Longitude of the central meridian (the line of longitude at the center of a map projection, generally used as the basis for constructing the projection), and when subfield $a contains the text Albers conical equal area, Azimuthal equidistant, Equidistant conic, Equirectangular, Lambert conformal conic, Mercator, Miller cylindrical, Polyconic, Sinusoidal, Transverse Mercator, or Van der Grinten. Horizontal coordinate system. Map projection.
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