Gridded Charts — Page 471, Lesson 471

Let’s pick this up right where the worked example leaves off. We’ve got a specific problem here, and I want to walk you through it carefully, because it ties together the whole idea of grid navigation and convergency. The question is: If the aircraft is 91 degrees east of the datum, and the aircraft is at 118°E, where is the datum? Let me unpack that. In grid navigation, we use a datum meridian — that’s the reference meridian from which we measure grid direction. The grid is drawn parallel to that datum, and the "east of the datum" tells us how far around the Earth, in longitude, the aircraft sits relative to that reference line. Here, the aircraft is 91 degrees east of the datum. That means if you start at the datum and move eastward 91 degrees of longitude, you arrive at the aircraft’s position, which is at 118°E. So to find the datum, we do the reverse: we take the aircraft’s longitude, 118°E, and subtract the 91 degrees of easterly displacement. 118°E − 91° = 027°E. So the datum meridian is at 027°E longitude. That’s the answer. Now, I want to be clear about the arithmetic and the direction. Because the aircraft is east of the datum, the datum must be west of the aircraft. So we subtract. If the aircraft had been west of the datum, we’d have added. The sign of the displacement tells you which way to apply the convergency. But the excerpt also gives you a diagrammatic method — Figure 27.21 — and I want to walk you through that, because it’s a really useful way to visualise what’s happening, especially when you’re not sure which way to apply the convergence. Here’s the procedure. First, plot the aircraft position, which is 28°S, 118°E. That’s the blue dot on the figure. So we’re in the southern hemisphere, at 118 degrees east. Next, draw in the direction of True North at that position — that’s the blue line. Remember, True North points toward the geographic North Pole, and on a chart, the direction of True North varies with position because of convergency. Then, draw in the True track of 042°(T) — that’s the green line. This is the actual direction the aircraft is travelling, measured relative to True North. So the green line makes an angle of 42 degrees with the blue True North line. Now here’s the key step. The Grid Track is 133°(G). That’s the direction of the aircraft’s track measured relative to the grid, not relative to True North. The difference between the True track, 042°, and the Grid track, 133°, is the convergency — the angular difference between True North and Grid North at that position. That Grid track of 133°(G) gives you the direction of the grid — that’s the red dotted line on the figure. So you draw a line at 133 degrees, and that line represents the direction of the grid lines at that point. Finally, you parallel the grid through the South Pole. That means you take that grid direction and draw a line parallel to it, but passing through the South Pole. Where that line crosses the equator — or rather, where it intersects the meridians — that gives you the datum meridian. So the diagram lets you see which way the convergence is applied. You can see whether the datum is east or west of the aircraft, and then you do the numerical calculation — which, in this case, gave us 027°E. Now, one important note from the excerpt: these diagrams are only sketches. They’re not meant to be scale drawings or precise solutions. Their purpose is purely to help you visualise the situation — to see which way to apply the convergence — and then you do the actual numerical calculation. So don’t try to measure angles off the sketch; use it to get the direction right, then do the arithmetic. Let me just recap the whole logic once more, because this is the heart of grid navigation. The grid is a set of parallel lines on the chart, all aligned with the datum meridian. True North, on the other hand, converges toward the pole. The angular difference between True North and Grid North at any position is the convergency. When you know the aircraft’s position relative to the datum — how many degrees east or west — you can find the datum by applying that convergency in the correct direction. Here, 91 degrees east of the datum, at 118°E, means the datum is 91 degrees west, at 027°E. That’s the complete worked example. Take a moment to let the diagram method sink in — it’s a skill you’ll use repeatedly in grid navigation problems.