The explicitly defined reference surface or zero‑elevation level (geodetic or project‑local) from which vertical positions, heights and depths are measured and reported in field recording.

Principle

Vertical measurements are meaningful only relative to a stated vertical datum; without the datum, numeric elevations lack an unambiguous reference and cannot be reliably integrated with other vertical data.

Demonstration

Illustrative scenario: Situation — A report lists feature elevations. Recognition — The archaeologist specifies that all elevations refer to the project's temporary datum, itself tied to a national vertical datum by a surveyed benchmark. Action — Other researchers convert elevations to their GIS by applying the documented offset. Consequence — Elevations integrate correctly across datasets because the datum relationship is recorded.

Misapplication

Reporting elevations without naming the vertical datum or its relationship to any national or project datum; the semantic error is treating elevation values as absolute when their reference surface is unspecified.

Consequence

Omitting the datum produces ambiguities when datasets are merged, can introduce systematic elevation offsets, and may invalidate comparisons of depth‑dependent analyses such as sediment accumulation or construction phases.

Reversal

For analyses confined to relative stratigraphy within a single, short‑term excavation area, a project‑local datum that is not referenced to a national system may be sufficient; conversely, long‑term monitoring, cross‑site comparison and integration with topographic data require a geodetically traceable vertical datum.

Boundary

Clearly within: elevations explicitly stated as referenced to a named vertical datum (national or project) with documented tie points. Boundary case: elevations given relative to an unnamed 'site zero' recorded only in field notes. Clearly outside: qualitative depth descriptions (e.g., 'deep', 'shallow') without numeric elevation or datum.

Semantic Tension

Absolute geodetic anchoring for interoperability ↔ sufficiency of relative stratigraphic levels for intra‑site interpretation and efficiency.

Synthesis

A vertical datum is the necessary semantic anchor that gives elevation numbers meaning; specifying its identity and any offsets is essential for integrating, comparing and reusing vertical data across surveys, projects and time.