The notion that there is a fixed line between cultures is itself a farce, but, historically the majority of such maps drawn for empires are written using the implied borders from latter-day interpretations of geography that was mentioned by an eventual victor as annexed. However, the reality on the ground is usually much different. In some cases, thousands of years different.
This is a difficult thing to describe for any pre-modern polity, where vassalage and weak frontiers make it difficult to describe if a central power actually controlled a territory. The Ottoman Empire de-facto exercised little control its North African vassals, the Roman Empire had increasingly weak control over its frontiers due to migrations and incursions, etc.
