The video above is from the September 2012 digital edition of National Geographic magazine.

Choosing a map projection is a major challenge for cartographers. Features such as size, shape, distance, or scale can be measured accurately on Earth. Once projected on a flat surface, however, only some of these qualities can be accurately represented. Every map has some sort of distortion. The larger the area covered by a map, the greater the distortion.

Depending on the map's purpose, cartographers must decide what elements of accuracy are most important to preserve. This determines which projection to use. For example, conformal maps show true shapes of small areas but distort size. Equal area maps distort shape and direction but display the true relative sizes of all areas. There are three basic kinds of projections: planar, conical, and cylindrical. Each is useful in different situations.

Cartographers at National Geographic chose to use a version of the Mollweide projection for their map highlighting ocean floors, published as the map supplement in the September 2012 issue of National Geographic magazine. This Mollweide projection is referred to as a pseudocylindrical projection. The specific version of the Mollweide projection used is called an interrupted Mollweide, because lines of longitude, or meridians, are interrupted. The map is pulled apart at specific meridians to minimize distortion in areas where the cartographer would like the map reader to focus their attention.

Find more interactive content, photos, and videos in the digital edition of National Geographic magazine.