General Chart Properties — Page 288, Lesson 255

Let me walk you through the properties of an ideal chart. This is the foundation of everything we do in navigation, so I want you to think of it as the checklist we measure every real chart against. We've already talked about how we project the spherical Earth onto a flat sheet of paper. Now we ask: what do we actually want from the result? Here's the list of ideal properties, split into two groups. First, representation of the Earth's surface. We'd like angles on the Earth's surface to be represented by the same angles on the chart. We'd like scale to be constant and "correct." We'd like areas to be shown with their true shape. And we'd like equal areas on the Earth to appear as equal areas on the chart. Second, navigation requirements. We'd like rhumb lines to be straight lines. We'd like great circles to be straight lines. We'd like latitudes and longitudes to be easy to plot. We'd like adjacent sheets to fit correctly. And we'd like worldwide coverage. Now here's the crucial reality check. Two of these properties can never be obtained, except on the globe itself. Scale can never be constant and correct. We can modify charts mathematically to give nearly constant scale in small areas, but not over large areas. And the shapes of large areas cannot be represented perfectly — though we can represent the shapes of small areas reasonably accurately. All the other ideal properties can be obtained on charts, but unfortunately not together on the same chart. You have to give something up. However, not all of them are essential for navigation. For instance, it really does not matter to a pilot whether areas are correctly represented or not. You don't make comparisons of area when flying. Even a reasonable amount of distortion of shape is acceptable, provided it's not too great and landmarks can still be recognized. Now, the key point. Of all the ideal properties, the only essential one is that navigation bearings must be "correct." The critical property is that angles on the Earth must be represented correctly on the chart. This is critical to aviation — indeed to navigation generally. If you draw a line joining two points on the chart and measure the angle, but then find that this does not correspond to the true direction on the Earth, the chart is useless for navigation. You might previously have thought that if you measure a track off any map, it will correspond to Earth direction. But this is not true for most charts. Those charts that do have this property are in the minority, and they are known as orthomorphic or conformal charts. Those two words mean the same thing: angle-preserving. Here's a practical note that ties this directly to your flying. Your ICAO 1:500 000 Topographical chart is called a "Lambert's Conformal Conic" — you'll see that name just above the graduated scale. So the chart you actually use in the cockpit is one of that minority of conformal charts, precisely because it preserves angles. Let me make sure the terminology is locked in. Orthomorphic and conformal are interchangeable — both mean the chart preserves angles correctly. That's the one property we cannot compromise on, because bearings are the language of navigation. Everything else — equal areas, true shapes of large regions, constant scale — those are nice to have, but they're not essential for flying.