area_sphere.c

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


static double M;


/*
 * r is radius of sphere, s is a scaling factor
 */

int G_begin_zone_area_on_sphere(double r, double s)
{
    return (M = s * 2.0 * r * r * M_PI);
}


double G_darea0_on_sphere(double lat)
{
    return (M * sin(Radians(lat)));
}


double G_area_for_zone_on_sphere(double north, double south)
{
    return (G_darea0_on_sphere(north) - G_darea0_on_sphere(south));
}