Spatially explicit bathymetric reconstruction of the Pertuis Charentais (France) from lead line depth soundings of the late 17th century

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.

历时性重复水深测量序列已被成功用于记录历史时期内海岸环境的地貌动力演化过程。绝大多数时序水深序列的时间覆盖范围有限，因水深测量记录极少能追溯至19世纪之前。本研究旨在探索更早期的测铅绳水深测量数据作为可靠水深数据源的潜力。本研究以17世纪晚期法国佩尔蒂沙朗泰（Pertuis Charentais）海域的海图为测试案例。海图图纸的高分辨率扫描件从公共在线数字图书馆获取。为明确该海图所采用的水深测量方法，研究团队从法国国家档案馆获取了一份与该海图同期的珍稀手稿。通过对约4600个地理参考水深测点进行插值，重建了覆盖1500平方千米的空间显式水深场。垂直误差通过假设17世纪晚期与1960年近期海床出露的潮下岩上方的水深值等效来估算。文献资料显示，该海图的作者并未主动开展水深测量工作，而是收集了当地水手与沿岸领航员在一段未知时长（可能为数年至数十年）内获取的实测数据。估算得到的±4.5米的较大垂直误差主要源于未进行潮汐校正。尽管无法绘制水深差值图，但即便存在这些局限性，本研究仍成功记录了该区域的大规模沉积物充填过程与一处河口沙坝的长期演化趋势，这与此前的研究结果一致。除服务于本地相关研究外，本文还为现代水文测量之前的早期水深数据处理提供了研究视角，并证明只要满足对其开展批判性评估的条件，这类古老海图仍具备重要的研究价值。