So, to give an example — Page 464, Lesson 461

Let’s start with the worked example at the top, because it ties together everything we’ve built so far. I’m going to read it as a chain of conversions, and I want you to see how each step hands off to the next. We begin with a grid heading of 090°(G). That’s the direction we want to steer, measured against Grid North on the chart. The first correction is convergence, and here it’s given as 10°W. Convergence is the angular difference between True North and Grid North at our position. Because it’s West, we subtract it from the grid heading to get True. So 090°(G) minus 10°W gives us 100°(T). Next, we apply variation. Variation is the angular difference between True North and Magnetic North, caused by the Earth’s magnetic field. Here it’s 8°W. Again, West means we subtract, so 100°(T) minus 8°W gives us 108°(M). Finally, we apply deviation. Deviation is the error introduced by the aircraft’s own magnetic fields, the difference between Magnetic North and what the compass actually reads. Here it’s 2°E. East means we add, so 108°(M) plus 2°E gives us 106°(C). That’s the compass heading we actually steer. So the full chain is: Grid 090, minus convergence 10°W, gives True 100, minus variation 8°W, gives Magnetic 108, plus deviation 2°E, gives Compass 106. Notice the pattern: West corrections subtract, East corrections add, all the way down the chain. Now, the key question the book poses: how do we know whether convergence is East or West in the first place? There’s a logical way to work it out, and it depends on two things — which hemisphere we’re in, and whether our position is East or West of the datum meridian. Let me set up the picture. The datum meridian is the reference meridian we chose for the grid — the line where Grid North and True North coincide. In the diagram at Figure 27.10, the vertical centre line is the datum meridian, and in this particular figure it happens to be the Greenwich meridian, but it doesn’t have to be. The red lines are other meridians on the Earth, and the green lines represent Grid North. Now look at the top left quarter of the diagram. That’s the Northern hemisphere, with the aircraft out to the left — that is, on a meridian somewhere to the West of the datum. In this quarter, except right at the Equator itself, True North leans to the right of Grid North. Right is East. So by definition, convergence must be East. Here’s the logic in plain terms. In the Northern hemisphere, west of the datum, True North is east of Grid North, so convergence is East. The book then tells you to work through the other three quadrants the same way, and you end up with the summary in Figure 27.12, which shows the direction of convergence for each combination of hemisphere and side of the datum. So the rule is: hemisphere tells you the sense of the lean, and your position relative to the datum tells you which way to apply it. Northern hemisphere, west of datum — convergence East. The other quadrants follow the same reasoning. Now, once we can convert from True to Grid on the chart, we need a method of actually steering that grid direction. The book gives us two options: we can use a compass, or we can use a gyro. That’s the bridge from the chart work into the cockpit — the grid gives us a direction on paper, and the compass or gyro lets us hold it in the air. Let me show you the diagram at Figure 27.11, which illustrates True North sitting east of Grid North — that’s the visual for the Northern hemisphere, west-of-datum case we just worked through. And Figure 27.10 shows the full grid laid over the Earth, with the datum meridian as the vertical centre line. So to pull it together: the example walks you through the full correction chain from grid to compass, and the convergence direction rule tells you whether that first correction is East or West based on hemisphere and position relative to the datum. Then, to steer the grid direction in practice, you use either a compass or a gyro.