Definition
Paper charts carry a source data diagram giving the year, surveying authority and method for each block of the chart. Electronic charts use ZOC categories A1, A2, B, C, D and U, each defining a position and depth accuracy and the degree of seafloor coverage; A1 is the best, U means unassessed. A nineteenth century lead line survey in category C or D may miss isolated pinnacles entirely, which is why prudent navigators avoid relying on soundings alone in such areas.
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