Albers Conical Equal Area

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The Albers Conical Equal Area projection is mathematically based on a cone that is conceptually secant on two parallels. There is no areal deformation. The North or South Pole is represented by an arc. It retains its properties at various scales, and individual sheets can be joined along their edges.

Construction

Cone

Property

Equal-area

Meridians

Meridians are straight lines converging on the polar axis, but not at the pole.

Parallels

Parallels are arcs of concentric circles concave toward a pole.

Graticule spacing

Meridian spacing is equal on the standard parallels and decreases toward the poles. Parallel spacing decreases away from the standard parallels and increases between them. Meridians and parallels intersect each other at right angles. The graticule spacing preserves the property of equivalence of area. The graticule is symmetrical.

Linear scale

Linear scale is true on the standard parallels. Maximum scale error is 1.25% on a map of the United States (48 states) with standard parallels of 29.5°N and 45.5°N.

Uses

Thematic maps.

Large countries with an east-west orientation.

Maps based on the Albers Conical Equal Area for Alaska use standard parallels 55°N and 65°N; for Hawaii, the standard parallels are 8°N and 18°N. The National Atlas of the United States, United States Base Map (48 states), and the Geologic map of the United States are based on the standard parallels of 29.5°N and 45.5°N.

This projection produces very accurate area and distance measurements in the middle latitudes. Thus, Albers Conical Equal Area is well-suited to countries or continents where north-south depth is about 3/5 the breadth of east-west. When this projection is used for the continental US, the two standard parallels are 29.5° and 45.5° North.

This projection possesses the property of equal-area, and the standard parallels are correct in scale and in every direction. Thus, there is no angular distortion (that is, meridians intersect parallels at right angles), and conformality exists along the standard parallels. Like other conics, Albers Conical Equal Area has concentric arcs for parallels and equally spaced radii for meridians. Parallels are not equally spaced, but are farthest apart between the standard parallels and closer together on the north and south edges.

Albers Conical Equal Area is the projection exclusively used by the USGS for sectional maps of all 50 states of the US in the National Atlas of 1970.

Albers Conical Equal Area Projection

map_projection_albers_conical

In the figure above, the standard parallels are 20°N and 60°N. Note the change in spacing of the parallels.