Gridded Charts — Page 464, Lesson 464

Let me walk you through this section on gridded charts, because it's really about how a pilot actually flies a Grid heading in practice — and it builds directly on something you already understand: how you fly a True heading using a magnetic compass. Let me start with the familiar case, because that's the foundation. Look at Figure 27.14. We have a rhumb line track — that's a straight line on a chart that crosses all meridians at the same angle. After correcting for drift, we need to maintain a True heading of, say, 080°(T). Now, the variation — the angular difference between True North and Magnetic North — is not constant along this track. To the west of the 3.5°W isogonal, the variation is, to the nearest degree, 3°W. An isogonal is a line on a chart joining points of equal magnetic variation. To the east of that 3.5°W isogonal, the variation is 4°W. Similarly, at the 4.5°W isogonal, tracking eastwards, the variation changes from 4°W to 5°W. So here's the flying procedure. The pilot starts by flying a Magnetic heading of 083°(M) in order to maintain a True heading of 080°(T). Why 083? Because with 3°W variation, your Magnetic heading is your True heading plus the westerly variation — 080 plus 3 gives you 083. As he crosses the 3.5°W isogonal, variation steps up to 4°W, so he alters Magnetic heading to 084°(M). Then as he crosses the 4.5°W isogonal, variation becomes 5°W, and he alters Magnetic heading to 085°(M) — all the while maintaining that same True heading of 080°(T). So the rule is simple: every time you cross an isogonal, you update your Magnetic heading by the change in variation. Now here's the key transfer of ideas. If a chart is constructed with isogrivs, then the same principles can be used to maintain a Grid heading when using a magnetic compass. An isogriv is the grid equivalent of an isogonal — it's a line joining points of equal grid variation, which is the angular difference between Grid North and Magnetic North. So instead of updating your heading for changes in magnetic variation relative to True North, you update it for changes in grid variation relative to Grid North. Let me show you what a gridded chart actually looks like. Figure 27.15 shows a printed grid chart. The blue graticule is latitude and longitude — that's your familiar lat/long graticule. The grey graticule is the grid — the grid lines running parallel to the datum meridian. The blue dotted lines are isogonals. The grey ones are isogrivs. And here's the reassuring part. You can see from the chart that the rate at which the isogrivs are changing is much the same as the rate at which the isogonals are changing. So it is no more difficult, in many parts of the world, to fly a Grid heading using a magnetic compass than it is to fly a True heading when using a magnetic compass. The same crossing-and-updating technique works, just with isogrivs instead of isogonals. Now, why would we bother with all this? That brings us to steering by gyro. One of the reasons we may be using Grid is because the aircraft is at high latitudes. In this case, a magnetic compass may be unusable in some parts of the world because the route — well, that's where the excerpt cuts off, but the point is that near the poles the magnetic compass becomes unreliable, and the grid system gives you a stable reference to steer by. So let me tie it together. You've got three references in play: True North, Magnetic North, and Grid North. Isogonals connect points of equal variation between True and Magnetic. Isogrivs connect points of equal variation between Grid and Magnetic. When you fly a True heading by compass, you update your Magnetic heading as you cross each isogonal. When you fly a Grid heading by compass, you do exactly the same thing, but you update as you cross each isogriv. And the beauty of it is that the isogrivs change at much the same rate as the isogonals, so the workload is no higher.