'Because distortion is generally greater toward the margins of a map projection, each projection shows the shape, area, or angles of some parts of the world more correctly than it does other parts. Consequently, in choosing a projection for a world map, one must identify the part of the world to be shown or emphasized and carefully match a map projection to that area. A world map of agricultural regions would benefit from comparatively low distortion in mid-latitude zones, whereas a map portraying the world distribution of tundra and permafrost should show areas north of 70° as accurately as possible.' (from ch. 5 of Matching the map projection to the need) Each projection is created for a specific purpose. The Mercator projection was created in the 16th century for navigation. It was designed so that a ship's navigator could draw a straight line on a chart that would be a line of constant direction. The 'cost' of this characteristic of the Mercator Projection is that areas at higher latitudes get enlarged. This is why a map based on the Mercator projection should not be used in a classroom. Compare the size of Greenland to Africa (or Alaska to Brazil) on the Mercator and the Mollwiede Projection (an equal-area projection pronounced 'molvida'). Alaska is bigger than N.J., but why we may not realize how big Alaska is. Some examples to illustrate how to choose an appropriate projection for your map. 'Special Flood Hazard Areas are subject to inundation by a flood that has a 1-percent or greater chance of being equaled or exceeded during any given year. This type of flood commonly is referred to as the 100-year flood or base flood. A 100-year flood is not a flood that occurs every 100 years! In fact, the 100-year flood has a 26 percent chance of occurring during a 30-year period, the length of many mortgages. The 100-year flood is used by the National Flood Insurance Program (NFIP) as the basis for insurance requirements nationwide'. Source.