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.
Maps of major regions, continents, or major nations should be centered locally whenever possible. One should never simply cut out a portion of a world map, especially if an area of interest is far from the projection's central axis and its lines of true scale. For example, a map of Japan extracted from an oval projection centered on the prime meridian would have a misleading and awkward bend to the left (Figure 5-1).
Suiting the Projection to the Region
Careful choice of the map projection can avoid much
unnecessary distortion. In some cases this choice is easy
because of the natural affinity of equatorial cylindrical
projections for the tropics, normal conic projections for
mid-latitude continents and continental subregions such as
Europe and Anglo-America, and polar azimuthal projections
for Antarctica or the Arctic. Africa, which extends from
roughly 37° N to 34° S, is well served by an equatorial
cylindrical projection, with a line of true scale at the Equator
or two lines of true scale, at 20° N and 20° S, say,
for still greater fidelity
(Figure 5-2).
The conterminous portion of the United States is similarly well
represented on conic projections, which preserve the Canadian
border from Minnesota to the state of Washington as a smooth,
gentle arc. Other large, mid-latitude countries commonly
portrayed on locally centered conic projections include
Australia, Canada
(Figure 5-3),
China, and the Soviet Union.
Maps illustrating concepts in oceanography, long-distance navigation, and strategic defense often have special centering requirements. A map of shipping routes between Europe and North America would normally employ a conic projection, a map concerned with direct flights between North America and Asia would benefit from a polar azimuthal projection, and a map of circum-Pacific navigation could benefit from an equatorial cylindrical projection.
In contrast, an equatorially centered azimuthal projection for a map of volcanoes and earthquake centers on the periphery of the Pacific Ocean would promote the well-known geologic concept of a "Pacific Ring of Fire" (see also Chapter 12).
Map projections can readily accommodate territories with a marked east-west or north-south elongation. Ideal for a low- or mid-latitude area with a pronounced east-west trend, a conic projection with two lines of true scale can be more or less optimized for a particular country or region. For the United States, as examples, the Albers Equal-Area Conic projection can be made with two lines of true scale (secant), called standard parallels (29°30' and 45°30' N) to reduce the effects of angular distortion, whereas the Lambert Conformal Conic projection (Figure 5-4) employs similar standard parallels (33° and 45° N) to control the effects of area distortion.
In contrast, transverse cylindrical projections provide low-distortion maps of areas with a wide range of latitude and a narrow range of longitude, such as Argentina (Figure 5-5), Chile, and New Zealand. These projections are often made with two "lines of strength" (lines of true scale) parallel to a vertical central meridian chosen to minimize distortion across the territory of interest.
Centering a World Map
On a map of the whole world, how the projection is centered
affects not only where distortion will be minimal but which
areas might be severed. Equatorially based world maps centered
on North America are rare because of their awkward partition
of Asia. More commonly, world maps are centered on the Greenwich
meridian (0°) and extend symmetrically from 180° W
to 180° E. Yet if strictly symmetrical, they not only
cut off the eastern tip of Siberia but separate the westernmost
Aleutian Islands from Alaska. Some map makers solve this problem
by alternately bulging and indenting portions of the map's
left and right edges, whereas others also show the proximity of
the U.S. and the U.S.S.R. by duplicating the Bering Strait's
marginal region on both the left and right sides of the map.
Centering can be especially critical for oval projections, on which maximum distortion occurs in the outer extremities. Greenwich-centered oval projections thus provide particularly distorted portrayals of Alaska, northeastern Siberia, Australia, and New Zealand. In 1953, Briesemeister, of the American Geographical Society, partially overcame this difficulty with an oblique equal-area projection that groups Europe, Asia, Africa, and the Americas toward the center, assigns the oceans to the outer quadrants, and severs only Antarctica (Figure 5-6). Yet, like other full-world map projections, the Briesemeister cannot treat all regions equally. Its vertical central meridian clearly favors Europe and Africa with only minimal distortion of angles and shape, whereas nearly horizontal meridians yield less familiar, more deformed views of western North America and eastern Asia.
Military planners and tourism officials might favor an azimuthal projection centered at an air base, a major city, or a resort. Such projections are commonly equidistant, to allow viewers to compare relative distances to other places (Figure 5-7). (An interesting and often useful variant is the linear cartogram, which omits coastlines and political boundaries and physical features but shows cities with distances to the center adjusted to portray travel time, transport cost, or some other measure of accessibility or relative distance.) If relative area is important, a locally centered equivalent (or equal-area) azimuthal projection is appropriate. In either case, distortion of shape will be minimal near the center and extreme at the margins.