Gnomonic¶

For a sphere, the gnomonic projection is a projection from the center of the sphere onto a plane tangent to the center point of the projection. This projects great circles to straight lines. For an ellipsoid, it is the limit of a doubly azimuthal projection, a projection where the azimuths from 2 points are preserved, as the two points merge into the center point. In this case, geodesics project to approximately straight lines (these are exactly straight if the geodesic includes the center point). For details, see Section 8 of [Karney2013].

Classification	Azimuthal
Available forms	Forward and inverse, spherical and ellipsoidal
Defined area	Within a quarter circumference of the center point
Alias	gnom
Domain	2D
Input type	Geodetic coordinates
Output type	Projected coordinates

Parameters¶

Note

All parameters are optional for the Gnomonic projection.

+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.

+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.

+ellps=<value>¶

The name of a built-in ellipsoid definition.

See Ellipsoids for more information, or execute proj -le for a list of built-in ellipsoid names.

Defaults to "GRS80".

+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.