A Look at the Mercator Projection

Filed in Maps and Cartography by on January 13, 2014
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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.

Next up: How the Mercator Projections Works and Criticisms of the Mercator Projection

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