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chart datum A generic term that is often used ambiguously, because any nautical chart involves the use of several datums. In most cases when used it is often intended to mean either the horizontal datum (such as WGS84 or NAD83) or it is meant to be the sounding datum, which for US charts is MLLW. Nautical charts also employ a vertical datum and an elevation datum. When we want to discuss a datum, it is best to be specific, and not use the term "chart datum." See horizontal datum, sounding datum, height datum, and elevation datum.

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