A sensible choice of graph to overcome the issues with nearest-neighbour graphs is a triangulation. A triangulation of a grid G is a set of simplices (N -dimensional analogues of triangles) that tile the convex hull H(G) (see, for example, N D in Fig. ). Any triangulation immediately satisfies the requirements given in section II C 1 and possesses high-quality numerical gradients, even for the pathological case of a random grid, as can be seen in Fig. .