General Chart Properties — Page 288, Lesson 252

I want to walk you through the opening of General Chart Properties. This is where we start building the foundation for every navigation chart you'll use as a professional pilot. Let's begin with the core idea: how do we get information from a round globe onto a flat piece of paper? That process is called projection. The name comes from the original technique — you'd place a light source inside a transparent globe and let it cast shadows of the latitude and longitude lines, the graticule, onto a flat sheet of paper. Today we mostly use computer models, but understanding that original light-projection method is more than adequate for grasping the properties of any chart, regardless of how it's actually produced. Now, there are two broad families of charts based on how they're created. Charts produced directly from a physical projection — like that light-source method — are called perspective or geometric projections. Charts produced by mathematical methods are called non-perspective charts. Here's the key point: most of the charts we actually use fall into this second, non-perspective category. But you can still think of them as perspective projections that have been modified mathematically. So the distinction is important, but in practice, the non-perspective ones dominate. Next, I want to introduce a term you'll see constantly: the Reduced Earth, often abbreviated RE. This means the scale model of the Earth on which the projection of the chart is based. For example, to make a 1:1,000,000 chart, you would use a one-millionth scale model of the Earth. So the Reduced Earth is simply the scaled-down globe that your projection starts from. Now let's look at the three general types of projection surfaces. There are exactly three: Azimuthal/Plane, Cylindrical, and Conical. Let's start with the first type: Azimuthal/Plane projections. An azimuthal projection — also called a plane projection — is produced by placing a flat sheet of paper against a point on the Earth. The most common use you'll encounter, and the only example taught and examined in the EASA syllabus, is for charts of the North and South polar regions. The resulting charts are called Polar Stereographic charts, and we'll cover those in detail in a later chapter. The principle of how the projection works is illustrated in Figure 17.1. So to summarise what we've covered: projection is the transfer from globe to flat paper; we have perspective and non-perspective charts, with most being non-perspective; the Reduced Earth is the scale model the projection is based on; and there are three surface types — azimuthal/plane, cylindrical, and conical — starting with azimuthal, which gives us polar stereographic charts.