General Chart Properties — Page 288, Lesson 252

I want to walk you through the very start of General Chart Properties, because everything we do in navigation — plotting, measuring, reading a chart — rests on this idea of projection. Here's the core problem. The Earth is a globe, a sphere. A chart is a flat piece of paper. You cannot lay a sphere flat without tearing or stretching it. So how do we get from the globe to the flat chart? The answer is projection. The word "projection" comes from the original technique. Imagine a light source inside the globe, shining out through the latitude and longitude graticule — that's the grid of parallels and meridians — and casting that grid's shadow onto a flat sheet of paper. That's literally projecting the graticule. Today, we mostly do this with computer models, not light. But here's the key point I want you to hold onto: understanding the old light-based technique is more than adequate for understanding the properties of charts, no matter how the chart is actually made today. Now, there are two big categories of charts. Charts produced directly from a projection — that is, by the physical, geometric method — are called perspective, or geometric, projections. Charts produced by mathematical methods are called non-perspective charts. And here's the practical reality: most of the charts we use are non-perspective. But you can think of them as perspective projections that have been modified mathematically. So the geometric idea is still the foundation; the math just refines it. Next, a term you'll see constantly: the "Reduced Earth," abbreviated RE. This means the scale model of the Earth on which the projection of the chart is based. Think about it this way — to make a 1:1,000,000 chart, you first build a one-millionth scale model of the Earth, and then you project that model onto the paper. So the Reduced Earth is that intermediate scale model. The chart is a projection of the Reduced Earth, not of the real Earth directly. Now, the types of projection. There are three general types of projection surfaces. The first is azimuthal, or plane. The second is cylindrical. The third is conical. Let me take the azimuthal one now, since it's first. An azimuthal, or plane, projection is produced by placing a flat sheet of paper against a point on the Earth. So instead of wrapping paper around the globe, you just touch a flat sheet to a single point. Its common use is to provide charts of the North and South polar regions. And I want to flag this — in the EASA syllabus, this is the only example of an azimuthal projection that's taught and examined. The resulting charts are called Polar Stereographic charts, and we'll cover them in detail in a later chapter. For now, just know the name and the principle. Let me show you the principle with a figure. That's Figure 17.3, the simple cylindrical projection and graticule — it illustrates the general idea of projecting the graticule onto a surface. And Figure 17.1, which I don't have rendered here, illustrates the azimuthal principle itself — the flat sheet touching the point on the Earth. So to summarize where we are: projection is how we transfer the globe's graticule to flat paper. Perspective projections are geometric; non-perspective are mathematical, and that's what we mostly use. The Reduced Earth is the scale model the projection is based on. And there are three projection surfaces — azimuthal, cylindrical, and conical — with azimuthal being the flat sheet touching a point, used for polar charts. That's our foundation.