NAME
pj_init - initialize cartographic projection
pj_init_plus - initialize cartographic projection
pj_fwd - forward cartographic projection
pj_inv - inverse cartographic projection
pj_transform - transform between coordinate systems
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 cartographic projection
with its argument control parameters. Argc is the number of elements
in the array of control strings argv that each contain individual
cartographic control keyword assignments (+ proj arguments). The list
must contain at least the proj=projection and Earth’s radius or
elliptical parameters. If the initialization of the projection is
successful a valid address is returned otherwise a NULL value.
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 radians. If a projection operation fails, both elements 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 transform points between the
two provided coordinate systems. In addition to converting between
cartographic 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 coordinate 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 longitude values in decimal
degrees, performs Mercator projection with a Clarke 1866 ellipsoid and
a 33° latitude of true scale and prints the projected 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(1U),
Cartographic Projection Procedures for the UNIX EnvironmentA Users
Manual, (Evenden, 1990, Open-file report 90-284).
HOME PAGE
http://proj.osgeo.org
2001/04/05 Rel. 4.4