Ptolemy’s coordinates are in many cases obviously inaccurate (Silchester and Exeter being two examples): the mistakes may have been his or later transcribers’ but, either way, we have to go by the manuscripts. I would think that combining a number of these coordinates would not necessarily result in a sort of “accurate” average, but it would be more accurate than the least accurate figures. It may be less accurate than the most accurate, but we can’t be sure which ones are the most accurate. We can, though, look for ‘patterns’ which work.
Bearing in mind that my own reconstruction hasn’t been realised with 100% accuracy (oh, no, it hasn’t: I was a bit flighty), I looked for alignments of three towns, one of which in each case was Iscalis and the other two were places whose positions we know. If these lines cross each other, how closely would they show Iscalis in the same place when transferred to a modern map?
So these were *approximate* (NB! Really!) alignments that I used on my reconstruction:
And this is how the lines converged:
It looks as if there’s a very rough agreement among Ptolemy’s Roman towns as to where Iscalis was located. But what happens if the alignments are drawn on a modern map? As it happens, we’re still swimming in the Bristol Channel, but not nearly as far adrift as we were with the previous attempt:
The two red circles show the mouth of the river Axe by Brean Down and, slightly further south, the mouth of the river Parrett. The two small black dots show Cheddar and, to the north-east, Charterhouse-on-Mendip. More follows …