In each of these examples the surface or grid can be thought of as a single valued function z=f(x,y). Mathematically derived functions that apply to an entire region or to sub-sections of regions may be used to create grid datasets. For example the surface shown in Figure 6‑3 is a representation obtained by fitting a linear regression surface of the form z=ax+by+c to the set of spot heights provided by the GB Ordnance Survey for tile NT04.