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

We're now looking at the idea of "constant scale" on the Lambert chart, and I want to start by being very precise about what that phrase actually means, because it's a term we throw around a lot in navigation. You remember that the Lambert chart is not perfectly constant scale. The scale expands outside the standard parallels and contracts inside them. That's the fundamental behaviour of the projection. But the whole point of Lambert's modification was to reduce the rate of that scale change — to make the distortion grow slowly enough that we can ignore it for practical flying. So here's the professional definition we use. For practical purposes, we regard a chart as being "constant scale" if the amount of scale distortion does not exceed 1%. That's the threshold. What does that mean in the real world? It means you can measure a distance with a ruler on the chart, and the answer you get will be within 1% of the true distance — no more than 1% in error. For most aviation applications, that level of accuracy is perfectly good enough. You can plan a leg, measure it, and trust the result. Now, here's the important part: not every published Lambert chart actually meets that 1% specification. Some do, some don't. Let me give you two concrete examples from the real world. The UK CAA 1:500,000 topographical maps — those are Lambert charts — you can measure distances on them with a ruler very accurately. The maximum error there is about 0.1%. That's well inside our 1% constant-scale criterion. But contrast that with the Jeppesen ED-6. On that chart, there is about 3% scale error. Three percent is outside our 1% threshold, so by our professional definition, the ED-6 is not a constant-scale chart. You can still use it, but you have to know that ruler measurements carry that extra error. So what drives this difference? The amount of scale error depends on the separation of the standard parallels. That's the key relationship. The wider the gap between the standard parallels, the more scale distortion you get. You are not required to know the formula or to calculate the amount of scale error — I want to be clear about that — but I do want you to have a feel for what is and isn't a constant-scale chart. Let me walk you through the table, because it gives you that feel. The table lists the change of latitude between the standard parallels, and the corresponding percentage scale error. If the standard parallels are separated by 5⅓ degrees of latitude, the scale error is 0.1%. That's tiny. If the separation is 16 degrees, the error is 1% — that's exactly our constant-scale threshold. At 23 degrees separation, the error is 2%. At 28 degrees, it's 3%. And at 32 degrees, it's 4%. So you can see the pattern: as the standard parallels move further apart, the scale error climbs. And you can see why the UK CAA maps are so good. Their standard parallels are at 49°40'N and 54°20'N — that's a separation of only 5 degrees. Five degrees of separation gives you that 0.1% error I mentioned. Now look at the Jeppesen series that includes the ED-6. Its standard parallels are at 37°N and 65°N — a separation of 28 degrees. And 28 degrees of separation, from our table, gives you 3% scale error. That explains exactly why the ED-6 carries that 3% error. So the takeaway is this: "constant scale" is a practical judgement, not an absolute. It means the distortion stays within 1%, and whether a given chart qualifies depends entirely on how far apart its standard parallels are. The tighter the separation, the more you can trust your ruler.