Information for PROJ user

The library is included as part of PROJ starting with version 4.9.0 (released in 2015), where it is used as the computational backend for geod(1) and to implement some projections.

If PROJ is installed on your system, that you can compiled your application with cmake. A skeleton CMakeLists.txt is

cmake_minimum_required (VERSION 3.13.0)
project (proj-example C)
 
find_package (PROJ REQUIRED)
 
add_executable (proj-example proj-example.c)
target_link_libraries (proj-example ${PROJ_LIBRARIES})

This compiles and links the sample code proj-example.c.

/**
 * @file proj-example.c
 * @brief An example of computing geodesics with the PROJ library
 **********************************************************************/
 
#include <stdio.h>
#include <proj.h>
#include <geodesic.h>
 
int main (void) {
  /* Compute approximately equally spaced way points between LAX, 33.94N
     118.41W, and CHC, 43.49S 172.53E. */
  double a, invf;
  struct geod_geodesic g;
  struct geod_geodesicline l;
  const int n = 30;
  double lat[n+1], lon[n+1], azi[n+1];
  int i;
  {
    /* Use PROJ to get the ellipsoid parameters */
    PJ_CONTEXT* C = proj_context_create();
    char* argv[3] = {"type=crs", "proj=longlat", "ellps=WGS84"};
    PJ* P = proj_create_argv(C, 3, argv);
    if (P == 0) {
      fprintf(stderr, "Failed to create transformation object.\n");
      return 1;
    }
    proj_ellipsoid_get_parameters(C, proj_get_ellipsoid(C, P),
                                  &a, 0, 0, &invf);
    proj_destroy(P);
    proj_context_destroy(C);
  }
  /* Initialize geodesic */
  geod_init(&g, a, invf != 0 ? 1/invf : 0);
  /* Don't need GEOD_DISTANCE_IN if using GEOD_ARCMODE */
  geod_inverseline(&l, &g, 33.94, -118.41, -43.49, 172.53,
                   GEOD_LATITUDE | GEOD_LONGITUDE | GEOD_AZIMUTH);
  printf("latitude  longitude  azimuth\n");
  for (i = 0; i <= n; ++i) {
    /* Use GEOD_LONG_UNROLL so longitude varies continuously */
    geod_genposition(&l, GEOD_ARCMODE | GEOD_LONG_UNROLL,
                     i * l.a13 / n, lat + i, lon + i, azi + i, 0, 0, 0, 0, 0);
    printf("%.5f %.5f %.5f\n", lat[i], lon[i], azi[i]);
  }
  return 0;
}

In this example, PROJ provides the following

• access to the header file geodesic.h;
• access to the geodesic code bundled into the PROJ library;
• a mechanism for retrieving the parameters (equatorial radius and flattening) defining the ellipsoid.

To compile and run this example, do

cmake -B BUILD -S .
cd BUILD
make
./proj-example

which will print out 31 way points from Los Angeles to Christchurch starting with

latitude  longitude  azimuth
33.94000 -118.41000 -136.41544
31.49945 -121.08843 -137.86373
29.00508 -123.62942 -139.14437
...

Other points to note for PROJ programmers:

• The shape of the ellipsoid is given by the flattening f. f = 0 denotes a sphere. f < 0 denotes a prolate ellipsoid.
• The size of the ellipsoid is given by the equatorial radius a. This is usually given in meters. However, any units may be used and these are the units used for all distance-like quantites computed by the geodesic routines.
• All angle like quantities are expressed in degrees, not radians.