man(1) Manual page archive

     MAP(3X)                      (local)                      MAP(3X)

          orient, normalize - map projections

          orient(lat, lon, rot)
          float lat, lon, rot;

          struct place {
               float latrad, latsin, latcos;
               float lonrad, lonsin, loncos;

          struct place *p;

          int (*projection())()

          int project(p, xptr, yptr)
          struct place p;
          float *xptr, *yptr;

          These functions are loaded with ld(1) option -lmap.  Most of
          them calculate maps for a spherical earth.  Each map projec-
          tion is available in one standard form, into which data must
          be normalized for transverse or nonpolar projections.  The
          projection is specified by invoking one of the following
          functions, known collectively as projection. Each function
          returns the name project of a function that will do the
          actual work of calculating coordinates, or 0 if the required
          projection is not available.  The short summaries of proper-
          ties pertain to standard maps of the earth in standard ori-

          Equatorial projections centered on the Prime Meridian (lon-
          gitude 0).  Parallels are straight horizontal lines.

               mercator() equally spaced straight meridians, confor-
               mal, straight compass courses
               sinusoidal() equally spaced parallels, equal-area
               cylequalarea(lat0) equally spaced straight meridians,
               equal-area, true scale on lat0
               cylindrical() central projection on tangent cylinder
               rectangular() equally spaced straight parallels and
               mollweide() (homalographic) equal-area, hemisphere is a

          Azimuthal projections centered on the North Pole.  Parallels
          are concentric circles.  Meridians are equally spaced radial

     MAP(3X)                      (local)                      MAP(3X)


               azequidistant() equally spaced parallels, true dis-
               tances from pole
               azequalarea() equal-area
               gnomonic() central projection on tangent plane,
               straight great circles
               perspective(dist) viewed dist earth radii from center
               of earth
               orthographic() viewed from infinity
               stereographic() conformal, viewed from opposite pole
               laue() radius = tan(2×colatitude), used in xray crys-

          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.

               conic(lat0) central projection on cone tangent at lat0
               bonne(lat0) equally spaced parallels, equal-area, par-
               allel lat0 developed from tangent cone
               lambert(lat0,lat1) conformal, true scale on lat0 and
               albers(lat0,lat1) equal-area, true scale on lat0 and

          Projections with bilateral symmetry about the Prime Meridian
          and the equator.

               polyconic() parallels developed from tangent cones uni-
               formly distributed along Prime Meridian
               aitoff() equal-area projection of globe onto 2-to-1
               ellipse, based on azimuthal equal area projection
               gilbert() conformal, maps whole sphere into a circle
               bicentric(lon0) points plotted at true azimuth from two
               centers on the equator at longitudes ±lon0, great cir-
               cles are straight lines (a stretched gnomonic projec-
               elliptic(lon0) points are plotted at true distance from
               two centers on the equator at longitude ±lon0

          Doubly periodic projections.

               guyou() conformal, W and E hemispheres are square
               square() conformal, world is square with Poles at diag-
               onally opposite corners
               tetra() conformal map onto regular tetrahedron with an
               edge tangent to the Prime Meridian at the South Pole,
               developed into equilateral triangle
               hex() conformal map onto a hexagon centered on North
               Pole, N and S hemispheres are equilateral triangles

     MAP(3X)                      (local)                      MAP(3X)


               mecca(lat0) equally spaced vertical meridians, straight
               line from any point to `Mecca' (lat0 on the Prime
               Meridian) has the true bearing of Mecca

          Maps based on the spheroid.  Of geodetic quality, these pro-
          jections do not make sense for tilted orientations (see
          below).  For descriptions, see corresponding maps above.


          The project functions return a value 1 for a good point, 0
          for a point on a wrong sheet (for example the back of the
          world in a perspective projection), and -1 for a point that
          is deemed unplottable.

          Each standard projection is displayed with the Prime Merid-
          ian being a straight vertical line, along which North is up.
          The orientation of nonstandard projections is specified by
          orient. 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 meridian rot. Project the desired map in the
          standard form appropriate to the overlay, but presenting
          information from the underlying globe.

          Normalize converts latitude-longitude coordinates on the
          globe to coordinates on the overlaid grid.  The coordinates
          and their sines and cosines are input to normalize in a
          structure pointed to by p. Transformed coordinates and their
          sines and cosines are returned in the same structure.

          The project functions convert normalized coordinates in the
          structure pointed to by p to x-y coordinates for the desired
          map.  Scaling may be determined from the x-y coordinates of
          selected points.

          Latitudes and longitudes are measured in degrees for ease of
          specification for orient and projection, but in radians for
          ease of calculation for normalize and project. In either
          case latitude is measured positive north of the equator, and
          longitude positive west of Greenwich.  Radian longitude
          should be limited to the range -pi <= lon < pi.

          map(7), map(5), plot(3)