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Tuesday, June 13, 2017

Map Projections Definition

Map Projections Meaning and Facts 
Literally thousands of map projections exist. Each one distorts one or more of the four major map attributes. The projection that a cartographer chooses depends on the size and location of the area to be projected and on the purpose of the map.

Cartographers often choose the Robinson projection. This projection is unique because it is a compromise projection. It maintains no single property but minimizes overall distortion.

If true shape is the most important objective of the cartographer, he or she will select a conformal projection, such as the Mercator projection.  On a map that uses such a projection, land masses are shown in their true shapes. Size relationships, however, are distorted. If the purpose of the map is to show correct relative size, an equal-area projection, such as the Molleweide projection or the Peters projection, will be used. Maps whose projections show correct distances between places are called equidistant maps. Equidistant maps work well for projecting areas of limited size, such as a city or a state. A map of the entire world could never be equidistant, however, because it is impossible to show the lengths of lines of latitude and longitude on a flat map as accurately as they appear on a globe. Finally, maps that show true distances and direction measurements from a central point on the map are called azimuthal maps.  These maps are often used to show the Polar Regions.

Each kind of projection has its strengths and weaknesses. On a conformal map, for example, angles and directions are correct, but size relationships are quite distorted. Compare, for example, the relative sizes of Africa and Greenland on the Mercator projection with their relative size on the Molleweide or Peters projection or on a globe.

Equal-area maps, on the other hand, are especially useful for comparing factors that may be affected by an area's size, such as temperature patterns, population size, or mineral production. The greatest drawback of equal-area maps is that they distort the shapes of the areas shown.

Equidistant projections are used for road maps because they allow the driver to get a clear and accurate picture of the distances to be traveled. Certain types of azimuthal maps, in contrast, are especially useful to pilots because they show the shortest distance between two places on Earth as a straight line.

In spite of their drawbacks, then, there is indeed a map projection to meet every need. In studying maps, however, always remember to note the type of projection that was used so that you will be aware of how it is different from a globe, the one precise model of Earth. To aid you in this task, each of the maps in this book identifies the map projection used.


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