The use of historical bathymetric charts in a GIS to assess morphological change in estuaries

Navigational charts are a valuable data source for the study of morphological change in estuaries, as they provide bathymetric information of many estuaries, and span a vast period of time. Charts are suitable for studying patterns of morphological change, such as shoal and channel migration. In addition, sequential bathymetric charts or bed surveys can be used to calculate sedimentation and erosion rates. However, a number of problems arise when using these data to identify morphological change. Sources of error and uncertainty are associated with surveying techniques used, density of depth sampling points, interpolation and averaging during compilation of the chart. Large systematic errors may stem from the non-uniformity or poorly defined levels to which the depths on the charts are reduced. Errors propagate in spatiotemporal operations using such charts. Analysis of historical bathymetric charts from the Ribble estuary (north-west England) illustrates the difficulties involved in assessing morphological change in a quantitative manner. The pattern of morphological development within the Ribble estuary over the last 150 years was clear, and a significant long-term net accretional trend was found. However, temporal variations in the rate of sedimentation on a decadal scale were generally not significant.