area_poly2.c

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#include <grass/gis.h>


double G_planimetric_polygon_area(const double *x, const double *y, int n)
{
    double x1, y1, x2, y2;
    double area;

    x2 = x[n - 1];
    y2 = y[n - 1];

    area = 0;
    while (--n >= 0) {
        x1 = x2;
        y1 = y2;

        x2 = *x++;
        y2 = *y++;

        area += (y2 + y1) * (x2 - x1);
    }

    if ((area /= 2.0) < 0.0)
        area = -area;

    return area;
}