Gridded Charts — Page 464, Lesson 459

Let’s start with the problem that forces us to use a grid in the first place. Up to now, when we navigate, we correct for track changes as we go. But at high latitudes, that technique breaks down. The reason is convergency — the meridians converge toward the poles, so the amount of change becomes too great to ignore. So we abandon True North as our direction datum. Instead, we use a grid and its associated Grid North on our chart. And we also have to align our compass to Grid North, not True North. Now, how do we choose the direction for Grid North? There are various methods, but for the EASA ATPL syllabus, the datum direction is always aligned with a nominated meridian of longitude. For example, the Greenwich meridian is used in the figures. If the Greenwich meridian is the chosen datum, the grid is called a Standard Grid. Standard grids are widely used in printed grid maps for the European area. Another favourite datum often used in the USA and Canada is 060W — that is, the meridian at 60 degrees West. At the datum meridian itself, there is no difference between Grid North and True North. But as longitude changes further east or west from the datum, the angle between True North and Grid North increases. Why? Because Grid North remains parallel to the datum meridian, while True North changes with the increasing convergency of the meridians. So the further you move from the datum, the bigger that angle gets. Let me show you the creation of the grid. Now, convergence. To get a constant straight-line track direction, we select a datum meridian and draw lines parallel to it at suitable intervals on the chart, forming a grid. The direction of the datum meridian is called Grid North. The direction of the track measured from Grid North is constant, and it’s called Grid direction. This is what lets an aircraft fly along approximate great circle tracks. The difference between True direction and Grid direction at any point is called convergence. And here’s the key relationship: convergence is equal to the chart convergence between the meridian at that point and the datum meridian. Because Grid North is parallel to the datum meridian, while True North has changed by the amount of chart convergence between the datum meridian and the local meridian at the point you’re considering. Let me show you that in the figure. Now, how do we annotate convergence? Convergence is Easterly when True North lies to the East of Grid North, and Westerly when True North lies to the West of Grid North. And that gives us the conversion rule. To convert Grid direction to True direction, we apply convergence in this sense: Convergence EAST, True LEAST. Convergence WEST, True BEST. Let me make that concrete with the figures. In Figure 27.8, we have convergence west. We have a grid direction of 105 degrees, and a convergence of 20 degrees West. Applying the rule — convergence West, True Best — we add, giving 125 degrees True. In Figure 27.9, convergence east. Again 105 degrees grid, but now convergence is 20 degrees East. Convergence East, True Least — we subtract, giving 085 degrees True. So the same grid direction converts to different true directions depending on whether convergence is east or west. Now, this conversion follows the same pattern as our previous conversions — True, Variation, Magnetic, Deviation, Compass — with West Best. But convergence goes on the left-hand side of the columns, if you’re going from True to Compass. So the full chain becomes: Grid, Convergence, True, Variation, Magnetic, Deviation, Compass. Convergence sits between Grid and True, and it follows the same West Best rule. That’s the complete picture — grid direction, convergence, and how it all ties into the conversions you already know.