Geometric Forms of Projection

Most forms of projection operate by projecting Earth coordinates onto a geometric shape that can be easily flattened to a two-dimensional image. Three geometric shapes are frequently used:

Shape	Description
Cylinder	Earth coordinates may be projected onto a cylinder. The cylinder is cut lengthwise and unrolled to make a two-dimensional map. This type of projection is called a cylindrical projection. Some characteristics of cylindrical projections include the following: § Lines of longitude are parallel to each other. § Lines of latitude are parallel to each other. § Lines of longitude are at right angles to lines of latitude. § Regions near the equator or selected standard parallels are minimally distorted. § Regions near the poles are highly distorted. Cylindrical projections in Didger are: Equidistant Cylindrical, Gauss/Gauss-Kruger, Mercator, Miller Cylindrical, Transverse Mercator, and Universal Transverse Mercator.
Cone	Earth coordinates may be projected onto a cone. The point of the cone is usually directly above the pole and the sides of the cone pass through the globe at two user-defined latitudes, called the Standard Parallels. At the standard parallels, there is no difference between the east-west and north-south scales. The cone is cut from tip to base and unrolled to make a two-dimensional map. This type of projection is called a conic projection. Some characteristics of conic projections include the following: § Lines of latitude form concentric arcs. § Lines of longitude are straight and radiate outward from the tip of the imaginary cone. Conic projections in Didger are: Albers Equal Area, Equidistant Conic, Lambert Conformal Conic, and Polyconic.
Plane	Earth coordinates may be projected directly onto a flat plane. This type of projection is called an azimuthal projection. Projections of this type are recommended for maps of polar regions because cylindrical and conic projections generally either have severe distortion in polar regions or are unable to project coordinates in polar regions. The most notable characteristic of azimuthal projections is that the side of the Earth that is facing away from the center of the projection is not visible. Planar projections in Didger are: Azimuthal Equidistant, Lambert Azimuthal Equal Area, Orthographic, and Stereographic.
Other	Projections in this category are pseudocylindrical, pseudoconic, or based on some other mathematical projection or mathematical tables. These projections include: Eckert IV, Eckert VI, Mollweide, Robinson, Robinson-Sterling, Sinusoidal, State Plane*, and Unprojected Lat/Long.

* The State Plane Coordinate System uses Transverse Mercator, Lambert Conformal Conic, or Hotine Oblique Mercator, depending on the zone.