Map Projections

Interrupted Maps

Maps wrapped on a globe show advantages and limitations of selected projections
Comparing equatorial maps at identical scales tangent to a globe is an interesting exercise. In particular, which map could be glued to the global surface more neatly? why is South America so similar in the second and fourth (but not the third) maps? why are Africa and Western Europe nearly identical in the second, third and fourth maps? which is the "best" version of Greenland (check the globe)?
Lambert's cylindrical : equal-area, with severe horizontal stretching and vertical compression near the poles (Greenland and Antarctica are barely visible but presented in entirety)	Equidistant cylindrical: has correct vertical scale (total height equals half a circumference), but severe areal exaggeration near the poles, where horizontal scale is infinite	Ordinary sinusoidal: pseudocylindrical and equal-area; total height and scale are correct along the central meridian, but shapes are badly skewed far from it	Symmetrically interrupted sinusoidal remains pseudocylindrical and equal-area; has correct scale along the central meridian of every lobe, but many discontinuities

Introduction

No map projection can preserve shape and size simultaneously, and the larger the mapped area, the more pronounced the total distortion. Rectangular world maps are prone to excessive area and distance stretching, while those using circular and elliptical projections usually present too much shape distortion at the periphery.

Interrupted maps attempt a compromise, "cutting" the terrestrial surface along some arbitrarily chosen lines and projecting each section, or lobe (or gore, in case interruptions repeat periodically along meridians), separately with lower stretching. Commonly lobe boundaries are designed to fall upon less important (for the map's purpose) areas, like oceans.

In a sense, interrupting a map creates another kind of distance distortion, since neighbor points on the sphere become widely spaced in the map; therefore, too many lobes would negate the benefits of interruption. As mentioned for oblique maps, such distortion actually happens, usually overlooked, at the edges of any ordinary projection.

Interrupted projections were used by Waldseemüller (1507) and Leonardo da Vinci (ca. 1514), among others. An early example was a variation of Werner's projection by Mercator (1538). Some designs, like the HEALPix projection, were explicitly created with interruption in mind.

Simple Interrupted Maps

The sinusoidal (also known as Sanson-Flamsteed) projection has a simple construction and interesting features: pseudocylindrical, equal-area and constant vertical scale (i.e., parallels are uniformly spaced). On a whole-world equatorial sinusoidal map, the polar regions at extreme longitudes suffer from strong shape distortion (shearing). Interrupting the map preserves its better features with lesser shearing.

Clearly there is a trade-off: increasing the number of lobes further reduces shape distortion as each lobe is centered around its own different meridian, until the discontinuities make the map more a curiosity than something useful in its planar form. However, a lobed map could, if printed on a sheet of flexible material, cut and joined at the borders, make up a fairly good globe; interestingly enough, ancient gore maps had exactly that purpose, albeit with a different lobe arrangement and much more primitive projection methods.

Maps with lobes in a row along the Equator make clear why cylindrical projections necessarily distort polar regions: they must horizontally stretch and fasten them together in order to force a rectangular map.

Finally, as usual, designing an interrupted map reflects the author's particular point of view. An asymmetrical arrangement of lobe boundaries can avoid cutting the three major oceans instead of land masses. In the case of a pseudocylindrical projection as the sinusoidal, all other properties still hold, including the mapped area. Asymmetrical lobes are featured in classic interrupted maps by Goode, Boggs and McBryde.

www.progonos.com/furuti    September 16, 2008