For maps of Constantinople, Gibbon refers to Anselmo Banduri (d 1743) Imperium Orientale (1729)
For the city of Rome, Gibbon cites Famiano Nardini (1660-1661) Roma Antica (1704)
Giambattista Nolli (1701-1756) published a large map of Rome, the Pianta Grande di Roma, engraved in 1748. “The map is composed of 12 copper plate engravings that together measures 176×208 cms and was published in response to the commission of Pope Benedict XIV to survey Rome in order to help create demarcations for the traditional 14 rioni or districts.”(Wikipedia) Gibbon repeatedly refers to it as “great”, “new and excellent”, and adds that it “would furnish a solid and accurate basis for the ancient topography of Rome.”(31n109, 41nn87 & 102, 71n85)
Use the zoom features of the University of Oregon’s Interactive Nolli Map Website.