Determining the neighbours of a given grid point, as was done in Eq. 3 for a uniform grid, is a necessary prerequisite to performing a topographical analysis. Even simple topographical objects, such as local maxima and minima, are defined with reference to the behaviour of the function when moving to "nearby" points. It is possible to encode the necessary information about the neighbours of a given grid point in the edges of a neighbourhood graph N with nodes given by the points in G, and edges connecting each node x ∈ G to a set of neighbours N (x) ⊂ G \ x. In this section, the construction of such graphs is investigated.