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horizontal datum Also called geodetic datum. It is the prescription or reference frame used on nautical charts to correlate a Lat and Lon taken from a chart with a specific point on the surface of the earth. The prescription depends on the assumed shape of the earth, which varies from a perfect sphere, being more of a doorknob, or with even more careful study, more of a pear shape. Common horizontal datums used on charts include WGS84 or NAD83. To have your GPS position of a precisely located point on earth agree with the Lat and Lon read from the chart, your GPS must be set to the same datum, usually an option in the set up. Specific parts of the world have more localized datums for even more precision. This is sometimes referred to as "the chart datum," but this is an ambiguous terminology. See chart datum.

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