Reference point is the "Sepulcrum Hadriani".
The axis of the mausoleum extends itself in a vast forum, the perimeter of which encloses two circuses: the Circus Hadriani and the Circus Domitiae; at the end there is a complicated grouping of halls, where the Bustum Hadriani is located.
Following the axis of Hadrian's complex, roughly coinciding with the axis of via Cornelia, are the Horti Neroniani terminated at the heights of the hill by a building having a dominating position. If the position of this building coincides with that of the Vatican Basilica, the circus of Gaius and Nero is shifted [sic], in Piranesi's scheme, towards the foot of the Janiculum.
An intermediary grouping acts as transition between the Mausoleum of Hadrian and the Horti Neroniani is named the Area Marti, a joining of two hexagonal forums, with related arcades, and a Naumachia.
A built connection is created by a triangular, wedge shaped arrangement, the Horti Agrippinae, which, moreover, connects along the bank of the Tiber to a Natatio (actual location the church of S. Spirito).
Entering the area of the actual ancient Campomarzio, that is, in the area of the Campus Tiberinus, tucked into the bend of the Tiber, a few certain typographical bench-marks, established with accuracy by Piranesi, permit us to orient ourselves in this complex arrangement of buildings. We can identify, besides the curve of the Tiber, the Mausoleum of Hadrian, the Isola Tiberina, the Theater of Marcellus, the Pantheon, and the Mausoleum of Augustus.
The southern boundary of the Campomarzio of Piranesi is marked by a city wall moenia urbis priora aurelianensibus which may very closely correspond to the Servian wall. Perhaps a casual correspondence, because Piranesi could not have known the exact route of the Republican city walls. The route of this wall grazes the Mons Capitolinus and the Valli Quirinalis, which is also topographically shifted with respect to the true depression of the Hill.
The route of the Via Flaminia appears arbitrary.
The straight north-south line of the [present] Via Consolare turns sinuous and broken inside Piranesi's monumental complex of the city. And yet its outline was clear and known to its author since it was the route between the Porta Flaminia (Porta del Popolo) and the Campidoglio and was well defined by the built fabric of the seventeenth and eighteenth centuries.
The Via Flaminia of this new Campomarzio has a rather east-west orientation, reaching the base of the Mons Capitolinus, indeed brushing up against the Horti Pinci, but these last situated in a totally different place than the heights of the Mons Pincius.
It is true that Piranesi assigns to the Horti Salustiani an architectural complex which is extensive and symmetrically arranged in zone which may correspond to that valley, defined by the edge of the Quirinal and a section of the Servian wall, where today are located precisely those Horti Salustiani, but this zone is then surrounded by other arrangements of areas for sacred and public enclosures, which have no typographic comparison, and it is difficult to find any correspondence with the natural state of that zone which was still, in the time of Piranesi, rich with vegetation and ruins.
Maybe that which Piranesi marks as Circus Apollinaris is the Circus Salustianus, of which the ruins, in sections, were still visible.
Save for the few certain monumental reference points which can be identified with relative accuracy in Piranesi's plan, all the remaining area between the Tiber, the Campidoglio, the Quirinal, and the Pincio (the "Campus Martius") is architecturally composed by means of associations and unions that are mostly arbitrary and for which is lacking any possibility of documentation, even if one wishes to track them in the acquisition of archeological facts relative to the state of knowledge of the time.
Proceeding from the Pantheon area, the complex of the Porticus Pompeiana has a corresponding location [to the real situation]. Not so the Circus Flaminius, however, which is differently oriented with respect to the real situation.
More accurate is the orientation of the Circus Agonalis--called "of Alexander"--and, in turn, is joined to the circular system of the Templum Martis and the Ginnasium Neronis, the latter of which perhaps re-echoes the memory of the Baths of Nero.
In the area of the bend of the river (Campus Tiberinus - Tarentum) there is another grand Stadium with the temples of Jove, Juno, Minerva, and Venus, plus a Porticus Hadriani. Moreover, a complex dedicated to the memories of emperors Gratian, Valentinian, and Theodosius appears in the location, probably correct, of the Arcus.
Proceeding northward, in the [middle] zone of the Campomarzio marked by the Mausoleum [of Augustus] as the point of reference, with related Trophea Augusti, Piranesi's urban composition connects existing facts with intuitions of character, conforming to the open countryside character that this zone must have had, as far back as the Empire.
Still in the dense monumental zone is the Forum Aureli, where Antoninus' Column is enclosed by a wall and a building, connecting to the Templum Solis, with architectural imagination, but also with a vague archeological idea.
To the site of our Montecitorio corresponds an Amphiteatrum Statili Tauri, a notion that follows the theories that locate a theater at that spot.
The Prugma Solstium, is the solar clock indicated at that spot by modern typographers.
Past this architectural demarcation, the complexes appear in arrangements where green spaces prevail, thinning out, or marked by secondary alignments.
Here we find the Septa Trigaria and the spaces adjacent, within which are irregularly distributed the sepulchers: Sepulcrum Hirty e Pansae Consolum; Sepulcrum Syllae; Sepulcrum Juliae fil. C. Caesaris.
The entire zone corresponding to the present trident of roads from Piazza del Popolo is composed on the axis of the Mausoleum of Augustus, which leads to the Nemus Caesaris, a complicated architectural layout made up of halls and porticoes, at the center of which is the Bustum Caesaris Augusti, and terminates at a Templum Manium Augustorum.
The preceding area is enclosed by a semi-circle for sepulchers and images of illustrious men: the Memoriae Vespasiani, Tiberi, Claudi, Titi; radially arranged is the Memoriae rerum Caesarum Augusti; and on the axis, the Colonna rostrata Augusti.
All the imagination of the artist is given to re-evoking for the figure of the emperor a citadel consecrated to him which unites sepulcher and memory in a majestic architectural unity.
This grand systemization reaches the foot of the Pincio.
Piranesi fills the heights of the Pincio with a villa complex, an architectural nucleus called Horti Lucullani and with a singular Villa Aruntii, which appears as an isolated nucleus that one would be hard pressed to locate in the built fabric of the late 1500's and of the 1600 and 1700's. (It may be the location of the Trinita' dei Monti, adjacent to the Horti Pinci and the Domus Pinci.)
The character of the green zone is completed by the Horti Anteri, an architectural game for the Exercitationes Militares, the Balnea Veneris, and the Viridarium Lucii.
On the plan reappears the Neronian memory of the Porticus Neronianae--what information this is based on is not clear--and the Horti Valeriani bordering, in Piranesi's vision, the areas of the Horti Salustiani.
The whole layout of the northeast zone, as we have noted, is a free urban composition: the names given to the monumental groups have no archeological correspondence. The whole ensemble has an open character, dominated by green spaces, in an ingenious intuition of what the periphery of Rome must have been like, dominated by "horti," by villas, and by public spaces like the portico called Porticus S.P.Q.R. amoenitati dicata annexed to a Gimnasium; and like the Naumachia Domitiani on the river bank. A vast systemization of green zones on a triangular framework, protected by the statuae viroium illustrium, occupies the major part of the area of open fields.
The compositional alignment in given constant orientation by a Porticus Vipsania, and by a Porticus Alexandri Severi at the extreme north, the facades of which follow and define the course of the Equiria, which forms, as we have noted, one of the axes of Piranesi's composition. Around these are arranged, but constantly oriented with the aforementioned axis, other minor edifices; temples and sepulchers, with which the architect re-invokes the character of the ancient built fabric.
In those heights which we have marked out as corresponding to our present-day Villa Balestra, are Villas: Villa Plauti; Horti Dolabellae; Horti Pomponi; Horti Plotii.
In the valley that today we call Giulia, other singular edifices; the Sepulcrum Julii Caesaris et Drusi (circular), a Sepulcrum Agrippae on a trilobate scheme, and some ex-defense towers, Turres espugnandae.
The Circus Caii, et Neronis (Circue of Caligula and Nero) is positioned correctly within the Ichnographia Campus Martius.