PJ_INIT(3U)PJ_INIT(3U)NAMEpj_init - initialize cartographic projection
pj_init_plus - initialize cartographic projec‐
tion
pj_fwd - forward cartographic projection
pj_inv - inverse cartographic projection
pj_transform - transform between coordinate sys‐
tems
pj_free - de-initialize projection
SYNOPSIS
#include <proj_api.h>
projPJ pj_init(int argc, char **argv)
projPJ pj_init_plus(const char *defn)
projUV pj_fwd(projUV val, projPJ proj)
projUV pj_inv(projUV val, projPJ proj)
int pj_transform(projPJ src_cs, projPJ dst_cs, long point_count,
int point_offset, double *x, double *y, double *z)
void pj_free(projPJ proj)
DESCRIPTION
Procedure pj_init selects and initializes a car‐
tographic projection with its argument control
parameters. Argc is the number of elements in
the array of control strings argv that each con‐
tain individual cartographic control keyword as‐
signments (+ proj arguments). The list must
contain at least the proj=projection and Earth's
radius or elliptical parameters. If the ini‐
tialization of the projection is successful a
valid address is returned otherwise a NULL val‐
ue.
The pj_init_plus function operates similarly to
pj_init but takes a single string containing the
definition, with each parameter prefixed with a
plus sign. For example "+proj=utm +zone=11
+ellps=WGS84".
Once initialization is performed either forward
or inverse projections can be performed with the
returned value of pj_init used as the argument
proj. The argument structure projUV values u
and v contain respective longitude and latitude
or x and y. Latitude and longitude are in radi‐
ans. If a projection operation fails, both ele‐
ments of projUV are set to HUGE_VAL (defined in
math.h).
Note: all projections have a forward mode, but
some do not have an inverse projection. If the
projection does not have an inverse the projPJ
structure element inv will be NULL.
The pj_transform function may be used to trans‐
form points between the two provided coordinate
systems. In addition to converting between car‐
tographic projection coordinates and geographic
coordinates, this function also takes care of
datum shifts if possible between the source and
destination coordinate system. Unlike pj_fwd
and pj_inv it is also allowable for the coordi‐
nate system definitions (PJ *) to be geographic
coordinate systems (defined as +proj=latlong).
The x, y and z arrays contain the input values
of the points, and are replaced with the output
values. The point_offset should indicate the
spacing the of x,y,z arrays, normally 1. The
function returns zero on success, or the error
number (also in pj_errno) on failure.
Memory associated with the projection may be
freed with pj_free.
EXAMPLE
The following program reads latitude and longi‐
tude values in decimal degrees, performs Merca‐
tor projection with a Clarke 1866 ellipsoid and
a 33° latitude of true scale and prints the pro‐
jected cartesian values in meters:
#include <proj_api.h>
main(int argc, char **argv) {
char *args[] = { "proj=merc", "ellps=clrk66", "lat_ts=33" };
projUV p;
projPJ pj;
if (!(pj = pj_init(3, args)))
exit(1);
while (scanf("%lf %lf", &p.v, &p.u) == 2) {
p.u *= DEG_TO_RAD;
p.v *= DEG_TO_RAD;
p = pj_fwd(p, pj);
printf("%.2f\t%.2f\n", p.u, p.v);
}
exit(0);
}
LIBRARY
libproj.a - library of projections and support
procedures
SEE ALSO
http://proj.osgeo.org/ProjAPI, proj(1),
Cartographic Projection Procedures for the UNIX
Environment—A User's Manual, (Evenden, 1990,
Open-file report 90-284).
HOME PAGE
http://proj.osgeo.org
2001/04/05 Rel. 4.4 PJ_INIT(3U)
