General Chart Properties — Page 288, Lesson 255

I want to walk you through the general properties of charts — what we, as pilots, need from a map that represents the spherical Earth on a flat sheet of paper. Let's start with the list of properties we might want in what we'd call an 'ideal' or perfect chart. This list isn't exhaustive, but it covers the main things. First, for representing the Earth's surface: we'd want angles on the Earth's surface to be shown by the same angles on the chart. We'd want scale to be constant and 'correct' everywhere. Areas should keep their true shape. And equal areas on the Earth should appear as equal areas on the chart. Then, for navigation requirements: we'd want rhumb lines — that's a line of constant bearing — to appear as straight lines. We'd also want great circles — the shortest path between two points on a sphere — to be straight lines. We'd want latitudes and longitudes to be easy to plot. Adjacent sheets of the chart should fit together correctly. And coverage should be worldwide. Now, here's the reality: two of these properties can never be achieved, except on a globe itself. Scale can never be constant and correct. We can mathematically modify charts to give nearly constant scale over small areas, but not over large areas. Also, the shapes of large areas cannot be represented perfectly. However, we can represent the shapes of small areas reasonably accurately. All the other ideal properties can be obtained on charts — but unfortunately, not all together on the same chart. You have to choose which properties to prioritise. And here's the key point for us as pilots: not all of them are essential for navigation. For instance, it really doesn't matter to a pilot whether areas are correctly represented or not. You don't compare areas when flying. Even a reasonable amount of shape distortion is acceptable, provided it's not too great and you can still recognise landmarks. That brings us to the most critical property: orthomorphism, also called conformality. 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 absolutely vital for aviation — and for navigation generally. If you draw a line joining two points on the chart, measure the angle, and find that it doesn't correspond to the true direction on the Earth, the chart is useless for navigation. You might have thought that if you measure a track off any map, it will correspond to Earth direction — but that's not true for most charts. Charts that do have this property are in the minority, and they're known as orthomorphic or conformal charts. As a practical example, your ICAO 1:500,000 Topographical chart is called a 'Lambert's Conformal Conic' — you'll see that name just above the graduated scale. That's a chart designed to preserve angles correctly, which is why we use it for navigation.