Imagine you're flying east or west — Page 471, Lesson 466

I want to walk you through how a gyro is used for direction-finding in polar regions, and why we switch from True North to a Grid. Let's start with the core problem. Imagine you're flying east or west. The gyro stays aligned to its original direction in space — that's its whole nature, it's a rigid wheel that holds its axis fixed. But here's the catch: while the gyro holds that direction, the meridians on the Earth are converging. The local direction of True North is changing underneath you as you move. So if you try to steer by True North, you'd have to keep correcting for that meridian convergence. The correction factor is change of longitude × sine mean latitude. That's the formula you'd use to stay in True. Now, to operate in Grid, we don't need that correction at all. Instead, we need a direction that does not change with change of longitude — that's what a grid is. So we use the gyro, but without any correction for Transport Wander, which is the same thing as meridian convergence in this context. There is a small catch, though. There's a subtle error caused by the difference between Earth Convergence, which is the gyro correction, and Chart Convergence, which depends on the projection and the Standard Parallels being used. A correction has to be applied for this, and it's called Residual Transport Wander. It's fed in either by slewing the gyro or as a fiddle-factor to the Latitude Control. Its use is beyond the scope of this course, so we won't go further into it. So, to operate with a gyro in Grid, here's what you need: align it to Grid North initially, then feed in a correction for Earth Rate. Real Drift will be small with a good gyro, and Transport Wander will be non-existent — or there may be a small amount of Residual Transport Wander to be corrected. Now let's talk about Polar Grids. A gridded chart is invariably used in polar regions. In both the north and south polar areas, the plotting charts are a Polar Stereographic or a Transverse/Oblique Mercator — though the Mercator types are not covered in the syllabus. On each of these charts, the 360° of longitude on the Earth are represented by 360° on the chart. Let me show you what this looks like. That's the standard North Polar Grid. You can see how the grid lines overlay the converging meridians. And here's the key point: because the chart represents all 360° of longitude as 360° on the chart, the grid direction stays constant regardless of where you are. That's why we use it — it gives us a stable reference that doesn't shift as we cross longitudes. So to summarize the whole picture: in True, you fight the meridian convergence with a correction. In Grid, you sidestep it entirely by using a direction that doesn't change with longitude. The gyro gives you that stable direction, you align it to Grid North, correct for Earth Rate, and you're set — with only a small Residual Transport Wander to worry about, which is beyond this course.