def check_surface_validity(cls, edges):
        """
        Check validity of the surface.

        Project edge points to vertical plane anchored to surface upper left
        edge and with strike equal to top edge strike. Check that resulting
        polygon is valid.

        This method doesn't have to be called by hands before creating the
        surface object, because it is called from :meth:`from_fault_data`.
        """
        # extract coordinates of surface boundary (as defined from edges)
        full_boundary = []
        left_boundary = []
        right_boundary = []

        for i in range(1, len(edges) - 1):
            left_boundary.append(edges[i].points[0])
            right_boundary.append(edges[i].points[-1])

        full_boundary.extend(edges[0].points)
        full_boundary.extend(right_boundary)
        full_boundary.extend(edges[-1].points[::-1])
        full_boundary.extend(left_boundary[::-1])

        lons = [p.longitude for p in full_boundary]
        lats = [p.latitude for p in full_boundary]
        depths = [p.depth for p in full_boundary]

        # define reference plane. Corner points are separated by an arbitrary
        # distance of 10 km. The mesh spacing is set to 2 km. Both corner
        # distance and mesh spacing values do not affect the algorithm results.
        ul = edges[0].points[0]
        strike = ul.azimuth(edges[0].points[-1])
        dist = 10.

        ur = ul.point_at(dist, 0, strike)
        bl = Point(ul.longitude, ul.latitude, ul.depth + dist)
        br = bl.point_at(dist, 0, strike)

        # project surface boundary to reference plane and check for
        # validity.
        ref_plane = PlanarSurface.from_corner_points(ul, ur, br, bl)
        _, xx, yy = ref_plane._project(
            spherical_to_cartesian(lons, lats, depths))
        coords = [(x, y) for x, y in zip(xx, yy)]
        p = shapely.geometry.Polygon(coords)
        if not p.is_valid:
            raise ValueError('Edges points are not in the right order')
Check validity of the surface.

        Project edge points to vertical plane anchored to surface upper left
        edge and with strike equal to top edge strike. Check that resulting
        polygon is valid.

        This method doesn't have to be called by hands before creating the
        surface object, because it is called from :meth:`from_fault_data`.