Equal-area projection
Definition
An equal-area projection (also known as an equivalent or authalic projection) is a type of map projection that preserves the property of area, meaning that all regions on the map are shown in correct proportional relation to their actual sizes on the Earth's surface; while this property allows for accurate visual comparison of the extent of geographic features�such as countries, continents, or ecosystems�without areal distortion, it necessarily distorts other properties like shapes, angles, and scale to achieve this equivalence, making it particularly valuable for thematic mapping where representing statistical data distribution (e.g., population density, land use, or resource allocation) accurately by area is more critical than maintaining precise shapes.
Application
A primary practical application of an equal-area projection is in thematic cartography for socioeconomic and environmental analysis. For instance, when creating a world map depicting global forest cover loss, per-capita carbon emissions, or population density, using an equal-area projection like the Mollweide or Goode's Homolosine ensures that the area of each country or region is represented in its true proportion. This prevents larger landmasses from being visually exaggerated (as they are in the common Mercator projection) and allows for an accurate, fair visual comparison of statistical data across different geographic units, enabling policymakers, researchers, and the public to correctly assess the relative magnitude and distribution of global phenomena without areal bias.
FAQ
What is the main advantage of an equal-area projection?
The primary and defining advantage is that it preserves the correct relative size of all geographic areas. A square kilometer in Greenland covers the same map area as a square kilometer in Africa. This makes it the only valid choice for maps where comparing the size, density, or extent of different regions is the goal, such as for population density, agricultural land, forest cover, or the distribution of a resource.
If it's so accurate for area, why isn't it used for all world maps?
To preserve area, equal-area projections must distort shape, angle, and scale. Continents and countries often appear stretched, compressed, or sheared, especially near the map edges. For navigation charts, where preserving compass bearings (rhumb lines) is critical, or for general reference maps where recognizable shapes are preferred, projections like Mercator are used instead. Equal-area maps sacrifice shape fidelity to achieve area fidelity.
What are some common examples of equal-area projections?
Several well-known projections are designed to be equal-area:
Albers Conic: Excellent for mid-latitude countries (e.g., USA, China);
Lambert Azimuthal Equal-Area: Ideal for mapping hemispheres or polar regions;
Mollweide & Goode's Homolosine: Often used for thematic world maps;
Sinusoidal: Simple projection used for global datasets, but with extreme shape distortion at the edges.
How do I choose the right equal-area projection for my map?
The choice depends on the location and extent of your area of interest:
For the entire world: Use a global equal-area projection like Mollweide or Goode's Homolosine (which interrupts the oceans to minimize shape distortion).
For a continent or large country: Use a conic projection like Albers with two standard parallels tuned to your region's latitude.
For a polar region: Use Lambert Azimuthal Equal-Area centered on the pole.
Most GIS software (QGIS, ArcGIS) will suggest appropriate equal-area projections based on your map's extent.
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