On this topography of Rome map, you'll see numbers telling the height of the area, in meters.
The map is labeled hydrography and chorography of ancient Rome. While hydrography may be intuitive -- writing about or mapping of the water system, chorography probably isn't. It comes from the Greek words for country (khora) and writing or -graphy and refers to delineation of districts. Thus this map shows the areas of ancient Rome, its hills, the walls, and more.
The book from which this map comes, The Ruins and Excavations of Ancient Rome, was published in 1900. Despite its age, it would be worth reading if you want to know about the topography of ancient Rome, including water, soil, walls, and roads.