Map projections

Projections are usually categorized by the type of the surface onto which the Earth is projected.

Surface	Description
Cylindrical	The projection is a cylinder wrapped around the globe (for example, the Mercator projection).
Conic	The projection is a cone placed over the globe (for example, the Lambert Conformal Conic projection).
Azimuthal	The globe is projected onto a plane (for example, the Stereographic projection).

Other projections can be formed by modifying the basic types (for example, pseudo-cylindrical). The projection type is defined under the "General class" heading of the Supported Projections table.

In projecting a sphere (the Earth) on a surface (the image), the geometry of the sphere changes. A given projection usually retains some characteristics of the sphere, usually at the expense of other characteristics. The following terms list "characteristics" for the projections, where:

Equal area implies that a feature on one part of the map covers the same area if it also occurs on any other part of the map.
Conformal implies the correct form or shape and infers that the relative local angles around every point of the map are correct. This only applies to smaller, local areas. All projections distort the shape for larger areas.
Equidistant projections show true scale between one point (or two points) and every other point on the map.
Rectangular coordinates mean that the grid of the projection can be used to approximate distances, directions, and areas of the Earth surface. Rectangular coordinates are also referred to as plane coordinates.

Note: For CATALYST Professional programs, the Earth model for the projection can be defined in the MAPUNITS string or in the ELLIPS parameter (see Earth models).

Some projections can process the Earth as an ellipsoid or a sphere, while some only process the Earth as a sphere. For more information, refer to the projection under the "Earth model" heading of the Supported Projections table. This is important if the Earth model is not supported and is defined as values in the ELLIPS parameter. For projections that use an ellipsoid or sphere, ELLIPS can be:

the semi-major axis (or equatorial radius or A) for the Earth
the semi-minor axis (or polar radius or B) or the eccentricity squared

If the unsupported Earth model is a sphere, it can be defined in one of the following ways:

Do not specify a second value (that is, only one radius is defined)
Specify the equatorial radius again (that is, the polar radius equals the equatorial radius)
Specify the second value as zero (that is, the eccentricity squared is zero)

In general, non-azimuthal projections (that is, cylindrical or conic) usually use an ellipsoid or sphere. If no Earth model is defined for a projection that uses an ellipsoid or sphere, the Earth model ellipsoid defaults to D000 (that is, WGS84).

Projections that use a sphere use the first value in ELLIPS as the radius of the Earth. If a second value is defined, it is ignored.

Azimuthal projections usually use a sphere, but not an ellipsoid. If no Earth model is defined for projections that use a sphere, the default is D000 (that is, WGS84).