Sequences of repeated bathymetric surveys through time have been successfully used to document the morphodynamical evolution of coastal environments within historical times. Most chronobathymetric sequences have a restricted temporal coverage, as surveys rarely date back earlier than the 19th century. The aim of this study is to explore the potential of older lead line depth measurements as a reliable source of bathymetric data. A late 17th century marine chart of the Pertuis Charentais (France) was used as a test case. High-resolution scans of the chart sheets were retrieved from a public online digital library. A rare manuscript contemporary to the chart was obtained from the French National Archives, in order to constrain the methodology used to acquire depth measurements. A spatially explicit bathymetry covering 1500 km² was reconstructed from the interpolation of about 4600 georeferenced depth soundings. Vertical error was estimated by assuming the equivalence of depth values between the late 17th century and the recent (1960) bathymetries above subtidal rocks outcropping on the seafloor. Documentary data indicated that the chart author did not acquire depth measurements on purpose but instead collected data taken by local sailors and coasting pilots over an unknown period of time, probably ranging from a few years to a few decades. The large vertical error estimated to ±4.5 m error margin mainly resulted from the lack of tide correction. Bathymetric difference maps could not be computed, but despite these weaknesses, the large-scale sediment infilling of the area and the long-term evolution of an estuarine sandbank could be documented, in agreement with previous works. Beyond its utility for local studies, this article provides insights into the processing of depth measurements acquired long before modern hydrography surveys, and demonstrates that such old marine charts deserve consideration, provided conditions are met for their critical assessment.
