Mercator Projection Map (Edited from Wikipedia)

SUMMARY

The Mercator projection is a cylindrical map projection presented by the Flemish geographer and cartographer Gerardus Mercator in 1569. It became the standard map projection for nautical purposes because of its ability to represent lines of constant course, known as rhumb lines or loxodromes, as straight segments that conserve the angles with the meridians [lines of longitude that are drawn from the north pole to the south pole].

[What this means in practical terms is this: the map is drawn in such a way that if you plotted your current position on it, and on the map you needed to turn your ship 35 degrees to change your direction, then in real life you could simply change your ship’s direction by 35 degrees. Angles on the map correspond to angles on the compass.

On different map projections, you might plot a course with an angle of 35 degrees on the map, but in real life you might have to turn 37 degrees. Or, you might have to first turn 42 degrees and travel for 35 miles, then change again to 36 degrees for 48 miles.]

Although the linear [straight] scale is equal in all directions around any point, thus preserving the angles and the shapes of small objects (which makes the projection conformal), the Mercator projection distorts the size of objects as the latitude increases from the Equator to the poles, where the scale becomes infinite. So, for example, landmasses such as Greenland and Antarctica appear much larger than they actually are relative to land masses near the equator, such as Central Africa.

HISTORY

Mercator's 1569 edition was large, printed in eighteen separate sheets. The parallels and meridians are straight and perpendicular [at right angles] to each other. In accomplishing this, the unavoidable east-west stretching of the map, which increases as distance away from the equator increases, is accompanied in the Mercator projection by a corresponding north-south stretching, so that at every point location the east-west

1 scale is the same as the north-south scale, making the projection conformal. Being a conformal projection, angles are preserved around all locations.

Because the linear scale of a Mercator map increases with latitude, it distorts the size of geographical objects far from the equator [like Greenland and Antartica] and conveys a distorted perception of the overall geometry of the planet. At latitudes greater than 70° north or south the Mercator projection is practically unusable, since the linear scale becomes infinitely high at the poles. A Mercator map can therefore never fully show the polar area.

All lines of constant bearing (rhumbs or loxodromes—those making constant angles with the meridians) are represented by straight segments on a Mercator map.

The map’s properties, conformality and straight rhumb lines, make this projection uniquely suited to marine navigation: courses and bearings are measured using wind roses or protractors, and the corresponding directions are easily transferred from point to point, on the map, with the help of a parallel ruler or a pair of navigational protractor triangles.

The name and explanations given by Mercator to his world map was "New and augmented description of Earth corrected for the use of sailors." This shows that it was expressly conceived for the use of marine navigation. Although the method of construction is not explained by the author, Mercator probably used a graphical method, transferring some rhumb lines previously plotted on a globe to a square graticule, and then adjusting the spacing between parallels so that those lines became straight, making the same angle with the meridians as in the globe.

The development of the Mercator projection represented a major breakthrough in the nautical cartography of the 16th century. However, it was much ahead of its time, since the old navigational and surveying techniques were not compatible with its use in navigation.

Two main problems prevented its immediate application: the impossibility of determining the longitude at sea with adequate accuracy and the fact that magnetic directions, instead of geographical directions, were used in navigation. Only in the middle of the 18th century, after the marine chronometer was invented and the spatial distribution of magnetic declination was known, could the Mercator projection be fully adopted by navigators.

Several authors are associated with the development of Mercator projection:

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Portuguese mathematician and cosmographer Pedro Nunes (1502–1578), who first described the loxodrome and its use in marine navigation. He suggested the construction of a nautical atlas composed of several large-scale sheets in the cylindrical equidistant projection as a way to minimize distortion of directions. If these sheets were brought to the same scale and assembled, an approximation of the Mercator projection would be obtained (1537).

English mathematician Edward Wright (c. 1558–1615), who published accurate tables for its construction (1599, 1610).

English mathematicians Thomas Harriot (1560–1621) and Henry Bond (c.1600–1678) who, independently (c.1600 and 1645), associated the Mercator projection with its modern logarithmic formula, later deduced by calculus.

USES

As on all map projections, shapes or sizes are distortions of the true layout of the Earth's surface. The Mercator projection exaggerates areas far from the equator. For example:

Greenland appears larger than Africa, when in reality Africa's area is 14 times greater and Greenland's is comparable to Algeria's alone. Africa also appears to be roughly the same size as Europe, when in reality Africa is nearly 3 times larger.

Alaska takes as much area on the map as Brazil, when Brazil's area is nearly five times that of Alaska.

The Mercator projection is still used commonly for navigation. The Mercator projection was, especially in the late 19th and early 20th centuries, perhaps the most common projection used in world maps.

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