Icosahedral Snyder Equal Area

Snyder's Icosahedral Equal Area map projection on an icosahedron polyhedral globe offers relatively low scale and angular distortion. The equations involved are relatively straight-forward. The interruptions remain a disadvantage, as with any low-error projection system applied to the entire globe [Snyder1992]. This projection is used as a basis for defining discrete global grid hierarchies.

Classification	Polyhedral, equal area
Available forms	Forward and inverse, spherical
Defined area	Global
Alias	isea
Domain	2D
Input type	Geodetic coordinates
Output type	Projected coordinates

proj-string: +proj=isea

Note

As the projection is only defined on a sphere, it should only be used with a spherical ellipsoid e.g., +R=6371007.18091875 for a sphere with the authalic radius of the WGS84 ellipsoid. For mapping coordinates on the WGS84 ellipsoid to the authalic sphere, the input latitude should be converted from geodetic latitude to authalic latitude. A future version may automatically perform this conversion when using a non-spherical ellipsoid.

Parameters

Note

All parameters are optional for the projection.

+orient=<string>

Can be set to either isea or pole. See Snyder's Figure 12 for pole orientation [Snyder1992].

Defaults to isea

+azi=<value>

Azimuth.

Not supported by the inverse.

Defaults to 0.0

+aperture=<value>

Not supported by the inverse.

Defaults to 3.0

+resolution=<value>

Not supported by the inverse.

Defaults to 4.0

+mode=<string>

Can be either plane, di, dd or hex.

Only plane supported by the inverse.

Defaults to plane

+lon_0=<value>

Central meridian/longitude of natural origin, longitude of origin or longitude of false origin (naming and meaning depend on the projection method).

Defaults to 0.0.

Note

The default convention is to interpret this value as decimal degrees. To specify radians instead, follow the value with the "r" character.

Example: +lon_0=1.570796r

See Projection Units for more information.

+lat_0=<value>

Latitude of natural origin, latitude of false origin or latitude of projection centre (naming and meaning depend on the projection method).

Defaults to 0.0.

Note

The default convention is to interpret this value as decimal degrees. To specify radians instead, follow the value with the "r" character.

Example: +lat_0=1.570796r

See Projection Units for more information.

+R=<value>

Radius of the sphere, given in meters. If used in conjunction with +ellps, +R takes precedence.

See Ellipsoid size parameters for more information.

+x_0=<value>

False easting, easting at false origin or easting at projection centre (naming and meaning depend on the projection method). Always in meters.

Defaults to 0.0.

+y_0=<value>

False northing, northing at false origin or northing at projection centre (naming and meaning depend on the projection method). Always in meters.

Defaults to 0.0.