Mercator Charts - Properties — Page 297, Lesson 260

I want to walk you through the Mercator chart — one of the most important projections in navigation history. Let's start with how the earliest flat charts were made. The first method for transferring the grid of meridians and parallels from a globe onto a flat sheet was called a cylindrical projection. Here's how it worked. First, you'd make a scale model of the Earth — that's called the Reduced Earth, or RE for short — at an appropriate scale. Then you'd wrap a cylinder of paper around that Reduced Earth, so that the cylinder touched the RE right at the Equator. With a light source placed at the centre of the RE, you'd project the graticule — that's the network of meridians and parallels — onto the cylinder. Finally, you'd 'develop' the cylinder, meaning you'd open it up into a flat sheet of paper. That technique produced what's called a perspective projection — a simple cylindrical projection. And you can see this illustrated in Figure 18.1, which shows the Simple Cylindrical Projection. Now, this projected graticule had one big advantage for early navigators: the meridians came out as equally spaced parallel lines. Because of that, a straight line drawn on the chart would have a constant direction — that straight line would be a rhumb line. With their basic compass systems, early navigators preferred to sail constant directions, and they had to accept that they would be sailing a rhumb line. Modern navigators, by contrast, use more advanced guidance systems and normally aim to fly the great circle track — we'll cover that later. But the simple cylindrical projection had a serious flaw. In the 16th century, a Flemish cartographer named Gerard de Kremer — who used the Latin alias 'Mercator' — recognised its limitations. The projected graticule did meet one requirement for what we call an orthomorphic or conformal chart: the meridians and parallels crossed at right angles. So a straight line was indeed a line of constant direction — a rhumb line. Unfortunately, it was not the correct direction. The shapes were clearly not correct, and therefore the angles on the chart were not correct either. So the simple cylindrical projection gave you a rhumb line, but it gave you the wrong one — the angles and shapes were distorted. That's the problem Mercator set out to solve, and that's what we'll look at next.