Space Oblique Mercator

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Space Oblique Mercator (SOM) projection is nearly conformal and has little scale distortion within the sensing range of an orbiting mapping satellite such as Landsat. It is the first projection to incorporate the Earth’s rotation with respect to the orbiting satellite.

Construction

Cylinder

Property

Conformal

Meridians

All meridians are curved lines except for the meridian crossed by the groundtrack at each polar approach.

Parallels

All parallels are curved lines.

Graticule spacing

No graticules.

Linear scale

Scale is true along the groundtrack, and varies approximately 0.01% within sensing range (Environmental Systems Research Institute, 1992).

Uses

Georectification of, and continuous mapping from, satellite imagery.

Standard format for data from Landsats 4 and 5 (Environmental Systems Research Institute, 1992).

The method of projection used is the modified cylindrical, for which the central line is curved and defined by the groundtrack of the orbit of the satellite.The line of tangency is conceptual and there are no graticules.

The SOM projection is defined by USGS. According to USGS, the X axis passes through the descending node for each daytime scene. The Y axis is perpendicular to the X axis, to form a Cartesian coordinate system. The direction of the X axis in a daytime Landsat scene is in the direction of the satellite motion—south. The Y axis is directed east. For SOM projections used by EOSAT, the axes are switched; the X axis is directed east and the Y axis is directed south.

The SOM projection is specifically designed to minimize distortion within sensing range of a mapping satellite as it orbits the Earth. It can be used for the rectification of, and continuous mapping from, satellite imagery. It is the standard format for data from Landsats 4 and 5. Plots for adjacent paths do not match without transformation (Environmental Systems Research Institute, 1991).

This projection contains these unique parameters:

Landsat vehicle ID (1-5)    Specify whether the data are from Landsat 1, 2, 3, 4, or 5.

Orbital path number (1-251 or 1-233)    For Landsats 1, 2, and 3, the path range is from 1 to 251. For Landsats 4 and 5, the path range is from 1 to 233.

Space Oblique Mercator Projection

map_projection_space_oblique_mercator

Source: Snyder and Voxland, 1989