Gridded Charts — Page 464, Lesson 462

Let’s pick this up right where the grid technique leaves off — because now we have to actually steer the aircraft, and that brings in the compass. When we use grid technique, it’s usually because meridian convergence is a problem at high latitudes. But here’s the catch: if you’re that far north, you may also be fairly close to one of the Magnetic Poles. And that creates a new problem. Near a Magnetic Pole, variation can change too rapidly, and the magnetic field strength can be too weak — so a magnetic compass may not be practical at all. But — and this is important — that’s not always the case. Flying in parts of Northern Canada would probably give you a problem, because you’re close to the Magnetic North Pole. 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 steer by magnetic compass even while using grid technique. Now, the problem of steering by compass on a grid is not too complicated, and it’s very comparable to the problem of steering a True heading with a magnetic compass. Let me walk you through that comparison, because it’s the heart of this. When you steer True, you apply Deviation to convert 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 Compass heading to Magnetic heading. Then you apply the difference between Magnetic direction and Grid direction. And we give that difference a special name: Grivation. Grivation is the algebraic sum of Variation and Convergence. So if you need a Magnetic direction from a Grid direction, you must apply both the Convergence and the Variation — and Grivation is exactly that combined correction. Let me show you the worked example from the text, because it makes this concrete. We start with a Grid direction of 090°. We apply Convergence of 10°E, which brings us to True — so True is 080°. Then we apply Variation of 10°E, which brings us to Magnetic — so Magnetic is 070°. In this example, Grivation is 20°E. And that’s the key point: Grivation can be applied directly to a Grid direction to give Magnetic direction, and vice-versa. So instead of doing two steps, you can do one: Grid 090° plus Grivation 20°E gives you Magnetic 070°. Now let’s look at the two worked examples of Grivation itself, because the algebraic sum can be tricky with signs. First example: Convergence is 17°W, and Variation is 4°E. Grivation is 13°W. Why? Because 17°W and 4°E partially cancel — 17 west minus 4 east leaves 13 west. Second example: Convergence is 11°E, and Variation is 4°E. Grivation is 15°E. Here both are east, so they add — 11 plus 4 is 15 east. So the rule is: Grivation is the algebraic sum of Variation and Convergence. You add them with their signs — east and west — and the result, with its sign, is Grivation. Then you apply that single Grivation directly between Grid and Magnetic. That’s the whole steering-by-compass story on a gridded chart: one combined correction, Grivation, instead of handling Convergence and Variation separately.