First, the big picture — Page 455, Lesson 447

This is a grid navigation chart — the kind of table you’d use to convert between grid and true directions. Let me walk you through what you’re actually looking at, because it’s dense but very logical once you see the pattern. First, the big picture. This is a grid convergence chart for a polar stereographic chart. The whole point of grid navigation is that when you’re flying near the poles, meridians converge so rapidly that using True North as a reference becomes impractical — your heading would change constantly even on a straight track. So we overlay a grid — a set of parallel lines, usually aligned with the Greenwich meridian — and we navigate relative to that grid instead. The chart you’re looking at gives you the conversion angle: the difference between Grid North and True North at any given position. Now, let me decode the structure. The chart is laid out with latitude on the vertical axis and longitude on the horizontal axis. Look at the left-hand column — you’ll see labels like N50, N20, N10, 0, S10, S30. Those are the latitude lines. N50 means 50° North, N20 is 20° North, N10 is 10° North, 0 is the Equator, S10 is 10° South, S30 is 30° South. So the chart covers from 50° North down to 60° South — that’s the polar region where grid navigation is relevant. Across the top, you have the longitude values. I can see 70W mentioned in the figure caption — that’s the reference. The longitudes run across the top, and the numbers in the body of the table are the conversion angles in degrees. Here’s the key relationship. The conversion angle is the angular difference between Grid North and True North at that specific point. On a polar stereographic chart, True North at any point is the direction toward the North Pole — that’s the line joining your position to the pole, as the figure shows. Grid North, on the other hand, is the direction of the grid lines, which are parallel to the chosen grid meridian — usually Greenwich. So the numbers in the table — like 18 10, 17 57, 16 58 — those are the conversion angles in degrees and minutes. For example, at N50 and longitude 70W, you’d read a conversion angle of about 18°10′. At N20, it drops to around 18°10′ as well, but the pattern shifts as you move east or west. Let me show you the pattern. Look at the row for N50. The values run from about 18 10 at the left, decreasing as you move right — 18 03, 17 57, 17 51, 17 45, all the way down to 16 55 at the right edge. So as you move eastward along the same latitude, the conversion angle decreases. That makes sense — the closer you get to the grid meridian, the smaller the difference between Grid North and True North. Now look at the N20 row. The values are higher — around 18 10 at the left, but they decrease more slowly. And at N10, they’re around 18 03 to 17 57. The 0 row — the Equator — shows values around 18 09 to 18 11. And as you go south — S10, S30 — the values start increasing again. At S30, you see values like 18 26, 18 27, 18 29, climbing to 19 03 and beyond. Here’s the critical thing to understand. The conversion angle is zero at the grid meridian itself — that’s where Grid North and True North coincide. As you move away from that meridian, the angle grows. And the rate of growth depends on latitude — it’s largest near the poles and smallest near the Equator. That’s why the N50 row shows a wider spread of values than the N10 row. Now, how do you actually use this in flight? You take your position — your latitude and longitude — find the intersection on this chart, and read the conversion angle. Then you apply it. If you’re converting from True to Grid, you add or subtract the conversion angle depending on whether you’re east or west of the grid meridian. If you’re going from Grid to True, you do the reverse. The sign convention is critical — get it wrong and you’re 18 degrees off, which at polar latitudes is a serious navigation error. Let me also point out the figure reference. Figure 27.1 shows the fundamental concept — point A at longitude 70W, and the line defining True North is the line joining A to the North Pole. That’s the geometric basis for everything on this chart. And Figure 27.7 shows the creation of the grid itself — how the parallel grid lines are overlaid on the polar stereographic projection. One more thing to notice. The chart has a symmetry about the grid meridian. If you look at the values, they’re roughly symmetric — the angle at a given longitude east of the meridian matches the angle at the same distance west. That’s because the grid is symmetric about its reference meridian. So to summarize what you need to remember: this chart gives you the conversion angle between Grid North and True North for any position in the polar region. You read it by latitude and longitude. The angle is zero at the grid meridian, grows with distance from it, and grows faster at higher latitudes. You apply it to convert between grid and true headings, and the sign depends on your position relative to the grid meridian. That’s the core of grid navigation — once you can read this chart, you can navigate in the polar regions with confidence.