Space Oblique Mercator (SOM)

The Space Oblique Mercator (SOM) projection is a generalization of the Oblique Mercator projection, itself an (oblique) aspect of the Mercator projection with origins in the 16th century. In the previously available map projections, ground tracks of satellites traced curved lines which complicated their use in representing satellite remote sensing data. In response, SOM was specifically designed to represent satellite ground tracks as straight lines. This had the effect of minimizing distortions of data along a ground track and allowing satellite data to be represented accurately as a rectangular array.



Available forms

Forward and inverse, spherical and ellipsoidal

Defined area






Input type

Geodetic coordinates

Output type

Projected coordinates

Space Oblique Mercator (SOM)

proj-string: +proj=som +inc_angle=98.303820000243860022 +ps_rev=0.06866666666666667 +asc_lon=64.412896137498847793

(gives same results as: +proj=som +inc_angle=1.7157253262878522r +ps_rev=0.06866666666666667 +asc_lon=1.1242171183417042r)




Longitude of the ascending node for the orbit (decimal degrees).


Inclination (vertical tilt) of the orbit with respect to the reference plane, measured at the ascending node (decimal degrees).


Time required for a single orbit (days).


To specify asc_lon or inc_angle in radians (instead of degrees). Follow the value with the "r" character.

As an example, to specify the ascending longitude as 1.1242171183417042 radians (~64.4129 decimal degrees): +asc_lon=1.1242171183417042r.

For more information see projections usage.



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.


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.


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".


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

See Ellipsoid size parameters for more information.


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.


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.

Further reading

  1. Space Oblique Mercator Wikipedia

  2. USGS Report on Space Oblique Mercator Mathematical Development