Mercator Charts - Properties — Page 304, Lesson 268

I want to walk you through the properties of Mercator charts. Let's start with the scale expansion you see in Figure 18.4 — those examples show how the scale changes as you move away from the Equator. Now, Figure 18.5 gives us a really important visual comparison. On the left, part (a) shows a convergent projection, which is close to the true shape on a globe — meridians converge toward the poles. On the right, part (b) shows the Mercator projection, and you can see the distortion of shape that happens because the meridians are drawn as parallel straight lines instead of converging. Here's the key point for you as a pilot: these distortions of shape are insignificant over small distances, so they have no effect on your ability to map-read. They also have no implications for rhumb line navigation at all. However, they can give a false impression of the most direct routing, especially at high latitudes. That's something we covered back in Chapter 2. Let's move on to chart convergence. You already learned about Earth Convergence, or Convergency, in Chapter 14. That's the angle of inclination of the meridians on the Earth — in other words, the change in direction of a great circle between two longitudes. But for each type of projection, we also have the concept of Chart Convergence. This is the angle of inclination between meridians on the chart — or the change in direction of a straight line between two longitudes. For a Mercator chart, because all meridians are parallel, their mutual inclination is zero. The change in direction of a straight line drawn on the map is also zero. That straight line will always cut all meridians at the same angle. And that's exactly why Mercator produced this projection in the first place — so that a straight line on the chart gives a single track angle. Now compare that to Earth convergency. Earth convergency is zero at the Equator, but nowhere else. So Mercator convergence is correct at the Equator only, but it's constant everywhere — always zero. Let's talk about rhumb lines. Because the meridians are parallel lines, a straight line track drawn on the chart will cut all meridians at the same angle. A straight line track on a Mercator chart is a rhumb line. Now for great circles. The rhumb line between two points will always be nearer to the Equator than the corresponding great circle. Conversely, the great circle between two points will always lie nearer the Pole than the rhumb line. This was covered in detail in Chapter 2, and Figure 18.6 gives you a visual simplification of that relationship.