robust_cross_product()

void robust_cross_product	(	const GEOGRAPHIC_POINT *	p,
		const GEOGRAPHIC_POINT *	q,
		POINT3D *	a
	)

Computes the cross product of two vectors using their lat, lng representations.

Good even for small distances between p and q.

Definition at line 634 of file lwgeodetic.c.

{
        double lon_qpp = (q->lon + p->lon) / -2.0;
        double lon_qmp = (q->lon - p->lon) / 2.0;
        double sin_p_lat_minus_q_lat = sin(p->lat-q->lat);
        double sin_p_lat_plus_q_lat = sin(p->lat+q->lat);
        double sin_lon_qpp = sin(lon_qpp);
        double sin_lon_qmp = sin(lon_qmp);
        double cos_lon_qpp = cos(lon_qpp);
        double cos_lon_qmp = cos(lon_qmp);
        a->x = sin_p_lat_minus_q_lat * sin_lon_qpp * cos_lon_qmp -
               sin_p_lat_plus_q_lat * cos_lon_qpp * sin_lon_qmp;
        a->y = sin_p_lat_minus_q_lat * cos_lon_qpp * cos_lon_qmp +
               sin_p_lat_plus_q_lat * sin_lon_qpp * sin_lon_qmp;
        a->z = cos(p->lat) * cos(q->lat) * sin(q->lon-p->lon);
}

References GEOGRAPHIC_POINT::lat, GEOGRAPHIC_POINT::lon, POINT3D::x, POINT3D::y, and POINT3D::z.

Referenced by clairaut_geographic(), edge_distance_to_point(), edge_intersection(), edge_point_side(), and sphere_angle().

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