Cartography Word of the Day: Projection

Second in an irregular series.

The Mercator Projection, illustrative of the paradigm. Courtesy Nationalatlas.gov

It is axiomatic that the only truly accurate map of the world is one which is drawn upon a spherical solid. The planet, after all, is a 3-dimensional shape. As a sphere, in total, it is of limited use, not appropriate for every application, and can be tough to carry–never mind the age-old problem of folding it back up and putting it back in the car’s glove-box.

Perforce maps, for popular as well as professional use, are typically plotted or drawn on a flat sheet. Since this spherical surface is being rendered flat, there is distortion inherent in every map made, even the common city street map. On that level, however, local distortion is so small as to practically negligible. But larger and larger areas–counties, states, countries, oceans–introduce severe distortions as one works out from the center. For this reason, cartographers have developed a rich system of rendering areas on a curved surface that minimize distortion for those areas. These displays are called projections.

Let the Light Shine

To understand the meaning of the term, think of a glass globe with a silhouette outline of the contents on. Now, at the center of this globe, place a bright light. Wrap the globe in a sheet of paper, taking care to ensure that the sheet touches the Equator at all points. Now, switch the light on, and trace the outline that would appear projected on the paper. The likely result is a map of the world that looks not too much unlike Mercator’s famous world map.

The actual process of constructing projections takes this basic concept and uses mathematics to model and construct grids based on planes, cylinders and conic sections. The projection surface can be tangent (touching at one point) or secant (cutting through at two points to the sphere).

A Whole-Earth Classification Catalog

Projections come in a flock of styles corresponding to different needs and scopes of display. They can be conformal, equal-area, or equidistant; wear names of great cartographers such as Goode, Mercator, Gall, and Molleweide; and have mathematical-sounding roots such as sinusoidal or homalosine. To define each of these in thier right is somewhat outside of the scope of this entry and will be taken up properly in entries of thier own. A good all-purpose definition is supplied by nationalatlas.gov, which start thus:

Further reading:

Map Projections: defined and discussed at nationaatlas.gov