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. Due to the nature of projecting a three-dimensional surface onto a two-dimensional surface, a projection cannot have all four characteristics at one time. Therefore, the purpose of the map must be considered when selecting a projection.

Some of these characteristics include the following:

Characteristic	Description
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, 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/Gauss-Kruger, Lambert Conformal Conic, Mercator, Stereographic, 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 and Equidistant Cylindrical projections are equidistant.
Azimuthal	A projection is said to be azimuthal when 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, Lambert Azimuthal Equal Area, Orthographic, and Stereographic.

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.

Some projections are not strong in any one of the four characteristics and are not listed above.

If your map has a projection to it, it is very important to specify the appropriate projection settings to get accurate length/area results.