Maurice Davies has identified Turner’s sources as the first, unnumbered plate and plate Bi of Viator (Jean Pélerin), De Artificiali Perspectiva, Toul 1505.1 There are further notes from this source on folio 26 verso (D07395), folio 27 recto (D07396) and folio 61 recto (D07458). See the entry for folio 27 for more on Viator and Turner’s difficulties in citing this source other than by date, due to the lack of a title page in the copy he apparently consulted at the British Museum (since transferred to the British Library, London).
Turner rather carelessly copied Viator’s quite straightforward diagram of a square in perspective within a circle, the middle of three diagrams in the first plate, twice here and once on folio 27 recto (D07396), but without properly indicating the actual square; he apparently reminded himself on folio 7 verso (D07367) to look ‘at 1505 for a square’, but Davies notes that ‘no successful copy of the diagram survives’.2
Andrea Fredericksen has linked the present sketch and that on folio 27 recto to Turner’s lecture diagram 27, showing concentric squares in perspective (see entry for Tate D17041; Turner Bequest CXCV 71); the diagram also seems to have been informed by the perspective plan of concentric steps noted from Viator on folio 26 verso.
The building at the top right, a simple church-like structure on a Latin cross plan, is taken from the top of Viator’s plate Bi, below which a similar building is shown from an elevated viewpoint, with two further buildings on different rudimentary plans below. In each case the plan is projected in perspective below the building, as Turner shows here.