The Forgery
For a discussion of the forgery's presentation and acceptance, see this "forgery" section of the article on Charles Bertram, the forger.
Once it had been accepted as genuine, De Situ Britanniae exerted a profound effect upon subsequent theories, suppositions, and publications of history. It was the premier source of information (and sometimes the only source) for well over 100 years.
It contained 18 "iters" (of the type found in the legitimate Antonine Itinerary), compiled from fragmentary accounts of a Roman general, adding over 60 new and previously unknown stations to those mentioned in the legitimate account. Best of all, it filled up the entire map of Scotland with descriptions and the names of peoples, the part of Britain about which the least was known with any certainty.
It would later be determined that it was actually a clever mosaic of information gleaned from the works of Caesar, Tacitus, William Camden, John Horsley, and others, enhanced with Bertram's own fictions.
Read more about this topic: De Situ Britanniae