's move to deviation. Deviation is the angle between Magnetic North and Compass North, measured in degrees east or west from Magnetic North. So while variation is the difference between True and Magnetic, deviation is the difference between Magnetic and Compass. Let's take another example. The aircraft's direction relative to True North is 100 degrees. Variation is 25 degrees west, so Magnetic heading is 125 degrees. Now, if the compass indicates 135 degrees, that means Compass North is west of Magnetic North — the compass reading is larger than the Magnetic heading. That's westerly deviation of 10 degrees. If instead the compass indicates 115 degrees, Compass North is east of Magnetic North — the compass reading is smaller — that's easterly deviation of 10 degrees. From these examples we get another rule: Deviation East — Compass Least. And Deviation West — Compass Best. When deviation is east, the Compass heading is smaller than the Magnetic heading. When deviation is west, the Compass heading is larger. Now, sometimes instead of using the words "east" or "west," we use plus and minus signs as prefixes. In that convention, east is treated as plus and west as minus. The rule then becomes: to convert from Compass heading to Magnetic heading, you apply the deviation with its sign as stated — so east is plus, west is minus. To go the other way — from Magnetic to Compass — you change the sign and then apply it. Let me show you with numbers. Suppose deviation is 3 degrees east. If Magnetic heading is 263 degrees, and we want Compass heading: deviation east means compass least, so Compass heading is 263 minus 3, which gives 260 degrees. Going the other way: if Compass heading is 260 degrees and deviation is 3 degrees east, then Magnetic heading is 260 plus 3, which gives 263 degrees. Using the plus/minus convention: deviation is +3 degrees. To go from Magnetic to Compass, you change the sign and apply it — so 263 minus 3 equals 260. To go from Compass to Magnetic, you apply the deviation as stated — so 260 plus 3 equals 263. There's another equivalent rule you'll see: From Compass to Magnetic, the signs are true — meaning east is plus, west is minus. So if you have a Compass heading and a deviation of 3 degrees east, you add 3 to get Magnetic. If deviation were 3 degrees west, you'd subtract 3. That's the same as saying the sign is applied directly when going from Compass to Magnetic. Let me show you the figures that illustrate all of this. So to summarise the full conversion chain: True heading plus or minus variation gives Magnetic heading. Magnetic heading plus or minus deviation gives Compass heading. And the two memory aids are: Variation West — Magnetic Best (add). Variation East — Magnetic Least (subtract). Deviation West — Compass Best (add). Deviation East — Compass Least (subtract).