Survey datum

A survey datum is the agreed zero-reference level that a site’s heights, or reduced levels (RLs), are measured against. Every finished level on the job, the slab, the floor, the drainage falls, the boundary heights, is expressed as a height above or below this single reference. There are two kinds:

• A true datum: the Australian Height Datum (AHD), the national vertical datum tied to mean sea level, so an RL of, say, 12.500 m AHD is a real height above the sea.
• An assumed (site) datum: an arbitrary local zero (often set as RL 100.000) tied to a temporary benchmark on the site, used when the absolute height does not matter, only the relative differences across the site.

The datum is the foundation of every level on the job, so getting it wrong gets every finished level wrong. Two things matter most. Never mix datums on the one job: a level read off the assumed site datum and another off AHD will not agree. And on a flood-affected site you usually cannot use an assumed datum at all, because the consent sets the minimum finished floor level in metres AHD and the certifier will require the temporary benchmark to be tied to AHD, not to a local zero.

Confirm the datum on the survey and the architectural set before you set out: which datum the RLs are in, where the temporary benchmark is, and whether the consent requires AHD. Protect the benchmark on site so it does not get knocked, because if you lose it you lose your reference. See site set-out and survey and building height controls.

Also known as: Datum, level datum, vertical datum, reference level.

Category: Surveying / Site setout.

Related

• Site set-out and survey
• AHD (Australian Height Datum)

See also

• Reduced level (RL)
• Temporary benchmark
• Surveyor
• Building height controls

References

• Geoscience Australia: Australian Height Datum (verified 2026-05-10)
• ICSM: vertical datums (verified 2026-05-10)

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Last updated: 2026-05-30. Verified: 2026-05-10. Quarterly review for currency.