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Small letters saying what the bottom is made of.
Also known as: Bottom characteristic, Holding ground abbreviation, Nature of seabed
Small italic letters beside a sounding say what the seabed is — S for sand, M for mud, R for rock, Co for coral. It is anchoring information: mud and sand hold an anchor, rock and weed generally do not.
States the composition of the seabed in abbreviated form beside a sounding, using a fixed vocabulary of letter codes.
Contexts: Navigation, Maps
Is anchoring information rather than geology: the codes exist because holding ground determines whether a vessel can safely lie there.
Contexts: Navigation, Maps
One or two small italic letters printed immediately beside a depth numeral on the chart face.