The main purpose of interrupting a map is moving significant regions to less-distorted places, usually near the center of each lobe. Several cartographic tricks enhance the usefulness of interruptions.
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 central 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 Philbrick's Sinu-Mollweide projection and 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. Recentering may resort to different aspects or even oblique maps.
|Recentering to exploit areas of lesser distortion|
|Ordinary equatorial map, centered on meridian 0°||Equatorial, meridians recentered on 137°E||Oblique, fully recentered|
|These three regional maps show the Japanese islands using the same Eckert IV projection. Analysis of the angular deformation patterns for this projection shows that, although it preserves area everywhere, only two small "sweet spots" at the intersection of the central meridian and the standard parallels 40°30' N and S (in the equatorial aspect) are free of angular distortion. All three map cover the same area but a slightly different region, which in the second version is nearer the optimal spot — Tokyo lies near 35°N 139°E. The oblique version on the right is actually more distorted, while losing the useful property of straight parallels.|
Above, map using the Eckert IV projection, interrupted with
meridians recentered, condensed at the Atlantic Ocean.
presented uninterrupted in an inset drawn with Lambert's azimuthal
equal-area projection at identical scale.
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 a larger scale and better detail in the same printed area.
|Brazil and its immediate neighborhood, in a main Mercator map and (shaded) in the inset.|
|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 distances between, e.g., Iceland and Greenland, or Siberia and Alaska, are greatly stretched.|
Like several cartographic techniques, interruption can be misused or thought of as purely editorial convenience. Hastily or carelessly prepared maps may suffer from:
Such maps may be good enough for advertisement, but unacceptable for didactic or scientific purposes.