A globe is the only correct map of the world. To create a flat representation of earth’s curved surface something has to "give". When mapping a small area, distortion is negligible. On a map of the United States, distortion may affect careful long distance measurements but is of little concern to non-technical users. Distortion on a world map however may may lead to drastic misconceptions. Every world map user should be aware of how his map departs from the truth. For instance, compare Greenland in the diagrams below. In some it looks bigger than South America and in others it doesn’t even look like Greenland.
When choosing a world map keep in mind that maps which show relative sizes correctly (called Equal Area projections) do an unreliable job showing shapes, and maps which faithfully depict shapes (called Conformal projections) manifest great errors of relative size. Most popular world maps employ a projection that compromises among these conflicting goals.
There are dozens of different world map projections. Most sold by the Map Center use one of the following five:
Mercator: Until a generation ago this was the ubiquitous general purpose world map. Now it is out of favor because it greatly exaggerates areas in high latitudes. This could be an advantage if you need extra room for maptacks in Europe and North America. It has other important properties: It is the only projection where a straight line drawn on the map correctly describes a compass bearing on earth’s surface. Navigators still depend on this. The map is conformal: The shape of any small segment resembles the same segment on a globe. North-South meridians of longitude and East-West parallels of latitude are all straight lines that cross at right angles. The poles can not be shown.
The illustration makes clear why we call the translation of earth’s surface onto flat paper a projection. Earth’s spherical surface has been projected onto a cylinder as if by a light at earth’s center.
Gall’s Stereographic projection is similar to Mercator but size distortion in the higher latitudes is reduced by compressing the north-south distances.
Rand McNally world maps use this projection.
Peters Projection: If you remember a news story about the new world map thatis "Fair to all Peoples" you probably heard about the Peters Projection, a rigorously correct equal area map. Latitude and longitude lines are straight and cross at right angles. Shapes are distorted except at 45 degrees north and south. The poles can not be shown.
Van Der Grinten: Earth’s entire surface is projected into a circle. Mapmakers delete the polar regions and repeat some coverage at the sides to provide a convenient rectangular presentation. Note how on the map below, Alaska is shown with North America on the left and repeated in the top right corner east of Russia.
This is a widely used compromise. Both size and shape are fairly accurate in a large central portion of the map but size distortion is very significant near the edges. Overall, this map is more conformal than Winkel Tripel.
Winkel Tripel: The world is shown as a flattened oval, and space at the corners is available for inset maps. It is much closer to a true equal area map than Van Der Grinten. Shape distortion does increase with distance from the center but is extreme only near the edges. The poles, which in reality are points, are represented as line segments at top and bottom.
National Geographic world maps use the Winkel Tripel projection.