The Petronia Amnis of the Ichnographia begins in the east and flows along the southern edge of the Hortulorum valley. Its northern bank in the valley contains the Sepulchrum Agrippae and the Sepulchrum Julii Ceasaris et Drusi, while its southern bank is the ridge of the hill on top of which is the Horti Lucullani and some Turres expugnandae (military assult towers). Eventually, the stream skirts the southern portion of the Equiria and the northern flank of the Bustum Augustii. Ultimately, the Petronia Amnis flows into the Tiber.
The Petronia Amnis of the Ichnographia begins in the east within the valley of the Hortulorum, then skirts the northern flank of the Bustum Augustii, and eventually flows into the Tiber. Piranesi dislocates the Petronia Amnis from its original situation to a more northern position.
Piranesi acquired a detailed knowledge of Bufalini's Ichnographia Urbis of 1551 via his direct involvement with Nolli's Pianta Grande di Roma of 1748. A decade later, in 1758, Piranesi began his Ichnographia Campus Martius where, in some instances, he utilized Bufalini's map/plan as source material for the redrawing of ancient Rome's urban plan. Bufalini's plan, especially in the open areas all around the built-up section of Medieval and Renaissance Rome, includes 'reconstructions' of the larger ancient edifices like the imperial baths and stadiums, and some temple complexes. There are also near countless unnamed, fragmentary plans of ancient remains; remains, moreover, that, after consulting Nolli's plan, appear to no longer exist in Piranesi's time. It is from a select group of plan fragments on the Mons Pinicus or Collis Hortulorum of the Ichnographia Urbis that Piranesi imaginatively redraws the Horti Luciliani, the Sepulchrum Neronis, a Basilica along the Via Flaminia, the Horti Pincii, and the Monumentum Comitis Herculis.
Piranesi's resultant redrawn plans suggest a methodology whereby the fragmentary plans of Bufalini were used as kernels of ancient fact that, in turn, galvanized newly interpreted redrawings of what once was. As suggested by Dixon, "as there were great gaps in the knowledge of the past, great leaps were then needed to supply the holistic vision of the past which was the aim of scholars--archaeologists and historians--like Piranesi. In the Ichnographia, Piranesi filled the gaps..." Futhermore, from a strictly design point of view, Piranesi used some the fragmentary plans of Bufalini as contiguous elements which, when mirror-copied and multiplied, manifest the beginnings of the new plans.
Besides Bufalini's plan delineations, Piranesi also makes use of Bufalini's labelings. Bufalini labels all his full plan reconstructions of ancient buildings, often labels the fragmentary plans, and even labels blank locations (indicating the spot of an ancient edifice although actual remains no longer then existed). In utilizing the labels within the area of the Mons Pinicus or Collis Hortulorum, however, Piranesi hardly remains faithful to Bulafini's data. For example, where Bufalini positions the Horti Salustiani and Domus Pincii, Piranesi places the Horti Luciliani and, in turn, places the Horti Salustiani and Domus Pincii further east; the street Bufalini labels Via Conlatina, Piranesi labels Via Flamina; where Bufalini positions the Sepulcr. Neronis, Piranesi places the Bustum Caesaris Augusti and, in turn, labels an unnamed fragment along his Via Flaminia Sepulchrum Neronis; a small round structure Bufalini labels T. Solis, Piranesi labels Aula within the Horti Luciliani and, in turn, labels a newly imagined round building further south Delubrum Solis. It is honestly difficult to discern whether Piranesi is here playfully inverting Bufalini's data or actually rectifying Bufalini's "facts" with advanced knowledge of the past. Like Bufalini, Piranesi groups the Domus Martialis, Ludus Florae and the Templum Florae together, but he positions the group further west and moves the Domus Martialis south rather than north of the Ludus Florae. And where Bufalini locates the Sepulch. Falimiae Domiciarum, Piranesi places a very small Sepulcr. Familiae Aenobarb. and a very large Sep. Cnei Domitii Calvini whose plan Piranesi bases on an unrelated fragment of the Forma urbis.