Gridded Charts — Page 471, Lesson 467

We're starting a new topic now: gridded charts. Let's get into it. I want to walk you through why we use a standard polar grid in the first place. The key property here is that these charts are ideally suited for a standard polar grid, which can be printed on the charts for all users. The reason we do this is to eliminate confusion over the alignment of those few VOR/TACAN stations that are not aligned with Magnetic or True North, but with Grid North. So, in the Northern regions, the datum meridian used is the Greenwich meridian. Now, here's the crucial relationship: at any True meridian, the chart convergency will be equal to the longitude, with the sign reversed. Let me give you the examples: at 45°W, chart convergency equals 45°E. At 45°E, chart convergency equals 45°W. So the sign flips. Let's look at a route from A to B in Figure 27.16. The Grid Track is 090°(T). At 45°W, the Track is 090°(G) minus 45°E convergence, which gives you 045°(T). At 45°E, the Track is 090°(G) plus 45°W convergence, which gives you 135°(T). So you see the arithmetic: west of the datum, you subtract; east of the datum, you add. Now, in the Southern hemisphere, the Greenwich meridian is again used as the datum meridian. But here's the difference: in the Southern hemisphere, the chart convergency is always equal to the longitude when a standard grid is used. No sign reversal—it's just equal. To maintain orientation with Grid North at the top, diagrams illustrating the standard south polar grid are normally drawn with the 180°E/W meridian at the bottom of the diagram. Let's work through Figure 27.17. The Grid Track from A to B is constant 070°(G). At point A, longitude is 45°W, so convergence is 45°W. So 070°(G) plus 45°W convergence equals 115°(T). At point B, longitude is 45°E, so convergence is 45°E. So 070°(G) minus 45°E convergence equals 025°(T). And at point X, the track is 090°(G), convergence is 180°E/W, so 090°(G) plus or minus 180°E/W equals 270°(T). The plus/minus there reflects that 180°E and 180°W are the same meridian. Now let's move to the Grid/True Conversion Problems for the Northern Hemisphere. Figure 27.18 represents a North Polar Stereographic Projection. The 'n' factor—that's the relationship between the inclination of the meridians and the change of longitude—is 1.00 for this projection. That's a key number to remember. Let's consider Aircraft No. 1. Just by inspection of the diagram, you can see its Grid heading is 360°(G), whilst the True heading is 270°(T). So the True heading is 90 degrees less than the Grid, by simple visual inspection. Now let's work it out using convergence. If Grid North is at the 12 o'clock position, then the datum meridian must be the Greenwich meridian—so we have a Standard Grid. Aircraft 1 is at longitude 090°W, which means the aircraft is west of the datum. From the diagram: Northern hemisphere, aircraft west of datum, therefore convergence east. And here's the rule: Convergence East, True Least. So the True heading should be 90 degrees less than the Grid—which it is. Both the convergence formula and the diagram give the same answer. Now, the exercise asks you to try the same with Aircraft Nos. 2 to 5. But for now, let's make sure you've got the core rules locked in: in the north, convergency equals longitude with sign reversed; in the south, it equals longitude directly. And remember that mnemonic: Convergence East, True Least. That's the heart of converting between Grid and True.