Map Projections: Interruption Devices

Map Projections

Interruption Devices

The main purpose of interrupting a map is moving significant regions to less-distorted places, usually near the center of each zone. Several cartographic tricks enhance its usefulness.

Recentering and Cropping

Interrupted Eckert IV map with lobes recentered: only one meridian (not necessarily the middle one) in each lobe is mapped to a straight line

Especially with pseudocylindrical projections, each lobe of an interrupted map may easily be projected with its own, arbitrary central meridian, not necessarily the same as the median one. This introduces asymmetrical angular distortion, privileging regions near the straight central meridian while detracting from others. The central meridian may even change with latitude, like in the Eurasian lobe of Boggs's eumorphic map.

Recentering is also an effective device for uninterrupted continental or regional maps. The region of interest is centered, minimizing distortion, and the remaining projected area is cropped off.

World map, ordinary World map, meridians recentered World map, all recentered

Detail, ordinary Detail, meridians recentered Detail, all recentered

Ordinary map Meridians recentered Fully recentered

Three aspects of the Japanese islands using the same Eckert IV projection. Every regional map covers the same area but slightly different regions. The second option is better for this particular projection, whose least-distortion parallel is not the Equator, but instead near the interest area. The oblique map is actually more distorted due to exaggerated latitudinal scale close to the center.

Condensing and Insets

Same previous map, condensed at the Atlantic Ocean. Part of Western Pacific is covered by an inset of uninterrupted Antarctica, using a polar aspect of the same projection and scale (an azimuthal projection would be more appropriate).

Some cartographic devices are actually editorial tricks, intended for clarity or printing convenience. They do not affect distortion patterns, and can be employed purely as lay-out tools.

Frequently applied simultaneously with interruption, condensing the map means removing unimportant areas and joining the remaining sections. It can save publishing space or, conversely, allow larger scales and better detail in the same printed area.

Mercator map with azimuthal orthographic inset

Part of South America, both in the main Mercator map and (shaded) in the inset.

Also an editorial tool, an inset is a small illustration detached or superimposed on the main map, useful:

for presenting an area strongly distorted or interrupted, or impossible to be shown (like in Mercator's projection) by the main projection; the azimuthal equidistant is commonly employed for polar regions
especially for large-scale maps of a small or lesser-known region, for quickly pinpointing its location on Earth; the azimuthal orthographic is frequently used in small hemispheres

Abusing Interruptions

Like several cartographic techniques, interruption can be misused or thought of as purely editorial convenience. Hastily or carelessly prepared maps may suffer from:

failure to clearly mark condensed areas
interruption gaps not marked, suggesting continuity instead

Such maps may be good enough for advertisement, but unacceptable for didactic or scientific purposes.

Interrupted Eckert IV map, not condensed but with lobe gaps colored like additional sea area; partial graticule and removal of Antarctica help hiding the flaw. Although continental shapes are better presented than in rectangular maps created by cylindrical projections, land/water area ratios are misleading and distance between, e.g., Iceland and Greenland is greatly stretched.



Ordinary map	Meridians recentered	Fully recentered
Three aspects of the Japanese islands using the same Eckert IV projection. Every regional map covers the same area but slightly different regions. The second option is better for this particular projection, whose least-distortion parallel is not the Equator, but instead near the interest area. The oblique map is actually more distorted due to exaggerated latitudinal scale close to the center.