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Map Projections

  Map projections are systems for transferring patterns from the Earth\'s surface to maps. They work as if a light inside the globe projected the surface patterns on to a flat sheet, a cone or a cylinder placed against the surface. Since the Earth is a sphere, it is impossible to make this transfer without distortion. Different map projections have different strengths and weaknesses: some preserve shape, others size, others distance or direction, but no projection can do all these things and so no projection is correct for all purposes.

Probably the most familiar projection is that originated by Mercator in the late 16th century. This is convenient for navigation since a straight line follows a constant compass course. It is still used for maps of small areas but it is not suited to long distance navigation (because a constant compass course does not indicate the shortest route), and it is not suited for atlas maps because of its distortions of size. These result from the stretching of all lines of latitude to the same length as the equator and become extreme in high latitudes, making Greenland, for example, seem larger than Australia though it is actually less than a third as large. In practice, Mercator maps, which were much used for atlases until recent decades, emphasize the mid latitudes over the tropics. They were also popular because, as usually laid out, they put the UK in a central position, so flattering the beliefs of imperial Britain.

Since 1945, several new perspectives have become important. First, the Cold War made strategists look again at relations between the USA and USSR: polar projections show them to be much closer than Mercator suggests, reflecting the new realities of air travel and missile trajectories. More recently, the emergence of Japan has led to questioning of world maps centred on Europe or North America, and to suggestions that the prime meridian should run through the Pacific and not through Greenwich. The desire to eliminate the relics of colonialism has also produced an overtly ideological projection the Peters Projection.

The Peters Projection was stimulated by a desire to give tropical, less-developed countries equal weight by depicting equal areas. At times, the protagonists of the Peters map seem to suggest that it is the only correct projection, but this can only be political correctness and not cartographic correctness. In fact, the Peters preserves equal area by stretching tropical shapes north-south and polar ones east-west. In relation to its own aims, it seems less satisfactory than some other equal area projections, such as the Eckert 4 or Mollweide, where shapes in the tropics were realistic at the cost of deformations in high latitudes as lines of longitude converged. This problem can be reduced by interrupting the projection, dividing it into several segments but that has the problem of reducing visual continuity.

There are now hundreds of map projections, and new ones can be quickly generated by computers, so the topic is detailed and technical. What matters to ordinary map users is not detailed techniques but understanding that all projections distort something significantly if the area covered is more than a few thousand miles across. If the complexities are too daunting, the remedy is to check the maps against a globe, which does preserve shape, area, distance and direction, even though it is not easily portable. PS



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