For the function f(X) the contour line of value z is represented as L(z) where L(z) = {X ∈ F : f(X) = z}.
A contour set whose boundary is a contour line is the set of all points having values of f(X) ≤ z.
Francis [7] has shown that for point-sized NF and EFs, the contour line is continuous, with the corresponding
contour set being convex. However the finite sizes of the NF and the EFs may present
complications as given below:
1. Contour lines may be intercepted by an EF and hence could be disconnected.
2. The finite-sized NF may interfere with the traversal path of flow between facilities in the region Q
associated with the affected traversal path. Therefore points inside Q may have an objective function
value higher than that of the nearby points. This can make the contour line disconnected.