Complex sentence: The Antonini Itinerarium is a valuable register, still extant, of the stations and distances along the various roads of the Roman empire, seemingly based on official documents, which were probably those of the survey organized by Julius Caesar, and carried out under Augustus.
Split into simpler sentences: The Antonini Itinerarium is a register, of the stations and distances along the various roads of the Roman empire. 'Antonini Itinerarium' is seemingly based on official documents, probably of the survey organized by Julius Caesar, and carried out under Augustus.