"The directional indices T-M-P-L suggest templa, however their use is fairly conventional. Tramontane, literally "between the montains", is the name of the north wind or the north star as seen over the Alps. Meridióne is south, from the Latin meridianis for midday when Dianus, god of light and the sun, and male counterpart of Diana, is highest in the sky. Pončnte is west or the west wind, possibly from the Latin ponere "to put, or set down" in relation to the setting sun, while Levante is east, possibly from levare "to raise" in relation to the rising sun."
R. James Aitken, Piranesi-Vico-Il Campo Marzio: Foundations and the Eternal City.
Not too long ago, countable days actually, I first learned of Melania the Younger, and how her husband's (enormously expensive) family estate [Horti Valeriani] just outside the walls of Rome at the Salarian Gate, was one of the great properties (along with the Gardens of Sallust) that were plundered when Alaric and his Visigoths broke into (at the Salarian Gate) and sacked Rome for the first time. The Visigoths initially camped for many months outside the walls of Rome (near the Salarian Gate) thereby starving the city by disrupting all deliveries of grain from Africa to the city. The Salarian Gate, the Gardens of Sallust, and the Gardens Valeriani are all delineated within Piranesi's Ichnographia Campus Martius right where they are supposed to be. Interesting, right next to this complex of buildings/structures, Piranesi also delineates a Porticus Neronianae, a completely fictitious building in the shape of a large cross within a circle. Within a day of assimilating all this new data, I came to see how the inner circle of the Porticus Neronianiae matches the circle of the compass/north arrow that Piranesi also delineated within the Ichnographia, and I came to see how if you rotate the cross of the Porticus Neronianae 45 degrees, its four points then correspond exactly to the four cardinal points of global direction. The Porticus Neronianae of Piranesi's Ichnographia Campus Martius is the X that marks the spot where the first attacking Visigoths camped.
The inner circle of the Porticus Neronianae matches the circle of the compass that Piranesi also delineated within the Ichnographia.
The Horti Valeriani is situated in the upper right corner, directly above the Horti Sallustiani, which occupies the entire bottom portion of the plan. Note that the Aurelian Wall is delineated via a bowing dotted line, which is visible directly above the words 'Horti' and 'Sallustiani'. The position of the Salarian Gate--Porta Salaria-- is indicated along the dotted line just before it meets the lower right border.