|From cylindrical to pseudocylindrical|
|Pseudocylindrical maps generalize the
"polycylindrical" concept, which can be presented as a
generalization of the cylindrical group of projections, applied to
discrete latitude ranges.
|Generalizing the flattened
discrete map above, the continuous limiting case with infinitely many
strips would be the pseudocylindrical sinusoidal projection;
since the strips would exactly cover the globe surface, it is
easy to see the final result is equal-area.
The concept of "polycylindrical" maps, comprising many thin cylindrical maps with correct parallel length but arbitrary height, was suggested by W.Tobler by analogy to polyconic projections being a generalization of the conic group. More generally, any pseudocylindrical map can be conceptually created by juxtaposing a (possibly infinite) number of partial cylindrical maps, not necessarily using the same scale for parallels.
Even though their geometric constraints are much more flexible than for cylindrical designs, a few decisions are still common to all pseudocylindrical projections: the shape of meridians (straight lines, conic sections, sinusoidals, arbitrary), distance distribution of parallels, linear or pointed poles. Often identical combinations of choices have been adopted by different authors, and several pseudocylindrical "projections" are actually synonyms. Given those alternatives, projections in this large group are classified somewhat arbitrarily; many designs could easily be assigned to more than one section. Of infinitely many possible pseudocylindrical projections, several are useful didactical devices and popular choices for world maps.
|Sinusoidal (Sanson-Flamsteed) map, graticule spacing 10°|
Despite the common name, the "Sanson-Flamsteed" projection was not first studied by either Nicholas Sanson (ca. 1650) or John Flamsteed (1729, published posthumously), but possibly by Mercator — at least it was included for maps of South America in later editions (1606) of Mercator's atlas, and has been referred to as the Mercator equal-area projection, or Mercator-Sanson. Since in the normal aspect all meridians are sinusoids, it is also called sinusoidal, and can be easily deduced.
The sinusoidal projection is equal-area and preserves distances along the horizontals, i.e. all parallels in an equatorial map are standard lines — but only the central meridian. Although the equatorial band is reproduced with little distortion, the polar caps suffer from poor legibility due to shearing. The partially constant scale and simple construction still recommend this projection for continents like Africa and South America, frequently after convenient recentering and, for world maps, interruption.
Both the Equator and central meridian are standard lines, thus the whole map is twice wide as tall, the same proportions of the Mollweide projection with which it has been frequently combined.
|Normal Mollweide map|
|Rescaled Mollweide projections by Bromley (above, with standard Equator) and Tobler (on the right, circular) are equal-area too.|
Created by the German Karl B. Mollweide, the eponymous pseudocylindrical projection is bounded by an ellipse; poles are points and its Equator is twice as long as the straight central meridian, but neither is a standard line. All other meridians are elliptical arcs, and parallels are unequally spaced in order to preserve areas. Only the intersections of the central meridian with the standard parallels 40°44'12"N and S are free of distortion. Even though its geometry is easily deduced, calculation is more complex than for the other classic pseudocylindrical still important today, the sinusoidal — this and the loss of uniform scale along the central meridian are the price paid for lesser crowding in polar areas.
Despite the equal area property and its pleasant shape, Mollweide's projection received little recognition since publication in 1805, becoming better known only after the French Jacques Babinet presented it as the homalographic in 1857. Historically, its other common aliases include elliptical, Babinet, homolographic (from the Greek homo for "same", thus equal-area).
Profoundly influential, this projection was combined with the sinusoidal in fused (John P. Goode's homolosine, Allen K. Philbrick's Sinu-Mollweide, György Érdi-Krausz) and averaged (Samuel W. Boggs and Oscar S. Adams's eumorphic; some authors also relate it to the Winkel II) designs. Interrupted variants, alone or combined, have also been popular.
Other variations include oblique aspects like John Bartholomew's Atlantis and simple rescaling by orthogonal reciprocal factors, which preserves areal equivalence while changing both aspect ratio and the angular distortion pattern. For instance, Waldo Tobler (1962) suggested making the whole map circular with standard parallels 73°7'43.85" N and S; Robert H. Bromley's projection (1965) elongates the ellipse merging the standard parallels at the Equator.
Mollweide's 2:1 ellipse is occasionally mistaken for Aitoff's and Hammer's projections, neither of which are pseudocylindricals, although the latter is also equal-area. Much more easily confused is an elliptical full-world extension of Apian's second globular projection, mathematically much simpler and not equal-area.
|Collignon map, symmetrical diamond form|
|Interrupted Collignon map||Collignon map in its most common form|
The graticule in the equatorial aspect of a few pseudocylindrical projections comprises only straight lines, possibly broken at the Equator. Those include the trapezoidal, one of the oldest projections, Eckert's I and II, composites like the HEALPix grid, and Snyder's cartographic pun with a serious message.
Édouard Collignon's projection, introduced in 1865, preserves areas but strongly distorts shape. In the equatorial aspect, both northern and southern hemispheres can be either a isosceles triangle with base on the Equator and height half the Equator's length, or a isosceles trapezium (British: trapezoid) with the small base at the Equator. All graticule lines are straight but the meridians are optionally broken at the Equator.
The two most common arrangements for the worldwide map are either the isosceles triangle (with unbroken meridians and a flat South Pole for base) or the diamond. The two complementary options (upside-down triangle and hourglass-shaped) and the interrupted variant in two or more diamonds are equally valid. In spite of its simple construction, this projection is regarded as little more than a curiosity; more recently, it has found practical application as part of the HEALPix format.
|Craster's parabolic projection|
In 1929, Lt.Col. John E.E.Craster presented the features of three pseudocylindrical equal-area projections with meridians based on conic sections. He rejected the elliptical and hyperbolic versions, and included an abbreviated table of ordinates for the parabolic design. Mathematical details were later presented by Charles H.Deetz and Oscar S.Adams (1934), including an interrupted version, and again by Adams (1945).
Although with the same 2:1 proportions and superficially resembling the sinusoidal, Craster's parabolic projection has more convex meridians, somewhat reducing shape distortion in high latitudes near the boundary meridians. Its mathematical development is an interesting application of solving cubic functions.
As part of a series (1934) of pseudocylindrical projections based on trigonometric and conic curves, Reinholds Putniņš duly noted his P4 projection as identical with Craster's parabolic.
|Loximuthal maps with central meridian 0°. The reference latitude is 0° (top) and 51.5°N, near Greenwich (above). Loximuthal maps must be created for each point of interest, like these examples.|
In prewar Germany, Karl Siemon developed four pseudocylindrical projections, the first one undoubtably the best known. All but one were later independently rediscovered by other authors.
Dubbed Wegtreue Ortskurskarte by Siemon, his first projection (1935) is today almost always referred to as the loximuthal, after W.Tobler who published it in 1966. The portmanteau nicely summarizes its unique feature: given a reference latitude, all straight lines touching its intersection with the central meridian are loxodromes. In addition, they are all standard lines with correct azimuth, analogous to straight lines through the center of an azimuthal equidistant map.
In contrast, on a Mercator map all loxodromes are straight lines, but scale quickly changes not only between different loxodromes but also along the same lines; azimuths are also not uniform.
The loximuthal projection is not equal-area, and is conformal only where the central meridian intersects the reference (also standard) parallel. Unlike most pseudocylindrical projections, it is not symmetrical with respect to the Equator, unless the Equator is selected to be the reference latitude.
|Quartic authalic map|
The quartic authalic is another equal-area (authalic from Greek autos ailos, "same area") projection, designed by Siemon (1937) and independently by Adams (1944). Its meridians are fourth-order curves, hence the name.
|Kavrayskiy's fifth projection|
Besides conic and cylindrical projections, the Russian Vladimir V.Kavrayskiy also proposed three pseudocylindrical designs, of which the V and VI are based on trigonometric functions and are equal-area. The fifth (1933) has pointed poles, standard lines at the 35°N and S parallels and meridians calculated from sinusoids, but not sinusoidal themselves. The sixth (1936) is identical to Wagner's first projection of 1932, with sinusoidal meridians.