Checks if first three points are collinear - in the spherical
        case this corresponds to all points lying on a great circle
        and, hence, all coordinate vectors being in a single plane.

def _is_collinear(self, lons, lats):
        """
        Checks if first three points are collinear - in the spherical
        case this corresponds to all points lying on a great circle
        and, hence, all coordinate vectors being in a single plane.
        """

        x, y, z = lonlat2xyz(lons[:3], lats[:3])
        pts = np.column_stack([x, y, z])

        collinearity = (np.linalg.det(pts.T) == 0.0)

        return collinearity