PROJ -- Projection Reference

Why are projections needed for image processing? Projections are required to tie-down an image to the earth's surface. Once the required projection information has been defined and the image has been corrected to overlay the projection bounds, image equirectangular pixel and line coordinates can be transformed to produce equirectangular projection easting and northing coordinates which can be transformed with the projection formulas to produce equivalent non-equirectangular longitude and latitude coordinates. Or the inverse can be done: longitude and latitude coordinates can be transformed to projection easting and northings and image database pixel and line coordinates.

Projections are useful to compensate for distortions in large-scale imagery that cover a large earth surface, where distortions are produced from earth curvature. These are the same distortions that occur in projecting a spherical surface (the earth) onto a flat surface (the image database). Although there are still distortions in all projections, some are removed or compensated for depending on the projection used.

Once an image is tied to one projection, it can automatically be projected to overlay another projection. This gives you options to easily process your data, by matching your goals to the appropriate projection.

The software used for the projection transforms is the GCTP (General Cartographic Transformation Package) produced by the U.S. Geological Survey (See the REFERENCES section.). It can be used to convert:

projection coordinates to geographic coordinates
geographic coordinates to projection coordinates
projection coordinates to coordinates in a second projection

Transforms between different earth models is discussed under the EARTH MODELS subtopic.

The supported projections are: (Note: The following information for a projection is repeated at the top of that projection subtopic in the PROJECTION section.)

Code      General Class     Name                            Earth Model
--------  ----------------  ------------------------------  -------------------
ACEA      conic             Albers Conical Equal Area       ellipsoid or sphere
AE        azimuthal         Azimuthal Equidistant           sphere
                            (or Zenithal Equidistant)
CASS      cylindrical       Cassini                         ellipsoid or sphere
                            (or Cassini-Soldner)
EC        conic             Equidistant Conic               ellipsoid or sphere
                            (or Simple Conic)
                            (or Conic)
ER        cylindrical       Equirectangular                 sphere
                            (or Equidistant Cylindrical)
                            (or Simple Cylindrical)
                            (or Rectangular)
                            (or Plate Carree)
GNO       azimuthal         Gnomonic                        sphere
                            (or Gnomic)
GOOD      pseudo-cylindric  Goode's Homolosine              sphere
GVNP      azimuthal         Gen Vert Near-Side Perspective  sphere
KROV      conic             Krovak                          Bessel 1841
LAEA      azimuthal         Lambert Azimuthal Equal-Area    sphere
                            (or Zenithal Equal-Area)
LBSG      azimuthal         Lebanese Stereographic          sphere
LCC       conic             Lambert Conformal Conic         ellipsoid or sphere
LONG/LAT  not a projection  Longitude/Latitude              ellipsoid or sphere
                            (or Geographic)
MC        cylindrical       Miller Cylindrical              sphere
MER       cylindrical       Mercator                        ellipsoid or sphere
MSC       modif azimuthal   Modif Stereographic Conformal   Clarke 1866 (NAD27)
                            (or Alaska Grid)
OG        azimuthal         Orthographic                    sphere
OM        cylindrical       Oblique Mercator                ellipsoid or sphere
                            (or Oblique Cylindrical)
                            (or Orthomorphic)
                            (or Hotine)
PC        conic             Polyconic                       ellipsoid or sphere
PS        azimuthal         Polar Stereographic             ellipsoid or sphere
ROB       pseudo-cylindric  Robinson                        sphere
RSO       cylindrical       Rectified Skew Orthomorphic     ellipsoid or sphere
SG        azimuthal         Stereographic                   sphere
SGDO      azimuthal         Stereographic Double            ellipsoid or sphere
SIN       pseudo-cylindric  Sinusoidal                      sphere
                            (or Sanson-Flamsteed)
SOM       modif cylindric   Space Oblique Mercator          ellipsoid or sphere
SPCS      usually conic     State Plane Coordinate System   Clarke 1866 (NAD27)
          or cylindrical                                    or GRS 1980 (NAD83)
TM        cylindrical       Transverse Mercator             ellipsoid or sphere
                            (or Gauss-Krueger)
UPS       azimuthal         Universal Polar Stereographic   ellipsoid or sphere
UTM       cylindrical       Universal Transverse Mercator   ellipsoid or sphere
VDG       miscellaneous     Van der Grinten                 sphere

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