基准线

Datum \Da"tum\, n.; pl. {Data}. [L. See 2d {Date}.]
1. Something given or admitted; a fact or principle granted;
that upon which an inference or an argument is based; --
used chiefly in the plural.

Any writer, therefore, who . . . furnishes us with
data sufficient to determine the time in which he
wrote. --Priestley.

2. a single piece of information; a fact; especially a piece
of information obtained by observation or experiment; --
used mostly in the plural.
[PJC]

3. pl. (Math.) The quantities or relations which are assumed
to be given in any problem.

4. (Surveying) a point, line, or level surface used as a
reference in measuring elevations. --RHUD
[PJC]

{Datum line} (Surv.), the horizontal or base line, from which
the heights of points are reckoned or measured, as in the
plan of a railway, etc.
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