Mercator Charts - Properties — Page 297, Lesson 260

I want to walk you through how navigators first got the Earth's grid onto a flat sheet of paper, and why that simple method wasn't good enough — because that's exactly the problem Mercator set out to solve. Let's start with the very first technique. The earliest way to transfer the graticule — that's the network of meridians, the lines of longitude, and parallels, the lines of latitude — from a globe onto a flat sheet was the cylindrical projection. Here's the setup. You take a scale model of the Earth, called the Reduced Earth, or RE for short, built at some appropriate scale. Then you wrap a cylinder of paper around that Reduced Earth, and crucially, the cylinder touches the RE at the Equator. Now place a light source at the centre of the RE, and project the graticule outward onto the cylinder. Finally, you 'develop' the cylinder — that just means you open it up and lay it flat as a sheet of paper. That whole process produces what we call a perspective projection, and it's illustrated in Figure 18.1. Now, why did early navigators care about this at all? Because this projected graticule had one very significant advantage. The meridians came out as equally spaced parallel lines. And that's a big deal. Think about what a straight line means on such a chart. If the meridians are all parallel and equally spaced, then a straight line drawn on the chart crosses every meridian at the same angle. That means the straight line has a constant direction — it's what we call a rhumb line. Early navigators, with their basic compass systems, preferred to sail constant directions. They would set a compass heading and hold it. So they were happy to accept that they'd be sailing a rhumb line, because that's what their instruments could manage. Modern navigators are different — we use far more advanced guidance systems, and we normally aim to fly the great circle track, which is the shortest path between two points on the globe. But we'll come back to great circles later. So the simple cylindrical projection gave them rhumb lines. But here's the catch, and this is where Mercator enters the story. In the 16th century, a Flemish cartographer named Gerard de Kremer — who used the Latin alias 'Mercator' — recognised the limitations of this simple cylindrical projection. Let me be precise about what was good and what was bad. The projected graticule did meet one of the requirements for an orthomorphic chart — and orthomorphic is another word for conformal. A conformal chart is one where angles on the chart are correct. The simple projection met one requirement for that: the meridians and parallels crossed at right angles. Because of that, a straight line on the chart was indeed a line of constant direction — a rhumb line. So far so good. But here's the problem. It was not the correct direction. The shapes on the chart were clearly not correct, and because the shapes were wrong, the angles on the chart were not correct either. So even though the lines crossed at right angles, the actual angles between features — the true bearings — were distorted. The simple cylindrical projection gave you a rhumb line, but it pointed you in the wrong direction. That's the key distinction I want you to hold onto. A chart can have meridians and parallels crossing at right angles, and that gives you constant-direction lines. But that alone doesn't make the chart conformal. For true conformality, the shapes must be correct, and therefore the angles must be correct. The simple cylindrical projection failed on that second requirement. And that's precisely the limitation Mercator recognised — and what he fixed, which we'll look at next.