This third and last of the chapters to examine the surviving copy marks a transition to making the lost original map the primary focus of attention. The issue addressed here – of how accurately our copy may be reckoned to reproduce the original – has the greatest importance, but in fact the confidence with which assessments can be offered must vary considerably. If the danger were not already obvious enough by its very nature, Ptolemy himself warns that repeated copying of a map leads to the compounding of errors. Moreover, even for the production of the original, it may be unrealistic to assume that the data was uniformly correct and complete as supplied or marked in the first instance.

Thereafter, omissions, slips, and distortions are all too easily introduced by copyists through oversight or inaccuracy, especially in the case of line work. Copyists are also liable to make deliberate omissions, adjustments, and additions. Where parchment is used – as in the case of our copy – certain adjustments may patently be mere shifts in the placement of names in order to avoid holes or cuts. More generally, however, there will often be no means for copyists' initiatives to be detected once made, let alone reappraised. Some may reflect worthy intentions in the interests of improving and updating the map. Others, by contrast, may at best be decorative licence on the part of copyists keen to embroider it in some way or other, and unconcerned that its integrity might suffer as a result.