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## Albers Conic Equal-Area Projection (-–Jb -–JB)

This projection, developed by Albers in 1805, is predominantly used to map regions of large east-west extent, in particular the United States. It is a conic, equal-area projection, in which parallels are unequally spaced arcs of concentric circles, more closely spaced at the north and south edges of the map. Meridians, on the other hand, are equally spaced radii about a common center, and cut the parallels at right angles. Distortion in scale and shape vanishes along the two standard parallels. Between them, the scale along parallels is too small; beyond them it is too large. The opposite is true for the scale along meridians. To define the projection in GMT you need to provide the following information:

  Longitude and latitude of the projection center
  Two standard parallels
  Map scale in inch/degree or 1:xxxxx notation (-Jb), or map width (-JB)

Note that you must include the ``1:'' if you choose to specify the scale that way. E.g., you can say 0.5 which means 0.5 inch/degree or 1:200000 which means 1 inch on the map equals 200,000 inches along the standard parallels. The projection center defines the origin of the rectangular map coordinates. As an example we will make a map of the region near Taiwan. We choose the center of the projection to be at 125 oE/20 oN and 25 oN and 45 oN as our two standard parallels. We desire a map that is 5 inches wide. The complete command needed to generate the map below is therefore given by:

    gmtset GRID_CROSS_SIZE 0
    pscoast -R110/140/20/35 -JB125/20/25/45/5i -B10g5 -Dl -G200 -W0.25p -A250 -P > GMT_albers.ps

Figure 5.6: Albers equal-area conic map projection

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