Dictionary.com Unabridged (v 1.1)[Based on the Random House Unabridged Dictionary]:

da·tum –noun,
plural da·ta for 1–3, da·tums for 4, 5.

1.a
single piece of information, as a fact, statistic, or code; an item of data.
2.Philosophy. a.any fact assumed to be a matter of direct observation.
b.any proposition assumed or given, from which conclusions may be drawn.

3.Also called sense datum. Epistemology. the object of knowledge as presented to the mind. Compare ideatum.
4.Surveying, Civil Engineering. any level surface, line, or point used as a reference in measuring elevations.
5.Surveying. a basis for horizontal control surveys, consisting of the longitude and latitude of a certain point, the azimuth of a certain line from this point, and two constants used in defining the terrestrial spheroid.

[Origin: 1640–50; < L: a thing given, neut. ptp. of dare to give]

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The Chambers Dictionary:
datum Sing of data (qv) --- datumline, -level ...

Bedeutung 1. in Dictionary.com Unabridged erfasst die Bedeutung ``Messweert'' von datum, daher Plural ``data''.

F"ur H"ohenmessumgen etc. ``datums'' scheint die Pluralform zu sein, Bedeutungen 4 und 5.

Ich bin auf dies gestossen, da diskutiert wurde ob ``data'' Singular oder Plural ist:
Siehe auch: data - Einzahl oder Mehrzahl?