Definition
A diagram or note on the paper chart, corresponding to the zone of confidence attribution carried on electronic charts, dividing the sheet into areas and stating the date, scale and method of the underlying survey. Areas surveyed by lead line in the nineteenth century may have wide unsounded gaps between sounding lines and poorly fixed positions, so apparently clear water proves nothing. Passage plans should widen margins, avoid least depth areas and increase under keel clearance wherever survey quality is stated as poor.
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