Contradicting Ptolemy
Ptolemy's placement of the Selgovae town of Trimontium was accepted to be somewhere along the southern coast of Scotland until William Roy (1726 – 1790) placed it far to the east at Eildon Hills, near Newstead. Roy was trying to follow an itinerary given in the 1757 De Situ Britanniae, and moving Ptolemy's Trimontium made the itinerary seem more logical according to his historical work, Military Antiquities of the Romans in North Britain (1790, published postumously in 1793). Roy did not alter Ptolemy's placement of the Selgovae in southern Scotland, but chose to assign Trimontium to a different people who were described in De Situ Britanniae.
When De Situ Britanniae was debunked as a fraud in 1845, Roy's misguided placement of Trimontium was retained by some historians, though he was no longer cited for his contribution. Furthermore, some historians not only accepted Roy's placement of Trimontium, but also returned the town to the Selgovae by moving their territory such that they would be near Eildon Hills. Ptolemy's placement of the Novantae in Galloway was retained, and since Ptolemy said that they were adjacent to the Selgovae, Novantae territory was greatly expanded beyond Galloway to be consistent with this thesis, which survives in a number of modern histories.
The result is that an 'error correction' to the sole legitimate historical reference (Ptolemy), made so that a fictional itinerary in De Situ Britanniae would seem more logical, is retained; and the sole legitimate historical reference is further 'corrected' by moving the Selgovae far from their only known location.
While Roy's historical work is largely ignored due to his unknowing reliance on a fraudulent source, his maps and drawings are untainted, and continue to be held in the highest regard.
Read more about this topic: Selgovae