#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)


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.


The following program reads latitude and longitude values in decimal degrees, performs Mercator projection with a Clarke 1866 ellipsoid and a 33\(de 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)))
	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);


libproj.a - library of projections and support procedures

RELATED TO proj…, proj(1),

Cartographic Projection Procedures for the UNIX Environment–A User's Manual, (Evenden, 1990, Open-file report 90-284).