Let me set the scene — Page 457, Lesson 456

I want to walk you through why grid navigation exists in the first place, because it all starts with a problem that gets serious at high latitudes. Let me set the scene. Imagine you're at point A, which sits at longitude 70° West. The line that defines True North for you is the meridian of 70°W — that's the line joining A to the North Pole. Now, you want to fly a track from A to B, and that track makes an initial angle of 020° True. So you set off using a compass based on True heading, and you keep applying the appropriate drift to maintain that track of 020° True. Sounds straightforward, right? Here's the catch. As you cross each meridian going eastwards, the direction of True North changes. Relative to the "top of the map" — the 12 o'clock position on the page — the direction of North is turning anticlockwise. That means your direction of 020° True will also turn anticlockwise on the map. So if you faithfully maintain a track of 020° True, your path over the ground won't be a straight line on the chart at all. It will curve, and it will look something like a bending path that drifts away from where you intended. Now, here's the important part: this exact same problem occurs at lower latitudes too — unless you're using a direct Mercator chart. It's just that at lower latitudes, the effect is so small you usually don't notice it. Let me give you a concrete example with a Lambert chart. Suppose you want to fly from C to D, and both points are at 53° North. The mean track is 090° True, but the initial track is 082° True. You now have a choice. Option one: you can maintain a track of 090° True. If you do, you will still finish up at D, but you will fly the rhumb line track. If you were to take a succession of fixes while flying, without altering track, those fixes would appear to drift on the chart — they'd trace out that curved path I described. That's the core problem grid navigation solves. Because at high latitudes, meridian convergence becomes significant — the meridians bunch together and True North keeps shifting under you. So maintaining a constant True track doesn't give you a straight line on the chart. Grid navigation gives us a way to define a constant direction that doesn't twist with the meridians. Let me show you the geometry with a figure. That's Figure 27.1 — A at longitude 70°W, with the line to the North Pole defining True North. Keep that picture in your mind, because everything about grid navigation builds on this idea of convergence and how we escape its effects.