Azimuthal projections


Azimuthal projectionWith azimuthal (planar) projections, the spherical (globe) grid is projected onto a flat plane, thus it is also called a plane projection. The poles are the "normal aspect" (the viewpoint or perspective) which results in the simplest projected grid for this family of projections. That is, the plane is normally placed above the north or south pole. Normally only one hemisphere, or a portion of it, is represented on azimuthal projections. When projected from the centre of the globe with the normal aspect, the typical grid appearance for azimuthal projections shows parallels forming concentric circles, while meridians radiate out from the centre. If the imaginary projecting lightsource is inside the globe a gnomonic projection results, if the light is antipodal a stereographic, and if at infinity, an orthographic.

If you have a VRML-compatible browser installed on your computer, you may want to look at the following 3D virtual model of a cube, made up of six separate gnomonic azimuthal projetcions. If you do not have a plugin capable of rendering Virtual Reality Models, you can download e.g. the Cosmo player or the Viscape player.
General characteristics

Perspective projections

Stereographic projection Stereographic projection
  • Conformal
  • Parallels get closer near the centre
  • Scale is true at the centre
  • Every circle is shown as circular arcs or straight lines
  • A perspective projection of the globe onto a tangent (or secant) plane from a point opposite the point of the projection centre
  • Used in the polar aspect for topographic maps of polar regions
  • Recommended for conformal mapping of regions that are circular in shape
Oblique orthographic projection Orthographic projection
  • Neither conformal nor equal-area (general distortion)
  • Parallels get closer near the centre
  • Scale is true at the centre and along any circle centred on the projection centre but only the circumference of the circle
  • A perspective projection of the globe onto a tangent plane from an infinite distance
  • All great or small circles are shown as elliptical arcs or straight lines
  • It is frequently used for views of the Earth as would be seen from space
Gnomonic projection Gnomonic projection
  • Neither conformal nor equal-area
  • A perspective projection from the centre of the globe onto a tangent plane at the central point of the projection
  • Is used by navigators and aviators because great-circle paths (shortest distances or orthodromes) are shown as straight lines
  • Less than one hemisphere can be viewed from a given origin
  • Scale is true at the centre
  • All great circles are shown as straight lines

Non-perspective projections

Postel azimuthal projection Postel azimuthal equidistant projection
  • Equidistant. The distance between any two points on a straight line passing through the centre of the projection is shown at true scale. Distortion of other properties increases away from the center point.
  • Can view the entire Earth, but more than a hemisphere is rarely depicted
  • Used in the polar aspect for maps of polar regions, and the northern and southern hemispheres. It is sometimes used to show air-route distances
Lambert azimuthal projection Lambert azimuthal equal-area projection
  • Equal-area
  • Scale is true only at the centre but in all directions
  • Can view the entire Earth, but angular distortions are extremely bad on the edges
  • Frequently used in the polar aspect in atlases for maps of polar regions and of the northern and southern hemispheres. The equatorial aspect is used for atlas maps of the eastern and western hemispheres. The oblique aspect is used for atlas maps of continents and oceans
  • Recommended for equal-area maps of regions that are nearly circular

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