Equal-Area Projection

Overview Equal-area projections, also called equivalent projections, maintain the proportional relationship between areas on the map and corresponding areas on the Earth's surface. If one region is twice the size of another on Earth, it will appear twice as large on an equal-area map. This property is achieved by systematically adjusting the projection to compensate for scale distortions, necessarily at the expense of shape distortion (angles and local shapes are not preserved).

Several widely used projections preserve area. The Albers Equal-Area Conic is the standard for mapping the contiguous United States and other mid-latitude regions with east-west extent. The Lambert Azimuthal Equal-Area is used for continental and hemispheric maps. The Mollweide projection creates an elliptical world map frequently used for global thematic data. The Sinusoidal projection preserves area with simple mathematics and is used for mapping tropical regions. The Goode Homolosine interrupts the projection surface to minimize distortion for world maps showing distributions.

Equal-area projections are required whenever maps are used for visual comparison of geographic extents or densities. Choropleth maps showing population density, election results, or economic indicators must use equal-area projections to avoid misleading the viewer. Environmental assessments comparing land cover or deforestation across regions depend on accurate area representation. Atlas maps presenting thematic data at global or continental scales typically use equal-area projections.

Preserving area requires distorting shapes, particularly at the periphery of the map. Features far from the projection's standard lines or center point may appear stretched or compressed. This distortion can affect the visual perception of features even though their areas are correct. For mapping at large scales or small geographic extents, the difference between equal-area and conformal projections may be negligible.