Stereographic projection



In geometry, the stereographic projection is a particular mapping (function) that projects a sphere onto a plane. The projection is defined on the entire sphere, except at one point &mdash; the projection point. Where it is defined, the mapping is smooth and bijective. It is conformal, meaning that it preserves angles. It is neither an isometry nor area-preserving: that is, it preserves neither distances nor the areas of figures.

Intuitively, then, the stereographic projection is a way of picturing the sphere as the plane, with some inevitable compromises. Because the sphere and the plane appear in many areas of mathematics and its applications, so does the stereographic projection; it finds use in diverse fields including complex analysis, cartography, geology, and photography. In practice, the projection is carried out by computer or by hand using a special kind of graph paper called a stereonet or Wulff net.

History


The stereographic projection was known to Hipparchus, Ptolemy and probably earlier to the Egyptians. It was originally known as the planisphere projection. Planisphaerium by Ptolemy is the oldest surviving document that describes it. One of its most important uses was the representation of celestial charts. The term planisphere is still used to refer to such charts.

It is believed that the earliest existing world map, created by Gualterious Lud in 1507, is based upon the stereographic projection, mapping each hemisphere as a circular disk. The equatorial aspect of the stereographic projection was commonly used for maps of the Eastern and Western Hemispheres in the 17th and 18th centuries.

François d'Aiguillon gave the stereographic projection its current name in his 1613 work Opticorum libri sex philosophis juxta ac mathematicis utiles (Six Books of Optics, useful for philosophers and mathematicians alike).

Definition


This section focuses on the projection of the unit sphere from the north pole onto the plane through the equator. Other formulations are treated in later sections.

The unit sphere in three-dimensional space R3 is the set of points (x, y, z) such that x2 + y2 + z2 = 1. Let N = (0, 0, 1) be the "north pole", and let M be the rest of the sphere. The plane z = 0 runs through the center of the sphere; the "equator" is the intersection of the sphere with this plane.

For any point P on M, there is a unique line through N and P, and this line intersects the plane z = 0 in exactly one point P

' . Define the stereographic projection of P to be this point P

' in the plane.

For the stereographic projection to be performed on a computer, it must be expressed by explicit formulas. In Cartesian coordinates (x, y, z) on the sphere and (X, Y) on the plane, the projection and its inverse are given by the formulas


 * $$(X, Y) = \left(\frac{x}{1 - z}, \frac{y}{1 - z}\right),$$


 * $$(x, y, z) = \left(\frac{2 X}{1 + X^2 + Y^2}, \frac{2 Y}{1 + X^2 + Y^2}, \frac{-1 + X^2 + Y^2}{1 + X^2 + Y^2}\right).$$

In spherical coordinates (φ, θ) on the sphere (with φ the zenith and θ the azimuth) and polar coordinates (R, Θ) on the plane, the projection and its inverse are


 * $$(R, \Theta) = \left(\frac{\sin \varphi}{1 - \cos \varphi}, \theta\right),$$


 * $$(\varphi, \theta) = \left(2 \arctan\left(\frac{1}{R}\right), \Theta\right).$$

Here, φ is understood to have value π when R = 0. Also, there are many ways to rewrite these formulas using trigonometric identities. In cylindrical coordinates (r, θ, z) on the sphere and polar coordinates (R, Θ) on the plane, the projection and its inverse are


 * $$(R, \Theta) = \left(\frac{r}{1 - z}, \theta\right),$$


 * $$(r, \theta, z) = \left(\frac{2 R}{1 + R^2}, \Theta, \frac{R^2 - 1}{R^2 + 1}\right).$$

Properties
The stereographic projection defined in the preceding section sends the "south pole" (0, 0, &minus;1) to (0, 0), the equator to the unit circle, the southern hemisphere to the region inside the circle, and the northern hemisphere to the region outside the circle.

The projection is not defined at the projection point N = (0, 0, 1). Small neighborhoods of this point are sent to subsets of the plane far away from (0, 0). The closer P is to (0, 0, 1), the more distant its image is from (0, 0) in the plane. For this reason it is common to speak of (0, 0, 1) as mapping to "infinity" in the plane, and of the sphere as completing the plane by adding a "point at infinity". This notion finds utility in projective geometry and complex analysis. On a merely topological level, it illustrates how the sphere is homeomorphic to the one point compactification of the plane.





Stereographic projection is conformal, meaning that it preserves the angles at which curves cross each other (see figures). On the other hand, stereographic projection does not preserve area; in general, the area of a region of the sphere does not equal the area of its projection onto the plane. The area element is given in (X, Y) coordinates by
 * $$dA = \frac{4}{(1 + X^2 + Y^2)^2} \; dX \; dY.$$

Along the unit circle, where X2 + Y2 = 1, there is no infinitesimal distortion of area. Near (0, 0) areas are distorted by a factor of 4, and near infinity areas are distorted by arbitrarily small factors.

No map from the sphere to the plane can be both conformal and area-preserving. If it were, then it would be a local isometry and would preserve Gaussian curvature. The sphere and the plane have different Gaussian curvatures, so this is impossible.

The conformality of the stereographic projection implies a number of convenient geometric properties. Circles on the sphere that do not pass through the point of projection are projected to circles on the plane. Circles on the sphere that do pass through the point of projection are projected to straight lines on the plane. These lines are sometimes thought of as circles through the point at infinity, or circles of infinite radius.

All lines in the plane, when transformed to circles on the sphere by the inverse of stereographic projection, intersect each other at infinity. Parallel lines, which do not intersect in the plane, are tangent at infinity. Thus all lines in the plane intersect somewhere in the sphere &mdash; either transversally at two points, or tangently at infinity. (Similar remarks hold about the real projective plane, but the intersection relationships are different there.)



The loxodromes of the sphere map to curves on the plane of the form
 * $$R = e^{\Theta / a},\,$$

where the parameter a measures the "tightness" of the loxodrome. Thus loxodromes correspond to equiangular spirals. These spirals intersect radial lines in the plane at equal angles, just as the loxodromes intersect meridians on the sphere at equal angles.

The stereographic projection relates to the plane inversion in a simple way. Let P and Q be two points on the sphere with projections P' and Q' on the plane. Then P' and Q' are inversive images of each other in the image of the equatorial circle if and only if P and Q are reflections of each other in the equatorial plane.

Wulff net


Stereographic projection plots can be carried out by a computer using the explicit formulas given above. However, for graphing by hand these formulas are unwieldy; instead, it is common to use graph paper designed specifically for the task. To make this graph paper, one places a grid of parallels and meridians on the hemisphere, and then stereographically projects these curves to the disk. The result is called a stereonet or Wulff net (named for the Russian mineralogist George (Yuri Viktorovich) Wulff ).

In the figure, the area-distorting property of the stereographic projection can be seen by comparing a grid sector near the center of the net with one at the far right of the net. The two sectors have equal areas on the sphere. On the disk, the latter has nearly four times the area as the former; if one uses finer and finer grids on the sphere, then the ratio of the areas approaches exactly 4.

The angle-preserving property of the projection can be seen by examining the grid lines. Parallels and meridians intersect at right angles on the sphere, and so do their images on the Wulff net.



For an example of the use of the Wulff net, imagine that we have two copies of it on thin paper, one atop the other, aligned and tacked at their mutual center. Suppose that we want to plot the point (0.321, 0.557, -0.766) on the lower unit hemisphere. This point lies on a line oriented 60° counterclockwise from the positive x-axis (or 30° clockwise from the positive y-axis) and 50° below the horizontal plane z = 0. Once these angles are known, there are four steps: To plot other points, whose angles are not such round numbers as 60° and 50°, one must visually interpolate between the nearest grid lines. It is helpful to have a net with finer spacing than 10°; spacings of 2° are common.
 * 1) Using the grid lines, which are spaced 10° apart in the figures here, mark the point on the edge of the net that is 60° counterclockwise from the point (1, 0) (or 30° clockwise from the point (0, 1)).
 * 2) Rotate the top net until this point is aligned with (1, 0) on the bottom net.
 * 3) Using the grid lines on the bottom net, mark the point that is 50° toward the center from that point.
 * 4) Rotate the top net oppositely to how it was oriented before, to bring it back into alignment with the bottom net. The point marked in step 3 is then the projection that we wanted.

To find the central angle between two points on the sphere based on their stereographic plot, overlay the plot on a Wulff net and rotate the plot about the center until the two points lie on or near a meridian. Then measure the angle between them by counting grid lines along that meridian.

Other formulations and generalizations
Some authors define stereographic projection from the north pole (0, 0, 1) onto the plane z = &minus;1, which is tangent to the unit sphere at the south pole (0, 0, &minus;1). The values X and Y produced by this projection are exactly twice those produced by the equatorial projection described in the preceding section. For example, this projection sends the equator to the circle of radius 2 centered at the origin. While the equatorial projection produces no infinitesimal area distortion along the equator, this pole-tangent projection instead produces no infinitesimal area distortion at the south pole.



In general, one can define a stereographic projection from any point Q on the sphere onto any plane E such that As long as E meets these conditions, then for any point P other than Q the line through P and Q meets E in exactly one point P&prime;, which is defined to be the stereographic projection of P onto E.
 * E is perpendicular to the diameter through Q, and
 * E does not contain Q.

All of the formulations of stereographic projection described thus far have the same essential properties. They are smooth bijections (diffeomorphisms) defined everywhere except at the projection point. They are conformal and not area-preserving.

More generally, stereographic projection may be applied to the n-sphere Sn in (n + 1)-dimensional Euclidean space En + 1. If Q is a point of Sn and E a hyperplane in En + 1, then the stereographic projection of a point P ∈ Sn &minus; {Q} is the point P&prime; of intersection of the line $$\scriptstyle\overline{QP}$$ with E.

Still more generally, suppose that S is a (nonsingular) quadric hypersurface in the projective space Pn + 1. By definition, S is the locus of zeros of a non-singular quadratic form f(x0, ..., xn + 1) in the homogeneous coordinates xi. Fix any point Q on S and a hyperplane E in Pn + 1 not containing Q. Then the stereographic projection of a point P in S &minus; {Q} is the unique point of intersection of $$\scriptstyle\overline{QP}$$ with E. As before, the stereographic projection is conformal and invertible outside of a "small" set. The stereographic projection presents the quadric hypersurface as a rational hypersurface. This construction plays a role in algebraic geometry and conformal geometry.

Complex analysis
Although any stereographic projection misses one point on the sphere (the projection point), the entire sphere can be mapped using two projections from distinct projection points. In other words, the sphere can be covered by two stereographic parametrizations (the inverses of the projections) from the plane. The parametrizations can be chosen to induce the same orientation on the sphere. Together, they describe the sphere as an oriented surface (or two-dimensional manifold).

This construction has special significance in complex analysis. The point (X, Y) in the real plane can be identified with the complex number ζ = X + iY. The stereographic projection from the north pole onto the equatorial plane is then


 * $$\zeta = \frac{x + i y}{1 - z},$$
 * $$(x, y, z) = \left(\frac{2 \mathrm{Re}(\zeta)}{1 + \bar \zeta \zeta}, \frac{2 \mathrm{Im}(\zeta)}{1 + \bar \zeta \zeta}, \frac{-1 + \bar \zeta \zeta}{1 + \bar \zeta \zeta}\right).$$

Similarly, letting ξ = X &minus; iY be another complex coordinate, the functions


 * $$\xi = \frac{x - i y}{1 + z},$$
 * $$(x, y, z) = \left(\frac{2 \mathrm{Re}(\xi)}{1 + \bar \xi \xi}, \frac{2 \mathrm{Im}(\xi)}{1 + \bar \xi \xi}, \frac{1 - \bar \xi \xi}{1 + \bar \xi \xi}\right).$$

define a stereographic projection from the south pole onto the equatorial plane. The transition maps between the ζ- and ξ-coordinates are then ζ = 1 / ξ and ξ = 1 / ζ, with ζ approaching 0 as ξ goes to infinity, and vice versa. This facilitates an elegant and useful notion of infinity for the complex numbers and indeed an entire theory of meromorphic functions mapping to the Riemann sphere. The standard metric on the unit sphere agrees with the Fubini-Study metric on the Riemann sphere.

Visualization of lines and planes


The set of all lines through the origin in three-dimensional space forms a space called the real projective plane. This space is difficult to visualize, because it cannot be embedded in three-dimensional space.

However, one can "almost" visualize it as a disk, as follows. Any line through the origin intersects the southern hemisphere $$ z \leq 0$$ in a point, which can then be stereographically projected to a point on a disk. Horizontal lines intersect the southern hemisphere in two antipodal points along the equator, either of which can be projected to the disk; it is understood that antipodal points on the boundary of the disk represent a single line. (See quotient topology.) So any set of lines through the origin can be pictured, almost perfectly, as a set of points in a disk.

Also, every plane through the origin intersects the unit sphere in a great circle, called the trace of the plane. This circle maps to a circle under stereographic projection. So the projection lets us visualize planes as circular arcs in the disk. Prior to the availability of computers, stereographic projections with great circles often involved drawing large-radius arcs that required use of a beam compass. Computers now make this task much easier.

Further associated with each plane is a unique line, called the plane's pole, that passes through the origin and is perpendicular to the plane. This line can be plotted as a point on the disk just as any line through the origin can. So the stereographic projection also lets us visualize planes as points in the disk. For plots involving many planes, plotting their poles produces a less-cluttered picture than plotting their traces.

This construction is used to visualize directional data in crystallography and geology, as described below.

Other visualization
Stereographic projection is also applied to the visualization of polytopes. In a Schlegel diagram, an n-dimensional polytope in Rn + 1 is projected onto an n-dimensional sphere, which is then stereographically projected onto Rn. The reduction from Rn + 1 to Rn can make the polytope easier to visualize and understand.

Arithmetic geometry
In elementary arithmetic geometry, stereographic projection from the unit circle provides a means to describe all primitive Pythagorean triples. Specifically, stereographic projection from the north pole (0,1) onto the x-axis gives a one-to-one correspondence between the rational number points (x,y) on the unit circle (with y ≠ 1) and the rational points of the x-axis. If (m/n, 0) is a rational point on the x-axis, then its inverse stereographic projection is the point


 * $$\left(\frac{2mn}{n^2+m^2}, \frac{n^2-m^2}{n^2+m^2}\right)$$

which gives Euclid's formula for a Pythagorean triple.

Cartography


The fact that no map from the sphere to the plane can accurately represent both angles (and thus shapes) and areas is the fundamental problem of cartography. In general, area-preserving map projections are preferred for statistical applications, because they behave well with respect to integration, while angle-preserving (conformal) map projections are preferred for navigation.

Stereographic projection falls into the second category. When the projection is centered at the Earth's north or south pole, it has additional desirable properties: It sends meridians to rays emanating from the origin and parallels to circles centered at the origin.

Crystallography


In crystallography, the orientations of crystal axes and faces in three-dimensional space are a central geometric concern, for example in the interpretation of X-ray and electron diffraction patterns. These orientations can be visualized as in the section Visualization of lines and planes above. That is, crystal axes and poles to crystal planes are intersected with the northern hemisphere and then plotted using stereographic projection. A plot of poles is called a pole figure.

In electron diffraction, Kikuchi line pairs appear as bands decorating the intersection between lattice plane traces and the Ewald sphere thus providing experimental access to a crystal's stereographic projection. Model Kikuchi maps in reciprocal space, and fringe visibility maps for use with bend contours in direct space, thus act as road maps for exploring orientation space with crystals in the transmission electron microscope.

Geology
Researchers in structural geology are concerned with the orientations of planes and lines for a number of reasons. The foliation of a rock is a planar feature that often contains a linear feature called lineation. Similarly, a fault plane is a planar feature that may contain linear features such as slickensides.

These orientations of lines and planes at various scales can be plotted using the methods of the Visualization of lines and planes section above. As in crystallography, planes are typically plotted by their poles. Unlike crystallography, the southern hemisphere is used instead of the northern one (because the geological features in question lie below the Earth's surface). In this context the stereographic projection is often referred to as the equal-angle lower-hemisphere projection. The equal-area lower-hemisphere projection defined by the Lambert azimuthal equal-area projection is also used, especially when the plot is to be subjected to subsequent statistical analysis such as density contouring.

Photography


Some fisheye lenses use a stereographic projection to capture a wide angle view. These lenses are usually preferred to more traditional fisheye lenses, which use an equal-area projection. This is probably a result of the conformal property of the stereographic: even areas close to the edge retain their shape, and straight lines are less curved. Unfortunately stereographic fisheye lenses are expensive to manufacture (none are currently being produced). Image remapping software, such as Panotools, allows the automatic remapping of photos from an equal-area fisheye to a stereographic projection.

The stereographic projection has been used to map spherical panoramas. This results in interesting effects: the area close to the point opposite to the center of projection becomes significantly enlarged, resulting in an effect known as little planet (when the center of projection is the nadir) and tube (when the center of projection is the zenith).

Compared to other azimuthal projections, the stereographic projection tends to produce especially visually pleasing panoramas; this is due to the excellent shape preservation that is a result of the conformality of the projection.