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Decision Support System for Map Projections of Small Scale Data

Albers Equal Area Conic

  • Usage
  • Albers Equal Area Conic Projection

    Usage:*

            The Albers projection is exclusively used by the USGS for sectional maps of all 50 U.S. states in the National Atlas of 1970, and for other U.S. Maps at scales of 1:2,500,000 or smaller. The latter maps include the base maps of the United States issued in 1961,1967, and 1972, the Tectonic Map of the United States (1962), and the Geological Map of the United States (1974), all at 1:2,500,000.

            The scale factor along the meridians is the reciprocal of the scale factor along the parallels in order to maintain equal area. An important characteristic of all normal conic projections is that scale is constant along any given parrallel. Like other normal conics, the Albers Equal-Area Conic projections have concentric arcs of circles for parallels and equally spaced radii as meridians. The parallels are not equally spaced, but they are fathest apart in the latitudes between the standard parallels and closer together to the north and south. The pole is not the center of the circles, but is normally an arc itself.

            If the pole is taken as one of the two standard parallels, Albers formulas reduce to a limiting form of the projection called Lambert's Equal-Area Conic. If the pole is the only standard parallel, the Albers formulas simplifies to provide the polar aspect to the Lambert Azimuthal Equal-Area. In both of these limiting cases, the pole is the point. If the Equator is the one standard parrallel, the projection becomes Lambert's Cylindrical Equal-Area, but the formulas must be modified. None of these extreme cases applies to the normal use of Albers, with standard parallels in the temperate zones, such as usage for the United States.

            To map a given region, standard parallels should be selected to minimize variations in scale. Standard parallels are correct in scale along the parrallel and they are correct in every direction. Thus, there is no angular distortion, and conformality exists along these standard parallels, even on an equal area projection.

    * Usage information source:

    Snyder, John P. Map Projections - A Working Manual Paper U.S. Geological Survey Professional Paper 1395. Washington: United States Government Printing Office, 1987.

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