In cartography and geometry, the stereographic projection is a mapping that projects each point on a sphere onto a tangent plane along a straight line from the antipode of the point of tangency (with one exception: the center of projection, antipodal to the point of tangency, is not projected to any point in the Euclidean plane. It is thought of as corresponding to a "point at infinity". One approaches that point at infinity by continuing in any direction at all; in that respect this situation is unlike the projective plane, which has many points at infinity.

Two notable properties of this projection were demonstrated by Hipparchus:

• this mapping is conformal, i.e., it preserves the angles at which curves cross each other, and

• this mapping transforms those circles on the surface of the sphere that do not pass through the center of projection to circles on the plane. It transforms circles on the sphere that do pass through the center of projection to straight lines on the plane (these are sometimes thought of as circles through a point at infinity).

Contents

1 Formula 2 Loxodromes on a stereographic projection 3 See also 4 External link

Formula

On a sphere, let φ be azimuth and θ be co-latitude (angular distance from the pole). Let R be the radius of the sphere. Let the points of the sphere be projected stereographically onto a plane which is tangent to the pole. Let the points of the projection have coordinates ρ_P (radial distance away from origin) and θ_P. Then the projection is

<math> \theta_P = \phi, \qquad \qquad (1) <math>

<math> \rho_P = 2 R \tan {\theta \over 2}. \qquad \qquad (2) <math>

If θ_L is, instead, the latitude, then the equation for ρ_P changes to

<math> \rho_P = 2 R \tan {{\pi \over 2} - \theta_L \over 2 } \qquad \qquad (3) <math>

or, equivalently,

<math> \rho_P = 2 R ( \sec \theta_L - \tan \theta_L). <math>

Loxodromes on a stereographic projection

It is possible to find the equations of loxodromes on the stereographic projection. A loxodrome on a sphere is described by

<math> \phi = a \ln \left| \tan \left( {\theta_L \over 2} + {\pi \over 4} \right) \right|. <math>

Substituting equation (1) we obtain

<math> \theta_P = a \ln \left| \tan \left( {\theta_L \over 2} + {\pi \over 4} \right) \right|. \qquad \qquad (4) <math>

Equation (3) can be solved for θ_L:

<math> \theta_L = {\pi \over 2} - 2 \arctan \rho_P. \qquad \qquad (5)<math>

Substitute equation (5) into equation (4), then simplify,

<math> \theta_P = a \ln \left| \tan \left( {\pi \over 2} - \arctan \rho_P \right) \right|. \qquad \qquad (6)<math>

Apply the following trigonometric identity

<math> \tan \left({\pi \over 2} - \theta\right) = { 1 \over \tan \theta } <math>

to equation (6), yielding

<math> \theta_P = a \ln \left| {1 \over \tan \left( - \arctan \rho_P \right)} \right|<math>

<math> \theta_P = a \ln \left| {1 \over - \rho_P} \right| = a \ln \left| {1 \over \rho_P} \right| = -a \ln \rho_P.<math>

Let b = −1/a; then

<math> \rho_P = e^{b \theta_P}, <math>

therefore a loxodrome on a stereographic projection is a equiangular spiral.

See also

• map projection
• Riemann sphere
• projective geometry

External link

• http://mathworld.wolfram.com/StereographicProjection.html

ru:Стереографическая проекция