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# Manual Reference Pages  -  GEODESIC (3)

### NAME

geod_init - initialize an ellipsoid
geod_lineinit - initialize a geodesic line
geod_position - a position on a geodesic line
geod_direct - the direct geodesic problem
geod_inverse - the inverse geodesic problem
geod_polygonarea - the area of a polygon

Synopsis
Description
Example
Library

### SYNOPSIS

```#include <geodesic.h>

and link against the proj library.

```

### DESCRIPTION

This library is a port of the geodesic routines in the C++ library, GeographicLib, to C. It solves the direct and inverse geodesic problems on an ellipsoid of revolution. In addition, the reduced length of a geodesic and the area between a geodesic and the equator can be computed. The results are accurate to round off for |f| < 1/50, where f is the flattening. Note that the geodesic routines measure angles (latitudes, longitudes, and azimuths) in degrees, unlike the rest of the proj library, which uses radians. The documentation for this library is included in geodesic.h. A formatted version of the documentation is available at http://geographiclib.sf.net/1.40/C

### EXAMPLE

The following program reads in lines with the coordinates for two points in decimal degrees (lat1, lon1, lat2, lon2) and prints out azi1, azi2, s12 for the geodesic line between each pair of points on the WGS84 ellipsoid. (N.B. azi2 is the forward azimuth at point 2.)
```

#include <stdio.h>
#include <geodesic.h>

int main() {
double a = 6378137, f = 1/298.257223563; /* WGS84 */
double lat1, lon1, azi1, lat2, lon2, azi2, s12;
struct geod_geodesic g;

geod_init(&g, a, f);
while (scanf("%lf %lf %lf %lf\n",
&lat1, &lon1, &lat2, &lon2) == 4) {
geod_inverse(&g, lat1, lon1, lat2, lon2,
&s12, &azi1, &azi2);
printf("%.8f %.8f %.3f\n", azi1, azi2, s12);
}
return 0;
}

```

### LIBRARY

libproj.a - library of projections and support procedures

Full online documentation for geodesic(3),
http://geographiclib.sf.net/1.40/C

geod(1)

GeographicLib, http://geographiclib.sf.net

The GeodesicExact class in GeographicLib solves the geodesic problems in terms of elliptic integrals; the results are accurate for arbitrary f.

C. F. F. Karney, Algorithms for Geodesics,
J. Geodesy 87, 43-55(2013);
DOI: http://dx.doi.org/10.1007/s00190-012-0578-z

The online geodesic bibliography,
http://geographiclib.sf.net/geodesic-papers/biblio.html