page_title: Stereographic projection
section_title: First formulation
hierarchical_section_title: Stereographic projection / Definition / First formulation
context_page_description: In geometry, the stereographic projection is a particular mapping that projects a sphere onto a plane. The projection is defined on the entire sphere, except at one point: the projection point. Where it is defined, the mapping is smooth and bijective. It is conformal, meaning that it preserves angles at which curves meet. It is neither isometric 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 stereographic net, shortened to stereonet, or Wulff net.
context_section_description: 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.
  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
  In spherical coordinates (φ, θ) on the sphere (with φ the zenith angle, 0 ≤ φ ≤ π, and θ the azimuth, 0 ≤ θ ≤ 2π) and polar coordinates (R, Θ) on the plane, the projection and its inverse are
  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
caption: Stereographic projection of the unit sphere from the north pole onto the plane z = 0, shown here in cross section