.\" @(#)geodesic.3
.nr LL 7.0i
.TH GEODESIC 3 "2014/12/17 Rel. 4.9.0" 
.ad b
.hy 1
.SH NAME
.B geod_init
\- initialize an ellipsoid
.br
.B geod_lineinit
\- initialize a geodesic line
.br
.B geod_position
\- a position on a geodesic line
.br
.B geod_direct
\- the direct geodesic problem
.br
.B geod_inverse
\- the inverse geodesic problem
.br
.B geod_polygonarea
\- the area of a polygon
.br
.SH SYNOPSIS
.nf
#include <geodesic.h>
.br
and link against the \fBproj\fR library.
.SH 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 |\fIf\fR| < 1/50,
where \fIf\fR is the flattening.  Note that the geodesic routines
measure angles (latitudes, longitudes, and azimuths) in degrees, unlike
the rest of the \fBproj\fR 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/1.43/C
.SH EXAMPLE
The following program reads in lines with the coordinates for two points
in decimal degrees (\fIlat1\fR, \fIlon1\fR, \fIlat2\fR, \fIlon2\fR) and
prints out \fIazi1\fR, \fIazi2\fR, \fIs12\fR for the geodesic line
between each pair of points on the WGS84 ellipsoid.  (N.B. \fIazi2\fR is
the forward azimuth at point 2.)
.nf
\f(CW

#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\en",
               &lat1, &lon1, &lat2, &lon2) == 4) {
    geod_inverse(&g, lat1, lon1, lat2, lon2,
                 &s12, &azi1, &azi2);
    printf("%.8f %.8f %.3f\en", azi1, azi2, s12);
  }
  return 0;
} \fR
.br
.fi
.SH LIBRARY
libproj.a \- library of projections and support procedures
.SH SEE ALSO
Full online documentation for \fBgeodesic(3)\fR,
.br
http://geographiclib.sf.net/1.43/C
.PP
.B geod(1)
.PP
\fBGeographicLib\fR, http://geographiclib.sf.net
.PP
The \fBGeodesicExact\fR class in GeographicLib solves the geodesic
problems in terms of elliptic integrals; the results are accurate for
arbitrary \fIf\fR.
.PP
C. F. F. Karney, \fIAlgorithms for Geodesics\fR,
.br
J. Geodesy \fB87\fR, 43-55 (2013);
.br
DOI: http://dx.doi.org/10.1007/s00190-012-0578-z
.br
http://geographiclib.sf.net/geod-addenda.html
.PP
The \fIonline geodesic bibliography\fR,
.br
http://geographiclib.sf.net/geodesic-papers/biblio.html
.SH HOME PAGE
http://proj.osgeo.org