# geodesic man page

**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

```
#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/1.44/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

## See Also

Full online documentation for **geodesic(3)**,

http://geographiclib.sf.net/1.44/C

**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/s00190-012-05…

http://geographiclib.sf.net/geod-addend…

The *online geodesic bibliography*,

http://geographiclib.sf.net/geodesic-pa…