Mercator Charts - Properties — Page 297, Lesson 266

Let’s pick this up right where the properties of the Mercator chart leave off. I want to walk you through the key numbers and the orthomorphic nature of this projection, because these are the details that will show up in your exam questions. First, that 8° figure. In examination questions, 8° is often approximated to 500 nautical miles. If you see the number 500 in a question, it’s usually a clue that the examiner is using it to mean 8°, and is referring to the 1% scale error band. So remember that connection: 500 NM is shorthand for 8° of latitude, and that ties directly to the scale error limit. Let me summarize the scale properties for you, because this is the core of the Mercator. The Mercator scale is correct — meaning it matches the Reduced Earth — at the Equator. Then, as you move away from the Equator, the scale expands as the secant of the latitude. And the practical limit: the Mercator scale is within 1% up to 8° from the Equator. So within that 8° band, the scale error stays under 1%; beyond that, the expansion grows. Now, orthomorphism. This is a critical term. All charts used for navigation must be orthomorphic. The Mercator chart is orthomorphic, or conformal, by mathematical construction — that’s Mercator’s adjustment of the parallels of latitude. And importantly, the projection is non-perspective. That means it’s not built by projecting from a single point; it’s a mathematical construction instead. Next, the graticule — that’s the network of meridians and parallels. On the Mercator, the graticule is rectangular. The meridians are equally spaced parallel lines. The parallels of latitude are also parallel lines, but the space between them increases as the secant of the latitude. So the meridians stay evenly spaced, but the parallels spread further apart as you go poleward. Now, here’s the trade-off. Mercator produced an orthomorphic projection, but the areas are not correctly represented. Correct representation of area is not required on an orthomorphic chart. This is the classic distortion. Greenland appears as large as Africa on the chart, even though the land area of Africa is approximately 18 times that of Greenland. In reality, Greenland’s land area is only the same as the small NE corner of Africa. Similarly, the land area of Scandinavia illustrated is only one third of the land area of India, but on the chart they appear to have similar areas. And here’s a concrete scale example: the chart length equivalent to 3000 NM at 60°N is twice the chart length for the same distance at the Equator. That doubling is exactly the secant of 60°, which is 2. So the scale expansion is not arbitrary — it follows that secant relationship precisely. Finally, this distortion of area and change of scale also leads to a change of shape. Land masses at high latitudes appear too wide for their height compared with the same land masses on a globe or on a chart with convergent meridians. Compare the shape of the North American continent on a Mercator with its shape on a globe. The continent appears too wide. The east–west distance from Labrador to Western Alaska seems too great compared with the distance from Northern Alaska to the Gulf of Mexico. So the shape is stretched horizontally at high latitudes, even though the chart preserves angles — that’s the orthomorphic compromise. So the takeaway: the Mercator gives you correct angles and a correct scale at the Equator, but it sacrifices area and shape fidelity as latitude increases, with the scale expanding as the secant of latitude and staying within 1% only up to 8° from the Equator.