 |  |  |  |  | Map Projections |
|
| Equidistant cylindrical projection (Plate Carrée case) |
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| The effect of changing the standard latitude (parallels highlighted) of equidistant cylindrical maps |
Suppose an arbitrary projection in whose equatorial aspect:
The rectangular result is the very simple cylindrical equidistant projection ("equidistant" only along meridians and one or two selected parallels), having a multitude of alternate names. It is neither conformal nor equal-area, and despite the resemblance to the stereographic cylindrical, it is not truly created by a perspective method.
Since scale is constant along all meridians, 
is simply
; two parallels are standard
at 
, with circumference
.
Constant scale along any single parallel means:
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Different standard parallels affect only the map's width/height
ratio. For the common special case of a standard Equator (usually
known as the Plate carrée, French for a flat square surface),
, and
longitude and latitude are linearly mapped into 
and respectively, therefore rendering a
world map into a 2:1 rectangle. The trivially fast computation has
turned this format into a favorite for storing images intended for
texturing spheres in computer graphics, and for panoramic scenes in
digital photography, despite the range of detail density and
shape distortion, both much higher at the poles than near the Equator.
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| Boundary meridians of Winkel's first proposal and its foundation projections |
The cylindrical equidistant projection can be calculated quickily and presents a few interesting properties like immediate determination of angular and linear distances from two points. However, it suffers from infinite horizontal stretching along the poles. On the other hand, the sinusoidal projection is difficult to read at the polar regions due to high shearing. In both, scale is constant along each parallel, and all parallels are uniformly spaced.
The Winkel I projection is an arithmetic average of the sinusoidal and equidistant cylindrical projections. Neither equal-area nor conformal, it is defined as:
|
The Eckert
V projection is a special case for
.
Winkel instead chose a standard parallel yielding a map
with total area in scale with its width.
Consider the upper right quadrant of the map; the boundary meridian is given by
|
| Winkel I map |
Because
If the area of an spherical Earth is 
,
and therefore the standard parallels are about ±50°27"35'.
As the basis for his second proposal, Winkel used another auxiliary pseudocylindrical projection in whose equatorial aspect:
Resembling Mollweide's but
not equal-area, this intermediary elliptical projection is
essentially an extension of Apian's
second globular design to a
whole-world map. Consider the equation of an ellipse centered on the origin with
horizontal major axis 
and minor axis
:
In the northeastern quadrant, the boundary meridian is
Equally-spaced parallels imply
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| Winkel II map |
therefore 
. Since meridians are also
equally spaced, horizontal scale is constant, so
and
the auxiliary projection is given by:
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The Winkel II projection is a simple arithmetical average of this elliptical projection and the cylindrical equidistant:
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Again, the result is neither conformal nor equal-area and the author preferred 50°27"35' N and S for standard latitudes.
| | | | | www.progonos.com/furuti May 13, 2014 |
