General Chart Properties — Page 288, Lesson 253

I want to walk you through the three fundamental families of map projections used in aviation navigation: azimuthal, cylindrical, and conical. These are the building blocks for every chart you'll use as a professional pilot. Let's start with the azimuthal projection. In this type, the graticule — that's the network of latitude and longitude lines — is projected onto a flat plane that touches the Earth at a single point. Figure 17.1 shows the projection itself, and Figure 17.2 shows what the resulting graticule looks like. Now, here's an important operational point: although this particular azimuthal projection has produced a chart covering the entire Southern hemisphere, in practice you would only use the central area of that chart. For this example, that central area is used for flights over Antarctica. The distortion gets too severe toward the edges to be useful for navigation. Next, let's move to cylindrical projections. These were first produced in the 16th century by a Flemish navigator named Gerard de Kremer, who used the Latin alias 'Gerardus Mercator' — you'll know his name from Mercator charts. His method involved wrapping a cylinder of paper around what we call the "Reduced Earth" — that's a scale model of the Earth used for projection purposes. The cylinder touches the Reduced Earth at the Equator. The projection technique is illustrated in Figure 17.3. Here's how it works: the graticule is projected onto the cylinder. Then the cylinder is opened up to produce a flat sheet of paper. Once that's done, cartographers can add the geographic detail — coastlines, cities, airspace, and so on. You'll notice the shapes on this projection look a little strange. That's explained in detail in Chapter 18 on Mercator Charts, but for now just be aware that distortion is present. Also note that most of the world except the Poles has been projected. In practice, you would only use a section of the chart — normally the area near the Equator, where distortion is minimal. Now let's look at conical projections. These involve placing a cone of paper over the Reduced Earth and projecting the graticule onto the cone. The cone is then slit along one side and opened to produce a flat sheet of paper. The technique is illustrated in Figure 17.4. Let me walk through the steps more carefully. 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 — in this case along the Greenwich meridian. The cone is then 'developed', which is the technical term for opening it, to give the flat sheet of paper and graticule illustrated in Figure 17.5. In this particular example, the chart will show the whole Northern hemisphere. But again, as a user you will need only a small section of the chart — for example, the rectangle suggested in the figure. In that section, the graticule will look more familiar, similar to the ICAO 1:500,000 topographical chart you'll use for visual navigation. This familiar-looking graticule is illustrated in Figure 17.6. The conical graticule in Figure 17.6 is typical of the Lambert's Conical Chart, which is discussed in detail in Chapter 20. That's a very important chart type for aviation, so we'll come back to it later. To summarise: azimuthal projections use a flat plane touching at one point, cylindrical projections use a cylinder touching at the Equator, and conical projections use a cone placed over the Earth. In every case, you only use a limited section of the projected chart where distortion is acceptable for navigation.