General Chart Properties — Page 288, Lesson 253

I want to walk you through the general properties of chart projections, and we're going to start with the three fundamental ways cartographers turn the curved Earth into a flat chart. Let me set the scene. We have this idea of the "Reduced Earth" — that's the model globe, the scaled-down sphere we use as the basis for all chart work. Every projection starts with that sphere, and the question is always the same: how do we get the graticule — that's the network of meridians and parallels — off the sphere and onto a flat sheet of paper? The first method is the azimuthal projection. Picture a flat sheet of paper touching the Reduced Earth at a single point. The graticule is projected straight onto that flat plane, and the result is what you see in Figure 17.1 and Figure 17.2. Now here's the key thing about azimuthal projections: because the paper touches at one point, the distortion grows as you move away from that point of contact. That's why, even though this particular projection has produced a chart of the entire Southern hemisphere, the chart would only be used in the central area — in this case, for flights over Antarctica. The usable part is right around the point of tangency, where shapes and distances are most faithful. Now let's move to the second method, the cylindrical projection. This is the famous one, and it has a wonderful history. The earliest chart projections were produced in the 16th century by a Flemish navigator called Gerard de Kremer, who used the Latin alias "Gerardus Mercator." His projections used cylinders of paper wrapped around the Reduced Earth, and the cylinder touched the Reduced Earth at the Equator. So the paper cylinder is wrapped around the globe, touching all the way around the Equator, and the graticule is projected outward onto that cylinder. That technique is illustrated in Figure 17.3. Once the graticule has been projected onto the cylinder, the cylinder is opened up to produce a flat sheet of paper. Then the cartographers can add the geographic detail — the coastlines, the towns, the airfields. Now, I want you to notice something in that figure: the shapes look a little strange. The graticule looks distorted, and that's not an accident. That's explained fully in Chapter 18, which deals with Mercator charts specifically. For now, just register that the shapes are odd because of how the projection works. And here's another important point about the cylindrical projection: most of the world, except the Poles, has been projected onto that cylinder. But the user would only use a section of the chart — in this case, normally near the Equator. Why? Because that's where the cylinder touches the Earth, so that's where the distortion is least. The Poles are the problem area for cylindrical projections, and that's why they're excluded. Now the third method, and this is the one that's going to matter most for your navigation work: the conical projection. Here we place a cone of paper over the Reduced Earth and project the graticule onto the cone. The technique is illustrated in Figure 17.4. Let me walk you through the sequence carefully, because it's a multi-step process. In Figure 17.4(a), the cone is placed over the Earth. The graticule is projected onto the cone. Then, in Figure 17.4(b), the cone is slit along one side — in this case, along the Greenwich meridian. Once it's slit, the cone can be "developed" — that's the technical term for opening it out — to give the flat sheet of paper and graticule illustrated in Figure 17.5. Now, in this particular case, the chart will show the whole Northern hemisphere. But just like with the other projections, the user will need only a small section of the chart — for example, the rectangle suggested in Figure 17.5. And here's the payoff: in that section of the chart, the graticule will look more familiar. It will look similar to the ICAO 1:500,000 topographical chart, and that's illustrated in Figure 17.6. That typical conical graticule in Figure 17.6 is characteristic of the Lambert's Conical Chart, which we'll discuss in detail in Chapter 20. So keep that name in mind — Lambert's Conical — because it's going to be a recurring theme in your navigation studies. Let me pull the thread together. All three projections — azimuthal, cylindrical, conical — start with the Reduced Earth and a piece of paper in some geometric shape: a flat plane, a cylinder, or a cone. The graticule is projected onto that shape, and then the shape is opened flat. In every case, the usable portion of the chart is only the section near where the paper touched the Earth, because that's where distortion is minimized. For the azimuthal, that's the central area around the point of contact. For the cylindrical, that's near the Equator. For the conical, that's the band where the cone sits over the sphere — and that's the one that gives us the familiar graticule we see on the ICAO 1:500,000 chart. One more thing to note: the conical projection is the one you'll see most in real navigation charts, so when we get to Chapter 20 and Lambert's Conical Chart, you'll already understand the geometry behind it.