path: root/static/v10/man3/proj.3

.TH PROJ 3X bowell
.CT 2 graphics math
.br
.SH NAME
orient, normalize \- map projections
.SH SYNOPSIS
.B orient(lat, lon, rot)
.br
.B float lat, lon, rot;
.PP
.B normalize(p)
.br
.B struct place *p;
.SH DESCRIPTION
Users of
.IR map (7)
may skip to the description of `Projection generators'
below.
.PP
The functions
.I orient
and
.I normalize
plus a collection of map projection generators
are loaded by
option
.BR -lmap 
of
.IR ld (1).
Most of them
calculate maps for a spherical earth.
Each map projection is available in one standard
form, into which data must be normalized
for transverse
or nonpolar projections.
.PP
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
.I 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
.I lon
on the globe.
Then tilt the North Pole of the
overlay along its Prime Meridian to latitude
.I 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's) meridian
.I rot.
Project the desired map in
the standard form appropriate to the overlay, but presenting
information from the underlying globe.
It is not useful to use
.I orient
without using
.IR normalize .
.PP
.I Normalize
converts latitude-longitude coordinates on the globe
to coordinates on the overlaid grid.
The coordinates and their sines and cosines
are input to
.I normalize
in a
.B place
structure.
Transformed coordinates and their sines and cosines
are returned in the same structure.
.PP
.EX
.nr xx \w'12345678'
.ta \n(xxu +\n(xxu +\n(xxu +\n(xxu +\n(xxu +\n(xxu
	struct place {
		float radianlat, sinlat, coslat;
		float radianlon, sinlon, coslon;
	};
.EE
.PP
The projection generators
return a pointer to a function that converts normalized coordinates
to 
.I x-y
coordinates for the desired map, or
0 if the required projection
is not available.
The returned function is exemplified by
.I proj 
in this example:
.PP
.EX
.ta \n(xxu +\n(xxu +\n(xxu +\n(xxu +\n(xxu +\n(xxu
	struct place pt;
	int (*proj)() = mercator();
	float x, y;
.EE
.PP
.EX
	orient(45.0, 30.0, 180.0);	/* set coordinate rotation */
.EE
.PP
.EX
	. . .				/* fill in the pt structure */
	normalize(&pt);			/* rotate coordinates */
	if((*proj)(&pt, &x, &y) > 0)	/* project onto x,y plane */
		plot(x, y);
.EE
.PP
The projection function
.B (*proj)()
returns 1 for a good point,
0 for a point on a wrong
sheet (e.g. the back of the world in a perspective
projection), and \-1 for a point that is deemed
unplottable (e.g. points near the poles on a Mercator projection).
.PP
Scaling may be determined from the
.I x-y
coordinates of
selected points.
Latitudes and longitudes are measured in degrees for
ease of specification for
.I orient 
and the projection generators
but in radians for ease of calculation 
for
.I normalize
and
.I proj.
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
.if t .I \-\(*p\(<=lon<\(*p.
.if n .I -pi <= lon < pi.
.SS Projection generators
Equatorial projections centered on the Prime Meridian
(longitude 0).
Parallels are straight horizontal lines.
.br
.ns
.IP
.B mercator()
equally spaced straight meridians, conformal,
straight compass courses
.br
.B sinusoidal()
equally spaced parallels,
equal-area, same as
.I bonne(0)
.br
.B cylequalarea(lat0)
equally spaced straight meridians, equal-area,
true scale on
.I lat0
.br
.B cylindrical()
central projection on tangent cylinder
.br
.B rectangular(lat0)
equally spaced parallels, equally spaced straight meridians, true scale on
.I lat0
.br
.B gall(lat0)
parallels spaced stereographically on prime meridian, equally spaced straight
meridians, true scale on
.I lat0
.br
.B mollweide()
(homalographic) equal-area, hemisphere is a circle
.PP
Azimuthal projections centered on the North Pole.
Parallels are concentric circles.
Meridians are equally spaced radial lines.
.br
.ns
.IP
.B azequidistant()
equally spaced parallels,
true distances from pole
.br
.B azequalarea()
equal-area
.br
.B gnomonic()
central projection on tangent plane,
straight great circles
.br
.B perspective(dist)
viewed along earth's axis
.I dist
earth radii from center of earth
.br
.B orthographic()
viewed from infinity
.br
.B stereographic()
conformal, projected from opposite pole
.br
.B laue()
.IR radius " = tan(2\(mu" colatitude ),
used in xray crystallography
.br
.B fisheye(r)
.IR radius " = log(" colatitude / r ):
.I New Yorker
map from viewing pedestal of radius
.I r
degrees
.PP
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.
.br
.ns
.IP
.B conic(lat0)
central projection on cone tangent at
.I lat0
.br
.B simpleconic(lat0,lat1)
equally spaced parallels, true scale on
.I lat0
and
.I lat1
.br
.B lambert(lat0,lat1)
conformal, true scale on 
.I lat0
and 
.I lat1
.br
.B albers(lat0,lat1)
equal-area, true scale on
.I lat0
and 
.I lat1
.br
.B bonne(lat0)
equally spaced parallels, equal-area,
parallel
.I lat0
developed from tangent cone
.PP
Projections with bilateral symmetry about
the Prime Meridian
and the equator.
.br
.ns
.IP
.B polyconic()
parallels developed from tangent cones,
equally spaced along Prime Meridian
.br
.B aitoff()
equal-area projection of globe onto 2-to-1
ellipse, based on 
.I azequalarea
.br
.B lagrange()
conformal, maps whole sphere into a circle
.br
.B bicentric(lon0)
points plotted at true azimuth from two
centers on the equator at longitudes
.I \(+-lon0,
great circles are straight lines
(a stretched gnomonic projection)
.br
.B elliptic(lon0)
points are plotted at true distance from
two centers on the equator at longitudes
.I \(+-lon0
.br
.B globular()
hemisphere is circle,
circular arc meridians equally spaced on equator,
circular arc parallels equally spaced on 0- and 90-degree meridians
.br
.B vandergrinten()
sphere is circle,
meridians as in
.I globular,
circular arc parallels resemble 
.I mercator
.PP
Doubly periodic conformal projections.
.br
.ns
.IP
.B guyou()
W and E hemispheres are square
.br
.B square()
world is square with Poles
at diagonally opposite corners
.br
.B tetra()
map on tetrahedron with edge
tangent to Prime Meridian at S Pole,
unfolded into equilateral triangle
.br
.B hex()
world is hexagon centered
on N Pole, N and S hemispheres are equilateral
triangles
.PP
Miscellaneous projections.
.br
.ns
.IP
.B harrison(dist,angle)
oblique perspective from above the North Pole,
.I dist
earth radii from center of earth, looking
along the Date Line
.I angle
degrees off vertical
.br
.B trapezoidal(lat0,lat1)
equally spaced parallels,
straight meridians equally spaced along parallels,
true scale at
.I lat0
and
.I lat1
on Prime Meridian
.PP
Retroazimuthal projections.
At every point the angle between vertical and a straight line to
`Mecca', latitude
.I lat0
on the prime meridian,
is the true bearing of Mecca.
.br
.ns
.IP
.B mecca(lat0)
equally spaced vertical meridians
.br
.B homing(lat0)
distances to `Mecca' are true
.PP
Maps based on the spheroid.
Of geodetic quality, these projections do not make sense
for tilted orientations.
For descriptions, see corresponding maps above.
.br
.ns
.IP
.B sp_mercator()
.br
.B sp_albers(lat0,lat1)
.SH "SEE ALSO
.IR map (7), 
.IR map (5), 
.IR plot (3)
.SH BUGS
Only one projection and one orientation can be active at a time.
.br
The west-longitude-positive convention
betrays Yankee chauvinism.