Definition
Charts carry a source data diagram or reliability diagram identifying the age, method and scale of the surveys used in each area, from modern multibeam coverage to nineteenth century leadline work. In electronic charts this is formalised as zones of confidence, categories A1, A2, B, C and D, each with stated position and depth accuracy and seafloor coverage. Navigating on a chart whose source is a sparse lead line survey demands much greater clearance than the printed soundings alone suggest.
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