Gridded Charts — Page 464, Lesson 459

Let’s start with the big picture. Up to now, you’ve been navigating with True North as your datum — your compass and your chart both referenced to the geographic North Pole. That works fine at low and mid latitudes because the change of track caused by convergency is small enough to fold into your normal navigation corrections. But at high latitudes, that convergency becomes too great to ignore. So we abandon True North as our direction datum and use a grid instead, with its own Grid North. And we align our compass to Grid North, not True North. Now, how do we choose the datum direction for Grid North? There are various methods, but for the EASA ATPL syllabus, the datum is always aligned with a nominated meridian of longitude — for instance, the Greenwich meridian in the examples. If Greenwich 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 060°W. Here’s the key relationship: at the datum meridian itself, there is no difference between Grid North and True North. But as you move 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 get from the datum, the bigger the angular difference. Let me show you the creation of the grid. Now let’s define our terms precisely. To obtain a constant straight-line track direction, we select a datum meridian and draw lines parallel to it at suitable intervals on the chart — that forms the grid. The direction of the datum meridian is known as Grid North. The direction of the track measured from Grid North is constant and is known as 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 known as convergence. And here’s the important part: convergence is equal to the chart convergence between the meridian at that point and the datum meridian. Why? Because Grid North is parallel to the datum meridian, while True North has changed by the amount of the chart convergence between the datum meridian and the local meridian at the point being considered. So the convergence at any point is simply the chart convergence between that point’s meridian and the datum. Let me show you that relationship. Now, convergence is annotated Easterly when True North lies to the East of Grid North, and Westerly when True North lies to the West of Grid North. This gives us the conversion rule: Convergence EAST, True LEAST. Convergence WEST, True BEST. Let me unpack that. If convergence is east, then to convert from Grid direction to True direction, you subtract — True is least. If convergence is west, you add — True is best. Let me show you both cases. Look at the examples. We have 105°(G) with 20°W convergence giving 125°(T) — that’s convergence west, so True is best, we add 20°. And 105°(G) with 20°E convergence giving 085°(T) — convergence east, True is least, we subtract 20°. Now, this conversion follows the same pattern as your previous conversions: True, Variation, Magnetic, Deviation, Compass. You remember the rule: West Best, East Least. The only difference here is that 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 is just another correction you apply before you get to True, and it follows the same West Best, East Least logic. So to summarise: at high latitudes, convergency becomes too great to ignore, so we switch from True North to Grid North as our datum. The grid is built on a nominated datum meridian, and convergence is the angular difference between True and Grid direction at any point. It’s east or west depending on which side True North lies, and you apply it with the rule: Convergence EAST, True LEAST; Convergence WEST, True BEST.