def segment_midpoints_by_vertices(self, vertices):
        """
        Add midpoints to any segment connected to the vertices in the
        list / array provided.
        """

        segments = set()

        for vertex in vertices:
            neighbours = self.identify_vertex_neighbours(vertex)
            segments.update( min( tuple((vertex, n1)), tuple((n1, vertex))) for n1 in neighbours )

        segs = np.array(list(segments))

        new_midpoint_lonlats = self.segment_midpoints(segments=segs)

        return new_midpoint_lonlats