Characteristics of Projections

Some projections are imbued with characteristics that tell us if certain types of measurements (e.g. measurements of distance, area, etc.) are accurate on the projected map. Some of these characteristics include the following:

Equal Area	A projection is said to be equal area when the area of any given part of the map covers the same area on the Earth as any other part of the map of the same size. For example, if a one inch diameter circle on the map covers a 100 mile diameter circle on the Earth's surface, then we know that a one inch diameter circle anywhere else on the map is known to cover another 100 mile diameter circle on the Earth. In order for a projection to be equal area, however, consistency in the shapes, scales, and/or angles across the map must be sacrificed. Equal area projections include Albers Equal Area, Bonne, Eckert IV, Eckert VI, Lambert Azimuthal Equal Area, Mollweide, and Sinusoidal.
Conformal	A projection is said to be conformal when the local angles for points on the map are represented accurately. This means that the angles between any given point and any nearby points are accurate, but are not necessarily accurate for widely separated points on the map. A side effect is that conformal projections preserve the precise perpendicular intersections between parallels and meridians on the map. When mapping smaller areas, relative shape is preserved. In order for a projection to be conformal, however, consistency in the surface areas, shapes, and/or scales across the map must be sacrificed. Conformal projections include Gauss-Kruger / Gauss Conformal, Hotine Oblique Mercator, Lambert Conformal Conic, Mercator, Oblique Mercator, State Plane Coordinate System Projections, Transverse Mercator, and Universal Transverse Mercator.
Equidistant	A projection is said to be equidistant when the scale between at least one specific origin point on the map with respect to every other point on the map is represented accurately. In order for a projection to be equidistant, however, consistency in the surface areas, shapes, and/or angles across the map must be sacrificed. The Azimuthal Equidistant, Equidistant Cylindrical, Equidistant Conic, and Cassini projections are equidistant.
Azimuthal	With a projection of the azimuthal form, the direction of (or angle to) all points on the map are accurate with respect to the center point of the projection. Azimuthal projections include Azimuthal Equidistant, Gnomonic, Lambert Azimuthal Equal Area, Orthographic, and Stereographic.
None of the Above	Some projections try to minimize the effects of all distortions and as a result do not minimize any one distortion in particular. These projections include Polyconic, Robinson and Robinson-Sterling, Unprojected Lat/Long, and Van der Grinten.

In addition to the characteristics described above, some projections have highly specialized characteristics that may be useful in certain applications. For example, on maps made with a Mercator projection, all lines of constant direction (rhumb lines) are known to be straight, thereby making such maps very desirable as navigational charts.