Mercator Charts - Properties — Page 304, Lesson 273

We're starting a fresh topic now: the properties of a Mercator chart. This is a summary page, and I want you to treat it as a checklist to memorise, because every single line here is examinable. Let's begin with scale. On a Mercator chart, the scale is correct on the Equator. Everywhere else, it increases as the secant of the latitude. Now, secant is a trigonometric function — it's 1 divided by the cosine of the angle. At the Equator, latitude is zero, cosine of zero is 1, so the secant is 1 — that's why the scale is correct there. As you move away from the Equator, the cosine gets smaller, so the secant gets bigger, and the scale increases. The practical limit here: the scale is within 1% of correct only up to 8° from the Equator. Beyond that, the distortion grows. Next property: orthomorphic. Yes, a Mercator chart is orthomorphic, meaning it preserves angles and shapes locally. And here's a strong statement — all charts used for navigation must be orthomorphic. That's a rule you should remember. Now the graticule — that's the network of meridians and parallels. On a Mercator chart, meridians are straight, parallel lines, evenly spaced. The parallels are also straight, parallel lines, but the space between them increases with the secant of the latitude. So as you go poleward, the parallels spread further apart, matching the scale increase. Shapes: they're reasonably correct over small areas. But there's distortion over large areas, especially at high latitudes. That's the trade-off for being orthomorphic. Chart convergence — this is the angle between meridians on the chart. On a Mercator chart, convergence is zero everywhere. It's correct at the Equator, and it's constant across the chart. So meridians never converge; they stay parallel. Now the two big ones: rhumb lines and great circles. A rhumb line — a line of constant true bearing — is a straight line on a Mercator chart. Always, everywhere. That's the whole point of the projection. Great circles, though, are different. The Equator and all meridians are straight lines on a Mercator chart, because they happen to also be rhumb lines. But every other great circle is a curve, and its track appears nearer the Pole — or, put another way, it's concave to the Equator. So if you draw a great circle between two points, it bows toward the pole on the chart. Now, I want to flag something from the answers section, because it's a classic exam trap. A great circle on a Mercator chart can be represented as a straight line, but only in the case of the Equator and all meridians. So if a question asks whether great circles are straight lines on a Mercator chart, the answer is 'no' — except for those two special cases. That's the tricky bit. There's also a note about a convergency and conversion angle problem that often appears as a Mercator problem. The Mercator graticule is a good way to visualise it. And one more point: if you have two places on the same parallel of latitude, the rhumb line track between them must be East or West — 090°(T) or 270°(T). An examiner may not give you that track; you're expected to recognise it yourself. So the core takeaway: Mercator gives you straight rhumb lines, zero convergence, and scale that grows with the secant of latitude. Memorise that table, and you'll be ready for the questions.