sphere_signed_area()

static double sphere_signed_area ( const GEOGRAPHIC_POINT *  a, const GEOGRAPHIC_POINT *  b, const GEOGRAPHIC_POINT *  c  )

static

Computes the spherical area of a triangle.

If C is to the left of A/B, the area is negative. If C is to the right of A/B, the area is positive.

Parameters

a	The first triangle vertex.
b	The second triangle vertex.
c	The last triangle vertex.

Returns

the signed area in radians.

Definition at line 737 of file lwgeodetic.c.

References edge_point_side(), GEOGRAPHIC_EDGE::end, sphere_angle(), and GEOGRAPHIC_EDGE::start.

Referenced by ptarray_area_sphere().

{
        double angle_a, angle_b, angle_c;
        double area_radians = 0.0;
        int side;
        GEOGRAPHIC_EDGE e;

        angle_a = sphere_angle(b,a,c);
        angle_b = sphere_angle(a,b,c);
        angle_c = sphere_angle(b,c,a);

        area_radians = angle_a + angle_b + angle_c - M_PI;

        /* What's the direction of the B/C edge? */
        e.start = *a;
        e.end = *b;
        side = edge_point_side(&e, c);

        /* Co-linear points implies no area */
        if ( side == 0 )
                return 0.0;

        /* Add the sign to the area */
        return side * area_radians;
}

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