A simple correction of the error, using the area defined by the dash-dot outline (Figure 4) can be calculated; however, this correction is almost linearly proportional to the maximum pore diameter (with a small squared component for the two quarter circle regions), so it only represents a re-scaling of results. Furthermore, this correction is insufficient, because it fails to recognise the effect of there being no neighbouring pores with centres outsize the defined zone, to provide an exclusion zone effect. The result of this is that pores are disproportionally more likely to appear near to the edges of the defined zone. The effect of such disproportional appearance of pores near the upper and lower boundary of the defined zone will have some influence on the appropriate value for the porosity calculation. A more significant effect arises from the disproportional appearance of pores near to the right hand side, i.e. immediately in the sub-surface area. It is an effect that will be difficult to quantify, and is an unintended consequence of the pore placement heuristic. Having pointed out this failing, it is also necessary to remark that all these effects become less significant for the placement of larger numbers of smaller pores.