Definition
The IHO scheme classifies survey quality as A1, A2, B, C, D and U for unassessed, each defining position and depth accuracy and the expected seabed coverage. A1 implies full seafloor search with position accuracy of about 5 metres plus 5 percent of depth, while C and D indicate large uncertainties and possible undetected hazards. ZOC is shown on paper charts as a diagram and in electronic charts as the CATZOC attribute, and should be checked before relying on charted depths.
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