So, to give an example — Page 464, Lesson 461

Let’s pick this up right where the conversion chain left off. We had the worked example on the page: 090°(G), then 10°W grid convergence gives 100°(T), then 8°W variation gives 108°(M), then 2°E deviation gives 106°(C). That’s the full path from Grid to Compass. Now I want to walk you through the one thing that example glossed over — how you actually decide whether that convergence correction is East or West. Because the sign of that 10° is not arbitrary; it’s determined by two things: which hemisphere you’re in, and whether you’re East or West of the datum meridian. Let me set up the picture. Imagine the Earth with a grid laid over it. The vertical centre line in the diagram is the datum meridian — in the figure it happens to be the Greenwich meridian, but it does not have to be. It can be any meridian you choose as your reference. The red lines are other meridians on the Earth, and the green lines are Grid North. So you’ve got the real converging meridians in red, and the parallel grid lines in green. Now look at the top left quarter of the diagram. That’s the Northern hemisphere, and the aircraft is out to the left — meaning it’s on a meridian somewhere to the West of the datum. In that quarter, except at the Equator itself, True North leans to the right — that is, to the East — of Grid North. And here’s the key definition: because True North is East of Grid North, convergence is, by definition, East. That’s the logical rule — convergence takes the sign of the direction True North sits relative to Grid North. So the method is: work through the other three quadrants the same way — Northern hemisphere East of the datum, Southern hemisphere West, Southern hemisphere East — and you finish with the pattern shown in Figure 27.12, which gives you the direction of convergence for every combination. The takeaway is that you never guess the sign; you reason it from hemisphere and position relative to the datum. Now, once we can convert from True to Grid on the chart, the next problem is practical: how do we actually steer a Grid direction in the aircraft? There are two methods — you can use a compass, or you can use a gyro. That’s the choice the syllabus gives you, and it’s the bridge between the chart work and the flying. So the full picture is: the grid gives you a constant reference on the chart, convergence tells you how to relate it to True, and then compass or gyro is how you hold that grid heading in the air.