Airways - Miscellaneous Charts — Page 188, Lesson 197

I want to walk you through how we use a North Polar stereographic chart, specifically the grid system that makes navigation near the North Pole practical. Let's start with the core idea: on a polar stereographic chart, lines of longitude converge toward the pole, which means a constant true track would keep changing as you fly across different longitudes. To solve that, we overlay a grid — a set of parallel lines aligned with the Greenwich Meridian — and we express our direction as a grid track instead of a true track. Now, look at Figure 11.1 with me. The excerpt gives us a worked example. At point A, the longitude is 40° East. The convergence value at that longitude is 40° West convergence. That's the key relationship: the convergence angle is numerically equal to the longitude, but the direction — West or East — depends on which side of the Greenwich Meridian you're on. So at point A, longitude 40° East gives us 40° West convergence. Our true track at A is 310° true. To convert that to a grid track, we subtract the West convergence. So 310° true minus 40° West convergence gives us 270° grid. Now move to point B, at longitude 40° West. Here the convergence is 40° East convergence. The true track into B is 230° true. This time, because the convergence is East, we add it. So 230° true plus 40° East convergence also gives us 270° grid. Notice what happened: two different true tracks — 310° and 230° — both become the same grid track of 270°. That's the whole point of the grid system. It lets you fly a constant grid heading while the true heading changes as you cross meridians. What about where the track crosses the Greenwich Meridian, longitude 0°? The convergence there is zero. So true track and grid track are identical — both 270°. Now, the chart itself has a handy rule printed in the bottom right-hand corner of panel 9. It gives you a simplified method for calculating a grid bearing from a true bearing. Here's the rule: Grid bearing equals true bearing, then you apply the sign of the longitude. If the longitude is West, you add it. If the longitude is East, you subtract it. So the formula is: GRID BEARING = TRUE BEARING, then + LONGITUDE WEST, or – LONGITUDE EAST. That's exactly what we did in the example. At 40° East, we subtracted 40°. At 40° West, we added 40°. The rule works for any bearing, not just tracks — it applies to bearings in general. Let me make sure you have the terminology clear. Convergence is the angular difference between the direction of true north at a given meridian and the direction of grid north, which is constant across the chart. On a polar stereographic chart, grid north is aligned with the Greenwich Meridian. So the convergence angle at any point equals the longitude of that point, with West longitudes giving East convergence and East longitudes giving West convergence. That's the complete method for converting between true and grid on a North Polar stereographic chart, exactly as the excerpt presents it.