Calculating distance between two GPS coordinates. Result in meters.

distance.c

#include <stdio.h>
#include <math.h>
#include <stdlib.h>

#define ER (6371007.2)
#define RAD(degrees) (degrees * (M_PI / 180))

main(int argc, char **argv)
{
   double dLa1, dLo1, dLa2, dLo2;
   double dLa, dLo, dA, dM;

   dLa1 = RAD(atof(argv[1]));
   dLo1 = RAD(atof(argv[2]));
   dLa2 = RAD(atof(argv[3]));
   dLo2 = RAD(atof(argv[4]));

   dLa = dLa2 - dLa1;
   dLo = dLo2 - dLo1;

   dA = sin(dLa/2) * sin(dLa/2) + cos(dLa1) * cos(dLa2) * sin(dLo/2) * sin(dLo/2);
   dM = (ER * (2 * atan2(sqrt(dA), sqrt(1-dA))));
   printf("%.5lf\n", dM);
}