This drawing is an atmospheric bird’s-eye view of part of the Valdichiana, with the hills receding to a misty horizon. Arezzo and the bend of the Arno are to the left, and the lake of Trasimeno is in the right distance. Below, Leonardo listed and then crossed through the distances between Castiglione, Foiano and other towns in the vicinity, perhaps indicating that he used them when constructing a related map of the Valdichiana (RCIN 912278) or some other map. Melzi's number 156.

The purpose of the two sheets is not evident, though RCIN 912278, highly finished and with conventional script, must have been made for someone else to see. They have been associated with a revolt in Arezzo against Cesare Borgia in June 1502, or with Leonardo’s plans for the Arno canal (RCIN 912279), which had involved damming the Valdichiana with sluices to feed the river and canal when low. But it is the lake of Valdichiana that is the focus of attention, and it seems likely that the maps were made in connection with a plan to drain the malarial marsh. A channel cut northwards to the Arno in the fourteenth century to drain the lake had been only partly successful, and a channel cut in 1490 between Trasimeno and Valdichiana (seen in both maps here), to regulate the level of Trasimeno, had worsened the swampiness of Valdichiana. But no scheme is indicated here, and it was not until 50 years later that the digging of the present Canale Maestro along the length of the Valdichiana began in earnest.

Text adapted from Leonardo da Vinci: A life in drawing, London, 2018