A Look at the Mercator Projection

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In GIS and other mapping it is important to choose an appropriate map projection to accurately depict the Earth’s surface. A map projection is defined as a tool that transforms the Earth’s surface into a flat plane that can be shown on paper and/or digital maps. Map projections are based on an arrangement of parallels and meridians that represent a geographic coordinate system (Chang, 2012).

Map projections used by cartographers are grouped into two major categories – those that preserve a specific property and those that use a specific type of projection surface. There are four classes of projection types that preserves a property. They are the conformal, equivalent, equidistant and azimuthal projections. A conformal projection is one that preserves an area’s local angles and shapes, while an equivalent projection shows areas in proportional size (Chang, 2012). An equidistant projection is based on a consistent scale along specific lines and an azimuthal projection maintains accurate directions (Chang, 2012).

In addition, there are three projection types that are based on different surfaces to represent the world. These are the cylindrical, conic and azimuthal projections. A cylindrical projection is when a cylinder is used to construct a projection, a cone for the conic and a flat plane for the azimuthal. In each of these cases the specific shape is essentially wrapped around a globe and the image of the world is then projected onto the shape.

The Mercator projection.  Source: USGS.

The Mercator projection. Source: USGS.

These different map projections are broad categories and within them there is a variety of different projection types that can show local areas or the entire world. One of the most common and controversial types of map projection within these categories is the Mercator projection. This projection is cylindrical and conformal. It was originally used for navigation purposes but later became a staple in classrooms to teach world geography. It is controversial today because it does not accurately depict the size of the Earth’s northern and southern latitudes.

History and Development of the Mercator Projection

The Mercator projection was originally developed in 1569 by the Flemish cartographer Gerardus Mercator. At this time, many of Europe’s top cartographers and explorers used elliptical projections derived from Ptolemy’s latitude and longitude grid. Although accurate, these projections were difficult for navigators and explorers to use because they required that bearing constantly be recalculated as they moved (Stockton, 2013).

Maps created prior to Mercator’s that were drawn on Ptolemy’s grid showed that each degree of latitude or longitude was the same size. As a result sailor’s rhumb lines (straight lines on the Earth used by navigators that follow a single compass bearing) curved and navigators would have to recalculate their bearing as they moved to account for the change (Israel, 2003). Mercator found that to keep the rhumb lines straight he had to make lines of latitude move away from each other as they moved north and south of the equator. In order to do this, he created a projection that preserved the 90° angles between the latitude and longitude lines.

Nova et Aucta Orbis Terrae Descriptio ad Usum Navigantium Emendate Accommodata ("New and more complete representation of the terrestrial globe properly adapted for use in navigation"). Mercator's map of the world, 1569.

Nova et Aucta Orbis Terrae Descriptio ad Usum Navigantium Emendate Accommodata (“New and more complete representation of the terrestrial globe properly adapted for use in navigation”). Mercator’s map of the world, 1569.

How the Mercator Projection Works

Many discussions of how the Mercator projection works say to imagine a cylinder with a globe inside (Stockton, 2013). The globe should have a light inside so that an image of the world is projected onto the cylinder. Because the cylinder only touches the globe at the equator points along that parallel are the only ones on the projection that are completely accurate. Additionally, because the cylinder is perpendicular to the globe, lines of longitude are straight, instead of curved as on a globe when they are transferred to the cylinder. This results in straight lines of latitude and longitude with consistent, 90° angles between them around the world.

Because of the preserved 90° angles and straight lines of latitude and longitude, rhumb lines are also straight on the Mercator projection. This meant that sailors using maps in that projection no longer had to recalculate their bearings on long journeys. Instead they could mark their starting and ending points and simply follow the line along their expeditions. Due to this, the Mercator projection made world exploration much easier and became a essential map projection for navigation.

Using a cylinder and a globe with light is a simplified explanation as to how the Mercator projection works. In reality it is very complicated to derive the projection and it takes complex mathematical formulas to fully explain it (Israel, 2003). In 1599, Edward Wright, an English mathematician, first explained the very complicated mathematics of the Mercator projection and throughout the 1600s several other mathematicians attempted to find easier explanations. Today, most basic explanations as to how the Mercator projection work use the cylinder and a globe with light description.

Criticisms of the Mercator Projection

To keep longitude lines straight and maintain the 90° angle between the latitude and longitude lines, the Mercator projection uses varying distances between latitude lines away from the equator. As a result, the Earth’s poles and landmasses closest to them are distorted. This distortion stretches landmasses like Greenland and Europe and they appear much bigger than places that are close to the equator such as South America and Africa. Despite these errors the popularity of the projection as a navigation aid and its easily readable rectangular grid meant that it was easy to reproduce in printed materials like atlases and wall maps. As a result, it became a standard map for classrooms.

Throughout the 1900s geographers and scholars have claimed that the Mercator projection is incorrect and that it should only be used for navigation. It was not until the 1980s though that the projection began to receive wide criticism. In 1989 for instance, seven professional geographic organizations in North America adopted a ban on this and other rectangular coordinate maps (Rosenberg).

Most of the main criticisms of the Mercator projection are that it gives people a false impression of the size of the world’s landmasses. Greenland, for instance is not bigger than South America, but it appears to be on Mercator maps. Other critics say that this projection and the large size of continents like Europe gave an advantage to the colonial powers because it made them appear larger than they really are. This advantage eventually led to the lack of development in many equatorial regions that appear smaller on the Mercator maps.

Despite these criticisms, there is use for the Mercator projection in sailing and world exploration because it does allow for easier navigation. In addition, it is an interesting example of how a map projections can make areas of the world appear in certain ways.

References

Chang, Kang-tsung. (2012). Introduction to Geographic Information Systems. McGraw-Hill: New York, 6th Edition.

Encyclopedia Britannica. (n.d.). Mercator Projection (Cartography) – Encyclopedia Britannica. Retrieved from: http://www.britannica.com/EBchecked/topic/375638/Mercator-projection (30 December 2013).

Israel, Robert. (20 January 2003). “Mercator’s Projection.” University of British Columbia Mathematics Department. Retrieved from: http://www.math.ubc.ca/~israel/m103/mercator/mercator.html (30 December 2013).

Lammie, Rob. (15 August 2008). “3 Controversial Maps.” Mental Floss. Retrieved from: http://mentalfloss.com/article/19364/3-controversial-maps (30 December 2013).

Rosenberg, Matt. (n.d.). “Peters Projection vs. Mercator Projection.” About.com Geography. Retrieved from: http://geography.about.com/library/weekly/aa030201b.htm (30 December 2013).

Stockton, Nick. (29 July 2013). “Get to Know a Projection: Mercator.” Wired Magazine. Retrieved from: http://www.wired.com/wiredscience/2013/07/projection-mercator/ (30 December 2013).



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