Mercator Charts - Properties — Page 297, Lesson 266

I want to walk you through the properties of the Mercator chart, which is one of the most important projections you'll use in navigation. Let's start with a practical tip that often appears in exam questions: 8 degrees of latitude is frequently approximated to 500 nautical miles. If you see that number in a question, it's a clue that the examiner is using it to represent 8 degrees, and they're referring to the 1% scale error band — the zone where the scale error is still within one percent of true. Let me summarise the key scale properties of the Mercator projection: - Scale is correct — meaning it matches the Reduced Earth — at the Equator. - Scale expands as the secant of the latitude. As you move away from the Equator, the scale increases proportionally to the secant of the latitude angle. - Scale is within 1% of true up to 8 degrees from the Equator. That's the 1% error band I just mentioned. Now, an essential concept: orthomorphism. Every chart used for navigation must be orthomorphic. This chart is orthomorphic, also called conformal, by mathematical construction — specifically through Mercator's adjustment of the parallels of latitude. Importantly, this projection is non-perspective; it's not created by projecting from a single point onto a flat surface, but by a mathematical transformation. Let's look at the graticule — the grid of meridians and parallels. On a Mercator chart, the graticule is rectangular. Meridians are equally spaced parallel lines. Parallels of latitude are also parallel lines, but the space between them increases as the secant of the latitude. So as you go north or south from the Equator, the gaps between latitude lines get wider. Now, shapes and areas. Mercator produced an orthomorphic projection, which means shapes are preserved locally. However, areas are not correctly represented. Correct representation of area is not required on an orthomorphic chart — it's a trade-off for preserving angles and shapes. Here's a dramatic example: 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 about the size of the small north-eastern corner of Africa. Similarly, the land area of Scandinavia illustrated on the chart is only one third the land area of India, but on the chart they appear to have similar areas. Also note this: the chart length equivalent to 3000 nautical miles at 60 degrees north is twice the chart length for the same distance at the Equator. That's the scale expansion in action. 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. For example, compare the shape of the North American continent on a Mercator chart 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 north-south distance from Northern Alaska to the Gulf of Mexico.