The quote indicates a Roman expansion of land. during the rule of Augustus
Step-by-step explanation:
During the prosperous reign of the king Augustus, the empire of Rome stretched along the lines that the quote has given thus it is a passage about the stretch of land that was under Roman control at that time.
The tone of the passage conveys that it is not just a survey of the land under Roman rule but a boast of the might of the Roman empires and its vast stretch from mythical rivers to far away lands as being described here.