Mercator Charts - Properties — Page 304, Lesson 273

I want to walk you through the properties of a Mercator chart — this is a core topic in navigation, and you'll see it tested frequently. Let's start with the summary. The Mercator chart has several key properties, all of which are examinable, so you need to know them thoroughly. First, scale. On a Mercator chart, scale is correct only at the Equator. Everywhere else, the scale increases as the secant of the latitude. That means as you move away from the Equator toward the poles, the scale gets larger — things appear bigger than they really are. Within 1 degree up to 8 degrees from the Equator, the scale error is within 1%, so it's very accurate close to the Equator. Next, orthomorphic — yes, the Mercator chart is orthomorphic. That means it preserves angles and shapes locally. In fact, all charts used for navigation must be orthomorphic, so this is a requirement. Now the graticule — that's the network of meridians and parallels. On a Mercator chart, meridians are straight parallel lines, evenly spaced. Parallels of latitude are also straight parallel lines, but the space between them increases as the secant of the latitude. So the spacing between parallels grows as you go north or south from the Equator. Shapes — over small areas, shapes are reasonably correct. But over large areas, especially at high latitudes, there is significant distortion. That's why Greenland looks enormous on a Mercator map compared to Africa, even though Africa is much larger. Chart convergence — this is zero everywhere on a Mercator chart. Convergence is correct only at the Equator, and it's constant across the chart. That means meridians don't converge toward the poles; they stay parallel. Rhumb lines — these are straight lines always, everywhere on a Mercator chart. A rhumb line is a line of constant bearing, and on a Mercator projection it appears as a straight line. That's the key advantage of the Mercator chart for navigation. Great circles — the Equator and all meridians are straight lines on a Mercator chart, because they are also rhumb lines. But all other great circles appear as curves, and the curve has a track nearer the Pole — or, put another way, it is concave to the Equator. So if you draw a great circle route between two points that aren't on the Equator or a meridian, it will curve toward the nearer pole. Let me also mention a few notes from the answers section that clarify common exam points. A tricky question: great circles on a Mercator chart can be represented as straight lines, but only in the case of the Equator and all meridians. That's a common trap — don't think all great circles are straight lines on a Mercator chart. Another point: when you have two places on the same parallel of latitude, say 30° South, the rhumb line track between them must be either 090° True or 270° True — due east or due west. An examiner may not always give you that information explicitly; they may expect you to recognize it. So in summary: Mercator chart — scale increases with secant of latitude, orthomorphic, parallel meridians and parallels with increasing spacing, zero convergence, rhumb lines are straight, great circles are curves except for the Equator and meridians. That's the core set of properties you need to know.