Concept reference

Datum, Stationing and Elevation

C.1Plain-language definition

A datum is the agreed zero from which elevations are measured; stationing measures distance along an alignment. Neither changes the physics, but both control whether independent drawings describe the same physical point. Elevation differences survive a datum shift; mismatched datums create fictitious static head and construction conflicts.

C.2Governing relation or decision logic

If every elevation moves by a constant C, then (z2 + C) − (z1 + C) = z2 − z1. Stationing is commonly written as accumulated length, such as 0+125 for 125 m in a metric project, but notation and reset rules are local drafting practice and must be defined on the sheet.

C.3The failure to name

“Local datum” without a recoverable benchmark can strand future survey work. Confusing pipe invert, crown, centreline and water-surface elevation is equally dangerous because all four may appear at one station but represent different heights.

C.4Field review checklist

• Name the vertical datum, units, benchmark and any project conversion.
• Define the stationing origin, direction and equations or resets.
• Label each elevation by object and reference point—not just a number.
• Reconcile key elevations across plan, profile, sections, equipment schedules and controls.

C.5Where it fits in the course

See §1.5 for drawing literacy and §5.2 for datum errors in energy calculations.