rhumbline.c

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#include <math.h>
#include <grass/gis.h>
#include "pi.h"


static int adjust_lat(double *);

#if 0
static int adjust_lon(double *);
#endif /* unused */

static double TAN_A, TAN1, TAN2, L;
static int parallel;


int G_begin_rhumbline_equation(double lon1, double lat1, double lon2,
                               double lat2)
{
    adjust_lat(&lat1);
    adjust_lat(&lat2);

    if (lon1 == lon2) {
        parallel = 1;           /* a lie */
        L = lat1;
        return 0;
    }
    if (lat1 == lat2) {
        parallel = 1;
        L = lat1;
        return 1;
    }
    parallel = 0;
    lon1 = Radians(lon1);
    lon2 = Radians(lon2);
    lat1 = Radians(lat1);
    lat2 = Radians(lat2);

    TAN1 = tan(M_PI_4 + lat1 / 2.0);
    TAN2 = tan(M_PI_4 + lat2 / 2.0);
    TAN_A = (lon2 - lon1) / (log(TAN2) - log(TAN1));
    L = lon1;

    return 1;
}


double G_rhumbline_lat_from_lon(double lon)
{
    if (parallel)
        return L;

    lon = Radians(lon);

    return Degrees(2 * atan(exp((lon - L) / TAN_A) * TAN1) - M_PI_2);
}


#if 0
static int adjust_lon(double *lon)
{
    while (*lon > 180.0)
        *lon -= 360.0;
    while (*lon < -180.0)
        *lon += 360.0;

    return 0;
}
#endif /* unused */


static int adjust_lat(double *lat)
{
    if (*lat > 90.0)
        *lat = 90.0;
    if (*lat < -90.0)
        *lat = -90.0;

    return 0;
}