Calculates the distance between two GPS points using the Law of Haversines

arc_distance.c

/****************************************************************************
 * arc_distance.c
 *
 * Calculates the distance between two GPS points using the Law of Haversines
 ***************************************************************************/
 
#include <stdio.h>
#include <math.h>

#define pi 3.14159265358979323846

double distance(double lat1, double lon1, double lat2, double lon2, char unit);
double deg2rad(double deg);
double rad2deg(double rad);

int main(void)
{
    double x = distance(-36.9050478, 174.9245144, 33.7676334, -84.5610344, 'K');
    printf("%f\n", x);
}

// Calculate the distance between two GPS points
double distance(double lat1, double lon1, double lat2, double lon2, char unit)
{
    double theta, dist;
    theta = lon1 - lon2;
    dist = sin(deg2rad(lat1)) * sin(deg2rad(lat2)) + cos(deg2rad(lat1)) * cos(deg2rad(lat2)) * cos(deg2rad(theta));
    dist = acos(dist);
    dist = rad2deg(dist);
    dist = dist * 60 * 1.1515;
    
    switch(unit)
    {
        case 'M':
            break;
        case 'K':
            dist = dist * 1.609344;
            break;
    }
    
    return dist;
}

// This function converts decimal degrees to radians
double deg2rad(double deg)
{
    return (deg * pi / 180);
}

// This function converts radians to decimal degrees
double rad2deg(double rad)
{
    return (rad * 180 / pi);
}