Gridded Charts — Page 464, Lesson 462

Let’s pick this up right where the grid technique leaves off: steering by compass. When you’re using grid technique because of meridian convergence at high latitudes, the aircraft may also be fairly near one of the Magnetic Poles. And that creates a real problem for a magnetic compass. Near a Magnetic Pole, variation may be changing too rapidly, or the magnetic field strength may be too weak for the compass to be practical. But—and this is important—even at high latitudes that isn’t always the case. Flying in parts of Northern Canada would probably give you a problem, but flying in Northern Europe or Northern Russia probably would not, because those areas are a greater distance from the Magnetic North Pole. So under some circumstances, you can still steer by magnetic compass even when using grid technique. Now, the problem isn’t too complicated, and it’s very comparable to the problem of steering a True heading when using a magnetic compass. Let me walk you through that comparison, because it’s the key to the whole thing. When you steer True, you apply Deviation to convert the Compass heading to Magnetic heading. Then you apply the difference between Magnetic direction and True direction—that difference is Variation. So: Compass → apply Deviation → Magnetic → apply Variation → True. For Grid steering, it’s the same shape. You apply Deviation to convert the Compass heading to Magnetic heading. Then you apply the difference between Magnetic direction and Grid direction. And that difference—the difference between Magnetic direction and Grid direction—we call Grivation. Grivation is the algebraic sum of Variation and Convergence. Let me make sure that’s crystal clear. Grivation = Variation + Convergence, added algebraically. So if you need a Magnetic direction from a Grid direction, you apply both Convergence and Variation. And if you need a Grid direction from a Magnetic direction, you apply them the other way. Here’s the little diagram I want you to see. It shows the relationship: GRID, then CONV, then TRUE, then VAR, then MAG. So going from Grid to True you apply Convergence; going from True to Magnetic you apply Variation. And the example runs: Grid 090°, Convergence 10°E gives True 080°, then Variation 10°E gives Magnetic 070°. So in that example, Grivation is 20°E. And Grivation can be applied directly to Grid direction to give Magnetic direction, and vice-versa. So Grid 090° with Grivation 20°E gives Magnetic 070°—same answer, one step instead of two. Now, the examples at the bottom show you how to combine Convergence and Variation into Grivation, and you have to watch the signs carefully because it’s algebraic. First example: Convergence 17°W plus Variation 4°E equals Grivation 13°W. Second example: Convergence 11°E plus Variation 4°E equals Grivation 15°E. So the whole point is this: when you’re in grid technique, you don’t have to chain Convergence and Variation separately every time. You combine them into a single Grivation, apply it directly between Grid and Magnetic, and you’re done. That’s the practical payoff of the whole idea.