Let’s pick this up right where the gyro is doing its job — Page 471, Lesson 466

Let’s pick this up right where the gyro is doing its job. We’ve got a gyro spinning, and if we fly east or west, the gyro stays locked to its original direction in space. But here’s the catch: the Earth’s meridians—the lines of longitude—converge toward the poles. So as we move east or west, the local direction of True North changes under us, even though the gyro hasn’t moved. So if we want to fly in True, meaning we want our heading referenced to True North, we have to correct for that meridian convergence. The correction factor is change of longitude multiplied by sine of the mean latitude. Let me unpack that. Change of longitude is how many degrees of longitude we’ve flown across. Mean latitude is the average latitude between where we started and where we are now. Multiply those two, and you get the amount the meridians have converged—the amount True North has swung. That’s the correction we feed in. But now, what if we don’t want to bother with that? What if we want a direction that does not change with change of longitude? That’s exactly what a grid is. So we use the gyro, but we apply no correction for Transport Wander, which is just another name for meridian convergence. The gyro stays put in space, and we call that direction Grid North. Now, there’s a subtlety here. There is a small error caused by the difference between Earth Convergence and Chart Convergence. Earth Convergence is what the gyro correction is based on—it’s the true convergence of the Earth’s meridians. Chart Convergence depends on the projection being used and the Standard Parallels chosen for that chart. Because those two don’t match exactly, a small correction is needed. That correction is called Residual Transport Wander. It’s fed in either by slewing the gyro—physically rotating it a tiny amount—or as a fiddle-factor to the Latitude Control. But the book says its use is beyond the scope of this course, so we don’t go deeper into it. So, to operate with a gyro in Grid, here’s the full recipe. First, align the gyro to Grid North initially. Then feed in a correction for Earth Rate—that’s the apparent movement of the gyro caused by the Earth’s rotation itself. Real Drift, which is the gyro’s own mechanical wander, will be small with a good gyro. And Transport Wander will be non-existent, or there may be just that small amount of Residual Transport Wander to correct. That’s the whole picture. 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 either a Polar Stereographic projection or a Transverse or Oblique Mercator. The book notes those last two are not covered in the syllabus, so we focus on the Polar Stereographic. On each of these charts, the 360 degrees of longitude on the Earth are represented by 360 degrees on the chart. That’s a key property—the full circle of longitude maps to a full circle on the chart, which is what makes the grid work cleanly up there. So the big takeaway: in True, you correct for meridian convergence with change of longitude times sine mean latitude. In Grid, you skip that correction, align to Grid North, correct only for Earth Rate, and let the gyro hold its direction in space. That’s how we navigate in the polar regions.