Each standard projection is displayed with the Prime Meridian (longitude 0) being a straight vertical line, along which North is up. The orientation of nonstandard projections is specified by the three parameters=c(lat,lon,rot). Imagine a transparent gridded sphere around the globe. First turn the overlay about the North Pole so that the Prime Meridian (longitude 0) of the overlay coincides with meridian lon on the globe. Then tilt the North Pole of the overlay along its Prime Meridian to latitude lat on the globe. Finally again turn the overlay about its 'North Pole' so that its Prime Meridian coincides with the previous position of (the overlay) meridian rot. Project the desired map in the standard form appropriate to the overlay, but presenting information from the underlying globe. In the descriptions that follow each projection is shown as a function call, if it requires parameters, these are shown as arguments to the function. The descriptions are grouped into families. Equatorial projections centered on the Prime Meridian (longitude 0). Parallels are straight horizontal lines. Azimuthal projections centered on the North Pole. Parallels are concentric circles. Meridians are equally spaced radial lines. Polar conic projections symmetric about the Prime Meridian. Parallels are segments of concentric circles. Except in the Bonne projection, meridians are equally spaced radial lines orthogonal to the parallels. Retroazimuthal projections. At every point the angle between vertical and a straight line to 'Mecca', latitude lat0 on the prime meridian, is the true bearing of Mecca. Maps based on the spheroid. Of geodetic quality, these projections do not make sense for tilted orientations. list with components named x and y, containing the projected coordinates. NAs project to NAs. Points deemed unprojectable (such as north of 80 degrees latitude in the Mercator projection) are returned as NA. Because of the ambiguity of the first two arguments, the other arguments must be given by name. Each time mapproject is called, it leaves on frame 0 the dataset .Last.projection, which is a list with components projection, parameters, and orientation giving the arguments from the call to mapproject or as constructed (for orientation). Subsequent calls to mapproject will get missing information from .Last.projection. Since map uses mapproject to do its projections, calls to mapproject after a call to map need not supply any arguments other than the data. Richard A. Becker, and Allan R. Wilks, 'Maps in S', AT&T Bell Laboratories Statistics Research Report, 1991. http://www.research.att.com/areas/stat/doc/93.2.ps M. D. McIlroy, Documentation from the Tenth Edition UNIX Manual, Volume 1, Saunders College Publishing, 1990. inside-R.org is a collection of resources about the open-source R Project for the R Statistics Community. This site is sponsored by Revolution Analytics. Source.