One of the images in Du Perac’s 1575 collection of sketches of Rome is of the forum of Nerva. Here’s what he saw:
The Temple of Minerva is at the front left.
From the Murray guidebook of 1843, p.271, I learn that
Paul V (Borghese) took down an entablature and pediment in the Forum of Nerva to build a fountain on the Janiculum,…
A modern map of the imperial forums is here. This extract shows the forum of Nerva:
Du Perac’s view is looking up the forum, and along the right-hand side of the temple at the top of it.
An 1869 view from here is this:
There are a series of images (including Du Perac’s) here.
But … I gather there may be some confusion between the forum of Nerva and the forum of Augustus? This site has an image by Piranesi, of what is plainly the same place.
But the text on the page says “The Forum of Augustus (erroneously called Forum of Nerva)”.
Finally on Flickr I find this image of what can be seen today.
The Forum of Nerva is adjacent to and South East of the Forum of Augustus.
The wall in Piranesi’s vedute is the firewall at the back of the Temple of Mars Ultor, which stood on the NE side of the Forum of Augustus (‘up’ on your map is NE; to align the map N – S turn it 45 deg. to the right).
The wall in shadow on the extreme left of the print is the NW extremity of the Temple of Minerva, which stood in a similar relationship to the forum of Nerva as the Temple of Mars Ultor to the Augustan Forum.
Behind the wall (that is, the street from which Piranesi draws) is the subura between the Esquiline and Quirinal hills, now the lowest part of the Monti district. I am not sure that the arch is contemporary with the rest of the wall; if it is, then it was the southernmost entrance to the Forum of Augustus and, passing through it, one would have been bounded to the left by a portico that gave access to the southern exedra of the Augustan Forum.
The building seen above and beyond the columns in the Piranesi vedute no longer exists, as can be seen in the contemporary photograph.
Thank you so much for this; and I apologise for the delay, but WordPress placed several comments into the spam box, where I did not see them. I appreciate you explaining what is what. The wall at the back of the temple of Mars Ultor is familiar to me, tho; I often stay in the Hotel Nerva on the other side of it.