A large-scale map of the phytobenthic communities and, in particular, of the Posidonia oceanica meadow in the Bay of Mondello (Palermo, Italy) was made. The map was set up from aerial photography, which was verified by divers along ground-lines. The upper limit of the underwater vegetation, with special reference to the Posidonia oceanica meadow, was accurately located using a geodimeter. This topographical instrument proved to be very useful in realizing such cartography.