We're starting a brand-new chapter now — Chapter 27, Gridded Charts — Page 457, Lesson 456

We're starting a brand-new chapter now — Chapter 27, Gridded Charts. And I want to begin by answering the most basic question first: why do we even need grid navigation at all? The main reason is that, at high latitudes, the amount of meridian convergence becomes significant. Let me unpack that. Meridian convergence is the fact that lines of longitude — the meridians — are not parallel to each other. They all meet at the North Pole and the South Pole. So as you move north or south, the meridians are converging, or coming together. At low latitudes, near the equator, they're spread far apart and look almost parallel. But up in the polar regions, they're squeezing together dramatically, and that convergence becomes a real problem for navigation. Let me set up a concrete example. Look at Figure 27.1. We have point A at longitude 70°W. The line defining True North at A is the line joining A to the North Pole — that is, the meridian of 70°W itself. So True North at any point is simply the direction along your meridian toward the pole. Now suppose we want to fly a track AB, and the initial track angle is 020°(T) — that's 020 degrees True. So we set off using a compass based on True heading, and we keep applying the appropriate drift to maintain a track of 020°(T). So far, so good. But here's the catch. As we cross each meridian going eastwards, the direction of True North changes. Think about what's happening on the map. Relative to the "top of the map" — the 12 o'clock position on the page — the direction of North is turning anticlockwise. That means our direction of 020°(T) will also turn anticlockwise on this map. So even though we're holding a constant true track, the path we actually trace over the ground is not a straight line on the chart. If we maintain a track of 020°(T), our path over the ground will look like a curve — a curved line bending on the map, as shown in Figure 27.2. Now, I want to be clear that this isn't some weird polar-only quirk. Exactly the same problem occurs at lower latitudes — unless a direct Mercator chart is being used — but it's usually just not noticed, because the convergence is so small down there that the effect is negligible. Let me look at a lower-latitude example to make the choice concrete. Consider Figure 27.3, a Lambert chart. We wish to fly from C to D, and both points are at 53°N. The mean track is 090°(T) — that's due east. But the initial track is 082°(T). So the track direction is changing as we go, because of convergence. We now have a choice. Option one: we can maintain a track of 090°(T) the whole way. If we do, we will still finish up at D, but we will fly the rhumb line track. A rhumb line is a line that crosses every meridian at the same angle — a constant true track. If we were to take a succession of fixes whilst flying, but not alter track, they would appear like this — a curved path on the chart, as in Figure 27.4. So we get to D, but the path on the map looks bent. That's the fundamental problem that grid navigation is designed to solve. Because when you're up high, that bending becomes so severe that it's impractical to navigate by true tracks alone. So the whole chapter is about how we set up a grid — a set of parallel lines — to give us a consistent reference direction, so we can fly a straight line on the chart. But that's the next step. For now, I want you to hold onto this core idea: meridian convergence is significant at high latitudes, True North is defined by your meridian to the pole, and maintaining a constant true track produces a curved ground path on a converging-meridian chart.