Albers equal-area conic projection
The Albers equal-area conic projection, is a map projection that uses two standard parallels to reduce some of the distortion of a projection with one standard parallel. Although neither shape nor linear scale is truly correct, the distortion of these properties is minimized in the region between the standard parallels. This projection is best suited for land masses extending in an east-to-west orientation rather than those lying north to south. It is used by the USGS for maps showing the conterminous United States (48 states) or large areas of the United States, as well as for many thematic maps.
Conic projections can either be tangent or secant to the spheroid. In the tangent case the cone just touches the Earth along a single line or at a point. In the secant case, as with the Albers projection, the cone intersects or cuts through the Earth as two circles. (The secant case for a plane intersects as a circle.) Whether tangent or secant, the location of this contact is important because it defines the line or point of least distortion on the map projection. This line of true scale is called the standard parallel or standard line.
In the simple (tangent) conic projection the scale is exaggerated both north and south of the standard parallel. To counteract this, the Albers projection uses two standard parallels, one in the upper and one in the lower part of the map. The greatest accuracy is obtained if the selected standard parallels enclose two-thirds the height of the map. The Albers projection combines very small scale error with equality of area.
The Albers projection is similar to the Lambert conformal conic projection, however, equal area (or equivalent) projections preserve the area (or the amount of space) within features. On a small-scale political map of the world, the areas within each country are preserved.
- ↑ Albers Equal Area Conic, ArcGIS Desktop 9.2 online help, Accessed 17 May 2010.
- ↑ Map Projections. USGS Publications. December 2000.
- ↑ Map Projections, in The Atlas of Canada, Natural Resources of Canada website, Accessed 17 May 2010.
- ↑ Raisz, Erwin. General Cartography (1938), p. 94-95. McGraw-Hill.
- ↑ Map processing. GIS Commons: An Introductory Textbook on Geographic Information Systems. Chapter 3. Accessed 17 May 2010.
- Geographic coordinate system
- Universal Transverse Mercator coordinate system
- Elements of Map Projection. (26 MB download) U.S. Coast and Geodetic Survey, Special Publication 68 (1938).
- Map Projections. USGS Publications. December 2000.
- What are map projections? ArcGIS 10 online help. Accessed 17 May 2010.
- Types of Projection