What is "datum line"

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.

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

   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. [1913 Webster] ||

n. (context engineering English) A line which serves as a reference or base for the measurement of other quantities.