How are different map projections used?

The method used to portray a part of the spherical Earth on a flat surface, whether a paper map or a computer screen, is called a map projection. No flat map can rival a globe in truly representing the surface of the entire Earth, so every flat map misrepresents the surface of the Earth in some way. A flat map can show one or more--but never all--of the following: 

• True directions
• True distances
• True areas
• True shapes

Different projections have different uses. Some projections are used for navigation, while other projections show better representations of the true relative sizes of continents.

For example, the basic Mercator projection yields the only map on which a straight line drawn anywhere within its bounds shows a true direction, but distances and areas on Mercator projection maps are grossly distorted near the map's polar regions. On an equidistant map projection, distances are true only along particular lines, such as those radiating from a single point selected as the center of the projection. Shapes are more or less distorted on every equal-area map. 

The scale of a map on any projection is often crucial to the map's usefulness for a given purpose. For example, the extreme distortion that is present at high latitudes on a small-scale Mercator map of the World disappears almost completely on a properly oriented Transverse Mercator map of a small area in the same high latitudes. A large-scale (1:24,000) 7.5-minute USGS topographic map based on the Transverse Mercator projection is nearly correct in every respect.

 The USGS Map Projections poster summarizes and compares eighteen common map projections and their uses.