There is a lot about ancient people that we simply don't know. The Ancient Romans were great sailors with mighty fleets of warships and merchant ships, that could navigate on the open ocean both in Meditteranean and the Atlantic using the sun and stars. Of course doing that reveals the curvature of the Earth. Marinus of Tyre who influenced Ptolemy devised a system of nautical charts. We only know this through Ptolemy and none of the charts exist. The Romans also knew how to survey land, and it doesn't take a large leap of logic to imagine that sailors, astronomers, and tax collecters used some kind of maps. We just don't have any of them.