Lambert’s Conformal Chart - 2 — Page 349, Lesson 311

We're now looking at the constant scale property of the Lambert chart. I want to start with a key point: the Lambert chart is not completely constant scale. As we saw, scale expands outside the standard parallels and contracts inside them. But the whole point of Lambert's modification was to reduce the rate of scale change — to make the scale change more gradual, so it stays close to true over a wider area. Now, for practical aviation purposes, we don't need perfection. We regard a chart as being 'constant scale' if the amount of scale distortion does not exceed 1%. That's the professional threshold. What does that mean in practice? It means you can measure distances with a ruler on the chart, and the result will be 1% or less in error. That's good enough for most applications in aviation — you're not going to be off by a meaningful amount over the distances you plan and fly. But here's the important caveat: some published Lambert charts meet this 1% specification, and others do not. Let me give you two concrete examples. On the UK CAA 1:500 000 topographical maps, you can measure distances with a ruler very accurately — there's about 0.1% maximum error. That's well within the 1% threshold. But on the Jeppesen ED-6, there's about 3% scale error. That's three times the acceptable threshold — so that chart is not constant scale by our definition. So what drives this difference? The amount of scale error depends on the separation of the standard parallels. You are not required to know the formula or the exact amount of scale error, but I want to give you a feel for what makes a chart constant scale or not. Let me walk you through the relationship. If the change of latitude between the standard parallels is 5⅓ degrees, the scale error is 0.1%. If the separation is 16 degrees, the error is 1% — that's right at our threshold. At 23 degrees, it's 2%. At 28 degrees, it's 3%. And at 32 degrees, it's 4%. So you can see the pattern: the wider the separation between the standard parallels, the greater the scale error. Now let's tie this back to our two examples. The UK CAA 1:500 000 topographical maps have their standard parallels at 49°40'N and 54°20'N — that's a separation of 5 degrees. That's why they have such a tiny error, about 0.1%. The Jeppesen series, which includes the ED-6, has standard parallels at 37°N and 65°N — a separation of 28 degrees. And that explains the 3% scale error we saw. So the takeaway is this: when you pick up a Lambert chart, you should know its standard parallels and understand what that separation means for your distance measurements. A narrow separation keeps you within that 1% constant-scale threshold; a wide separation pushes you well beyond it.